Measuring & Testing of Components - Andhra Pradesh · PDF file · 2015-07-29Digital voltmeter gives a numerical value of voltage on display system. ... fsd) A basic D‘Arsonval

Structure1.1 Measurement of AC/DC voltage and currents using voltmeter and current meters

1.2 Regulated power supply

1.3 Measurement of voltages, currents & resistance using analog & digital multimeter & continuity test

1.4 Test & measure the values of capacitor using R.L.C meter & com pare with the marked / color code value

1.5 Transformer testing

1.6 Test the given loud speaker and measure the voice call resistance using multimeter

1.7 Test the working of different types of switches relays connectors

Learning ObjectivesAfter studying this uint, student will be able to

• Able to measure AC DC voltage and DC current by using voltmetre and Ammeter

• Application of regulated DC power supply

• Measurement of voltage, current and resistance by using Analog and Digital Multimeter

1UNIT

Measuring & Testing of Components

Electronics Engineering Technician274

• Measurement of Resistance, Inductance, Capacitance by using digi tal LCR meter, Color Code.

• Testing of Transformer, Measurement of DC resistance

• Measurement of Voice coil resistance of the loudspeaker.

• Study of Switches, Relays, Connectors and Cables.

1.1 Measurement of AC/DC voltage and currents using voltmeter and current metersVoltage Definition

Voltage can be defined as potential difference between two points in aelectric circuit. A voltmeter is used for measuring a voltage in a electrical circuitthe units for voltage are “volts”.

Analog Voltmeter

Analog voltmeters move a pointer across a scale in proportion to thevoltage of the circuit.

Digital Voltmeter

Digital voltmeter gives a numerical value of voltage on display system.

AC Voltmeter

To measure ac voltage the output ac voltage is rectified by half waverectifier before the current passes through the meter across the meter the otherdiode serves as a protection. The diode conducts when a reverse voltage appearsacross the diode. So that current by passes the meter in the reverse direction.

Fig 1.1 AC Voltmeter

1000 V

500 V

250 V

50 V

10 V

1 M

500K

400K

80K

53 2.2

1540

638

326

D2

+ -

D1

Paper - III Measuring Instruments, Consumer & Power Electronics 275

Multirange AC Voltmeter

Fig 1.2 Multirange AC Voltmeter

This is the circuit for measuring different ac voltages resistancesR1,R2,R3,R4&R5 from a chain of multiplier for voltage ranges of1500V,1000V,250V,50V&10V respectively on the 2.5V range, resistance R6acts as a multiplier and corresponds to the multiplier Rs. Rsh is the meter shuntand acts to improve the rectifier operation.

DC Voltmeter

Dc voltmeter is necessary, to know the amount of current required todeflect the basic meter to full scale. This current is known as full scale deflection(Ifsd)

A basic D‘Arsonval movement can be converted into a dc voltmeter byadding a series resistor known as multiplier. The function of the multiplier is tolimit the current through the movement so that the current dosenot exceed theFSD value.

Fig 1.3 DC Voltmeter

Im = fsd current of the movement Ifsd

Rm = initial resistance of movement

Rs = multiplier resistance

D2

D1R2 R3

1000 V

R4 R5R1 R6

1500 V

250 V

50 V10 V2.5 V

AC i/p

Rsh Rm

RsIm

RmV


V= full range voltage of the instrument

V = Im (Rs+Rm)

Im = _V___ (Rs+Rm)

Rs = V- ImRm Im

Problem 1

A basic D‘Arsonval movement with a fsd of 50µA and internal resistanceof 500&is used as a voltmeter. Determine the value of the multiplier resistanceneeded to measure a voltage range of (0-10V)?

Sol

Rs = V Rm Im

= 10V 500 50µA

= 0.2×106 -500 = 200K – 500

Multirange Voltmeter

To obtain a multirange voltmeter, we connect a number of resistors alongwith a range switch to provide a greater number of workable ranges.

The multipliers are connected in a series string and range selector selectsthe appropriate amount of resistance required in series with the movement.

Fig 1.3 Multirange Voltmeter

Im

Rm

+

+

-

-


Fig 1.4 Multipliers connected in series string

This arrangement is advantageous compared to the previous one becauseall multiplier resistance value and are also easily available in precision tolerance.

The first resistor or low range multiplier R4 is the only special resistorwhich has to be specially manufactured to meet the circuit requirements.

Problem 2

Convert a basic D‘Arsonval movement with an internal resistance of50&and a fsd Current of 2mA into a multirange dc voltmeter with voltageranges of 0-10V, 0-50V,0-100V,0-250V?

Sol

Fig 1.5

10V range(V4 position of switch) the total circuit resistance

Rt = = 5K&

R4 = Rt-Rm =5k-50=4950&

R1 R2 R3 R4

Im

Rm

V 1 V 2 V 3

V 4+

-

V 1 V 2 V 3

+

-

0-250 V 0-100 V 0-50 V 0-10 V


for 50V range (V3 position of switch) the total circuit resistance

Rt = = = 25K&

R3 = Rt - (R4+Rm) = 25×103-(5×103)

R3 = 20K&

for 100V range (V2 position) Rt = = 50K&

R2 = Rt – (R3+R4+Rm)

= 50×103 – [4950+20K& +50K&]

R2 = 25K&

for 250V range (V1 position)

Rt = = 125k&

R1 = 125×103 - [4950+25×103+20×103+50]

= 125×103 - [5×103+25×103+20×103]

= 125×103 – 50K = 75K&

Rt = R1+Rm+R2+R3+R4

= 75+25+20+4950+50

= 125K &

Extending Voltage range

The voltmeter can be extended to measure high voltages by using a highvoltage probe or by using an external multiplier resistor. In most meters thebasic movement can be used to measure very low voltage. However great caremust be used not to exceed the voltage drop required for full scale deflection ofthe basic movement.

Fig 1.6 Extending Voltage range

Externalmultiplier

Meter set to lowestcurrent range To leads


Measurement of Current using Ammeter

Fig 1.7 Measurement of Current using Ammeter

DC Current Measurements

Electronic voltmeters are frequently constructed to act as multipurposeinstruments so that they can be used to measure current as well as voltage. Theunknown current is made to flow through a known standard resistance. Thevoltage drop across this resistance is proportional to the current and is measuredby a VTVM or a TVM. The scale of the meter is calibrated in terms of current

AC Current Measurement

When alternating current is to be measured a rectifier to change thealternating current into a corresponding direct current .Which is then measuredby VTVM ot TVM.

Another method employs an AC current probe which enables the ACcurrent to be measured without disturbing the circuit under test. The AC currentprobe clips around the wire carrying the current and in effect makes the wire aone turn primary of a current transformer(C.T). The C.T has a ferrite core andthe secondary consists of a large number of turns.

The voltage induced in the secondary winding is amplified and theamplifier’s output can be measured by any suitable AC voltmeter. Normally theamplifier is designed so that 1mA current in the wire being measured produces1mV at the amplifier output. The current is then read directly on the voltmeterusing the same scale as for voltage measurements.

Battery

Ammeter

Lamp


DC Ammeter

Fig 1.8 DC Ammeter

The basic movement of a dc ammeter is a pmmc galvanometer. Sincethe coil winding of a basic movement is small and light, it can carry only verysmall currents. When large currents are to be measured, it is necessary to bypassa major part of the current through a resistance called a shunt.

The resistance of shunt can be calculated by using conventional circuitanalysis.

Rin = Internal resistance of the movement of the coil

Rsh = Resistance of the shunt

Im = Full scale deflection current of the movement

Ish = shunt current

I = full scale current of the ammeter including the shunt

Since the shunt resistance is in parallel. With the meter measurement thevoltage drop across shunt and movement must be same.

Vsh = Vin

Ish×Rsh = Im×Rm

But

Ish = I - Im

Hence

For each required value of full scale meter current. We can determinethe value of shunt resistance.

RshIsh

Rm

Im+

-

I

-

+


Ex : A 1mA meter movement with a an internal resistance of 100&! isto be converted into a 0-100mA. Calculate the value of shunt resistancerequired?

Sol

Given Rn = 100&

Im = 1mA, I = 100mA

The shunt resistance used with a basic movement may consist of a lengthof constant temp resistance wire within the case of the instrument. Alternatively,these may be an external (manganin or constantan) shunt having a very lowresistance.

The general requirements of a shunt as follows

1. The temperature coefficients of the shunt and instrument should be low and nearly identical.

2. The resistance of the current shunt should not vary with time.

3. It should carry the current without excessive temperature rise.

4. It should have a low thermal emf with copper.

Extending the Range of DC Ammeter

Fig 1.9 Extending the Range of DC Ammeter

The current range of the dc ammeter may be further extended by anumber of shunts, selected by a range switch, S- such a meter is called “MultirangeAmmeter” shown in fig.

D’Arsonval

movementRm

R1 R3R2 R4

+

+ Im

-

-


The circuit has four shunts R1,R2,R3&R4 which can be placed in parallelwith the movement to give four different current ranges I1,I2,I3&I4.

Switch S is a multi position, make before break type switch this switchprotects the movement from being damaged without a shunt during rangechanging.

The Aryton shunt or universal shunt

Fig 1.10 Aryton shunt or universal shunt

The Aryton shunt eliminates the possibility of having the meter in thecircuit without a shunt. This advantage is gained at the price of slightly higheroverall meter resistance. Fig shows a circuit an Ayrton shunt ammeter. In thiscircuit, when the switch is in position resistance Ra is in parallel with the seriescombination of Rb, Rc and the meter movement, hence the current through theshunt is more than the current through the meter movement, thereby protectingthe meter movement and reducing its sensitivity.

If the switch is connected to position ‘2’, resistance Ra and Rb aretogether in parallel with the series combination of Rc and the meter movement.Now the current through the meter is more than the current through the shuntresistance,

D’ArsonvalmovementRm

Rc

Rb

Ra

1

23

+

-

S+

-


If the switch is connected to position ‘3’, Ra, Rb, Rc are together inparallel with the meter. Hence maximum current flows through the metermovement and very little through the shunt. This increases the sensitivity.

Precautions to be taken while using an Ammeter

1. As the ammeter resistance is very low, it should never be connected across any source of e.m.f always connect an ammeter in series with the load.

2. The polarities must be observed correctly. The opposite polarity deflects the pointer in opposite direction against the mechanical stop and this may damage the pointer.

3. While using Multirange ammeter, first use the highest current range and then decrease the current range until sufficient deflection is obtained. So to increase the accuracy, finally select the range which will give the reading near full scale deflection.

1.2 Regulated Power SupplyA regulated power supply is an embedded circuit or stand alone unit the

function of which is to supply a stable voltage (or less often current), to a circuitof device that must be operated with in certain power supply limits.

The output from the regulated power may be alternating or unidirectional,but is nearly always DC (direct current).

Applications

1. Dc variable bench supply usually refers to a power supply capable of supplying a variety of output voltages useful for bench testing electronic circuits possibly with continuous variation of the output voltage or just some preset voltages.

2. A laboratory power supply normally implies an accurate bench power supply, while a balanced or tracking power supply refers to twin supplies for use when a circuit requires both positive & negative supply rails.

3. In mobile phone power adaptors.

4. Regulated power supplies in appliances.

The earlier regulated power supplies includes batteries, resonanttransformer, nonlinear resistors, loading resistors, neon stabilizer tubes, vibratingregulators, power control lines, discrete circuit.


A Test bench power supply circuit

Fig 1.11 A Test bench power supply circuit

1.5 Volt power supply using the LM 17 T

Fig 1.12 (1.5 Volt power supply using the LM 17 T)

1N4001

Heat sink

1000uF 16 V

10uF

220

47 1uF

1.52 Volts (calculated)

12.6 Volts center tapped secondary 450 MA

1N4001

x RlVour

1.25+ (ADJ x R2) R2 =

R2

R11 +Output voltage = 1.25 x - 1

Transformerseconadry18 vac - 2 Amp

Bridge rectifier(50 volt / 2 Amp)

R1 = 220D1, 2 =1N4001 R2 = 2000

Line

Neutral

Ground

Fuse 0.5 Amp

3300F35 V

LM317T

10 F 25VAC

AC

0-15V

Com-mon


1.3 Measurement of Voltages, Currents & Resistance using analog & digital multimeter & continuity test

For the measurement of d.c as well as a.c voltage and current resistance,an electronic multimeter is commonly used. It is also known as Voltage OhmMeter (VOM) or multimeter.

The features of multimeter are

1. The basic circuit of VOM includes balanced bridge d.c amplifier.

2. To limit the magnitude of the input signal, range switch is provided by properly adjusting input attenuator input signal can be limited.

3. It is also includes rectifier section which converts a.c input signal to the d.c voltage.

4. It facilitates resistance measurement with the help of internal battery and additional circuitry.

5. The various parameters measurement is possible by selecting required function using Function switch.

Use of multimeter for D.C voltage measurement

The fig shows the arrangement used in multimeter to measure the d.cvoltages. For getting different ranges of voltages, different series resistances areconnected in series which can be put in the circuit with the range selector switch.

We can get different ranges to measure the d.c voltages by selectingproper resistance in series with the basic meter.

Fig 1.13 Use of multimeter for D.C voltage measurement

R1 R2 R3 R4 R5

1000 V

250 V50 V

10 V

2.5 V

5000 V d.c D.C Voltage


Use of multimeter as Ammeter

To get different current ranges, different shunts are connected acrossthe meter with the help of range selector switch. The working is same as that ofPMMC ammeter.

Fig shows the arrangement used in the multimeter to use is as an ammeter.

Fig 1.14 Arrangement used in the multimeter to use is as an ammeter

Use of multimeter for measurement of A.C Voltage

Fig shows voltmeter section of a multimeter

Fig 1.15 Voltmeter section of a multimeter

The rectifier used in the circuit rectifies a.c voltage into d.c voltage formeasurement of a.c voltage before current passes through the meter. The otherdiode is used for protection purpose.

R1

R2

R3

R5R4 R6

Rangeselectswitch

250 mA 500 mA

50 mA

R1 R4 R5R2 R3

R6

D1

D2M

1000 V

250 V50 V

10 V

2.5 VS switch

A-c voltage i/p


Use of multimeter for resistance measurement

The fig shows ohm meter section of multimeter.

Fig 1.16 Ohm meter section of multimeter

For a scale multiplication of 1 before any measurement for a scale isshort circuited and “zero adjust” control is varied until the meter reads zeroresistance i.e. it shows full scale current.

Now the circuit takes the form of a variation of the shunt type ohmmeter.Scale multiplication of 100 and 10,000 can also be used for measuring highresistance. Voltages are applied to the circuit with the help of battery.

Digital Multimeter

Digital multimeter is an instrument used for the measurement of voltage,current and resistance. Fundamentally it is a digital voltmeter.

Working

The block diagram of digital multimeter is shown in fig.

All quantities other than d.c voltage are first converted to an equivalentd.c voltage by some device. To measure a.c voltage, the input a.c is convertedinto a d.c voltage by means of a rectifier. Attenuator is used to bring the inputsignals to the level acceptable by the multimeter.

The digital multimeters generally use compensated attenuator for botha.c and d.c measurement “for resistance measurement” a constant current ispasses through the resistance to be measured and the voltage developed acrossit is measured and displayed in ohms.

R1 R2 R3 R4

Resistance tobe measured

Zero adjustBattery

M


Fig 1.17 Block diagram of digital multimeter

For “current measurement”, the unknown current is passes through aresistor. The voltage developed across the resistor is measured. The value ofresistor is changed in steps according to the range. As shown in block diagram,a current to voltage converter is used for measurement of current. The circuit forcurrent to voltage converter is shown in fig b. The current to be measured isapplied to the summing junction ‘A’ at the i/p of operational amplifier (Op-Amp). The ideal Op-Amp has very high i/p impedance, hence the current throughthe resistor IR is equal to the i/p current.

Fig 1.18

Attenuator

Compen-sated attenuator

Currentto voltage converter

Rectifier

A/DConverter

Counter

Display unitCon

stantcurrentsoruce

Ohm

DCV DCVDCV

DCMAInput

OhmDCMA

ACV

IiIr

A

To A/D con-verter of theDMM

Op-Amp


The current IR causes a voltage drop across one of the resistor, which isproportional to the current Ii. Different resistors are used for different ranges.

The analog quantities to be measured are converting into a train of pulsesby A/D converter and fed to the counter. These pulses are counted by the counterand displayed by the display unit in decimal number. The decimal number asindicated by the readout is a measure of the value of the i/p quantity.

Specification of Digital Multimeter

The specifications are normally defined in a way that enables differentDMMS from different manufacturers to be compared.

Specifications

1. Maximum voltage between terminals and earth ground 600V.

2. DC voltage ranges : 200m/2/20/200/1000V

(i) Accuracy : 0.5% of rgd2 digits (0.8% for 600V)

(ii) Over voltage protection : 600V dc

3. AC voltage ranges : 200/600

(i) Basic Accuracy : 1.2% of rgd2 digits

(ii) Freq range : 40Hz to 4000Hz average, calibration in rms of sine wave

(iii) Over load protection : 600Vrms AC

4. Dc current ranges : 200µ/2m/20m/200m/10A

(i) Basic Accuracy : 1% of rgd2 digits (1.5% for 200mA)

(ii) Over load protection : F 200mA/250V (no fuse for 10A range)

5. Resistance range : 200/2K/20K/200K/2M

(i) Basic Accuracy : 0.8% of rgd2 digits (1.0% for 2M&! range)

(ii) Over load protection : 250V dc or rms AC for all ranges

6. Diode and Continuity : Continuity check: if continuity exists (less than 1.5K&!) built in buzzer will sound.

7. LCD display size : 1.8 in0.7 in (4.57 cm 1.78 cm)


( An analogue meter moves a needle along a scale each type of meterhas its advantages. Used as a voltmeter, a digital meter usually better because itsresistance is much higher 1 M&! or 10 M&! compared to 200 K&! for aanalogue multimeter on a similar range. On the other hand, it is easier to followa slowly changing voltage by watching the needle on an analogue display. Usedas an ammeter, an analogue multimeter has a very low resistance and is verysensitive, with scales down to 500 µA. more expensive digital multimeter canequal or better performance)

1.4 Test & Measure the values of capacitor using R.L.C meter & compare with the marked / color code valueBasic Concepts

Resistor

It is a passive electronic component mainly used for controlling flow ofelectronic current and providing desired amount of voltage in electronic circuits.

Resistors are based on type Nonlinear & Linear

The function of a resistor is controlling flow of electric current and alsoit provide desired amount of voltage in electronic circuit.

Unit – ohm (& )

Capacitor

Capacitors are passive electronic components which have ability tocharge or store energy. It is made up of two parallel conducting plates separatedby same dielectric material.Capacitor types are fixed & variable.

The function of capacitor are block DC & it allows AC

Units are Farads (F)

Inductor

Inductors are passive electronic components used to minimize thealternating current while permitting the flow of direct current alternating currentwhile permitting the flow of direct current.

Inductor types are fixed & variable.

Symbol is

Symbolically represented as fixed

variable


The function of a inductor is minimizes AC blocks & permit DC.

Units are Henry (H)

RLC meter

RLC meter is the instrument which measures the value of passivecomponents like resistor, inductor and capacitor

The value of the component is displayed directly on the front paneldisplay.

Procedure to calculate the value of capacitor

Electrolytic capacitor

Fig 1.19 Electrolytic capacitor

It is easy to find the value of electrolytic capacitor because they areclearly printed with their capacitance, voltage rating and polarity as shown in theabove figure.

By using numerals

Unpolarized capacitors (small values, up to 1µF)

Fig 1.20 Unpolarized capacitors (small values, up to 1µF)

Examples

Small value capacitors are Unpolarized so it can be. Connected anyway they are not damaged by heat while soldering. Many small value capacitorshave their value printed but it is without a multiplier.

220 F25 V

10 F63 V

+++

Symbol is

0.1 102


For example 0.1 means 0.1µF = 0.1×106 F = 100nF

Sometimes the multiplier is used in place of the decimal point

For example 4n7 means 4.7nF

Capacitor Number Code

A number code is often used on small capacitor where printing is difficult.

The 1st number is the 1st digit, the 2nd number is the 2nd digit and the 3rd

number is the number of zeros to give the capacitance in pF. Ignore anyletterthey just indicated the tolerance and voltage rating.

For example 102 means 1000pF = 1nF (not 102pF)

472J means 4700pF = 4.7nF (J means 5% tolerance)

1.5 Transformer testingThe Transformer Ohmmeter is a line-operated, field-portable instrument

designed specifically to measure the dc resistance of all types of magnetic windingssafely and accurately. Its predominant use is the measurement of the dc resistanceof all types of transformer windings within the defined ranges of current andresistance. It can also test rotating machine windings and perform low-currentresistance measurements on connections, contacts and control circuits. Threefeatures combine to make this instrument unique: dual measurement, load tap-changer testing and safety shutdown.

1st digit

0

1

2

3

4

6

7

8

9

2nd digit

0

1

2

3

4

6

7

8

9

0

1

2

3

4

8

9

1

10

100

1,000

10,000

0.01

0.1

B

C

D

F

G

J

K

M

Z

0.1pF

0.25pF

0.5pF

1%

2%

5%

10%

20%

80%/-20%

Multiplier Tolerance

104 K


The dual set of potential inputs measure the resistance of the primaryand secondary windings of a single- or three-phase transformer simultaneously.The dual reading characteristic will speed up the measurement when it is used totest windings on delta-delta connected windings on three-phase transformers.

Due to circulating currents induced when the test current is applied tothe primary winding, this type of measurement is countered by the same currenton the secondary winding. This action attenuates the circulating current and thereading time is improved tenfold. The Transformer Ohmmeter is extremely usefulwhen testing the windings and contact resistance on tap changers with make-before-break contacts and voltage regulators.

The internal shutdown circuit will be triggered by a voltage kickback ofa few microseconds if the tap-changer contacts are opened when the tap-changercircuit is operated through all of the tap positions. This action will check forpitted or misaligned contacts as the instrument will shut down if either conditionoccurs. Users are protected by the shutdown circuit safety feature any inadvertentdisconnection of a test lead or loss of power to the instrument will safely dischargethe energy stored in the test sample.

Application

The Transformer Ohmmeter is used

• To verify factory test readings

• As part of a regular maintenance program

• To help locate the presence of defects in transformers, such as loose connections

• To check the make-before-break operation of on-load tap-changers

A regular maintenance program that includes winding resistancemeasurements is the most effective way to use this instrument. Once a benchmarkis established, subsequent test results can be compared to determine if changesare occurring in the transformers, instrument transformers and associated controlwiring, voltage regulators, motors, generators, breaker contacts, all types ofconnections (bolted, soldered, crimped, etc.).


Fig 1.21 Transformer testing

T ap-changers are mechanical devices and the most vulnerable part of atransformer. Tap-changers result in more failures and outages than any othercomponent and so require frequent testing and attention to ensure proper, reliableoperation. The Transformer Ohmmeter can be used to check the make-before-break operation of on-load tap-changers and also to measure the contactresistance of each tap position.

1.6 Test the given loud speaker and measure the voice call resistance using multimeter

Measuring the impedance or resistance of a speaker can be importantskill to have when wiring car audio system. The speaker resistance potentiallyhas a big effect on the performance of amplifiers connected to the gear, and inworst case scenarios, it is possible to damage the amplifier if the resistance ofthe speaker is too low. Speaker typically came in three different impedanceratings:4 ohms,8 ohms and 16 ohms. The impedance of the speaker you needdepends upon the output impedance of your amplifier and the number andconfiguration of speakers you connect to that amplifier.

One-windingmeasurerment Low - voltage

windingHigh - voltagewinding

Simultaneoustwo - windingmeasurement


How To Measure Speaker Impedance with a Multimeter

Knowing the resistance of a speaker is important when pairing it with anamplifier, If the resistance is too low, the amplifier can run too hot, creating thepotential for damage in the circuits. If the resistance is too high, the output of theamplifier will be restricted, distributing audio performance.

First step to measure speaker’s resistance. Put the probe of themultimeter into the sockets on the multimeter. Set the range for your multimeterfor readings in the range 2-16 ohms. Touch each of the probe to a differentspeakers terminal and check the reading this is the speaker’s resistance.

If speaker runs in series

When the speaker are connected “end-to-end” that is in a daisy chainwith each wire respectively. Connected to the previous and following speakers.The speakers are in series, the total electrical resistance of the circuit is increasedresistance is measured in ohms. Calculate individual speaker resistance and toget resultant value add all those values.

1.7 Test the working of different types of switches relays connectorsSwitch

A switch is an electrical device used to connect and disconnect a circuit.Switches cover a industrial plant switching mega watts of power on high voltagedistribution lines. An ideal switch has zero resistance when closed and infiniteresistance when open.

Types of switches

1. Toggle switch

2. Rocker switch

3. Rotary switch

4. Micro switch

5. Push button switch

6. Proximity switch

7. Switch according topple/throw

8. Thumb wned switch

9. Membrance switch


10. Slide switch

11. Selector switch

12. Joystick switch

Toggle Switch

Toggle switches are actuated by a level angled in one of two or morepositions. Then common light switch used in house hold wiring is an example ofa toggle switch. Most toggle switches will come to rest in any of their positions.While others have an internal spring mechanism returning the level to a certainnormal position, allowing for what is called “momentary” operation.

Fig 1.22 Toggle switch

Push button Switch

Push button switches are two-position devices actuated with a buttonthat is pressed and released. Most push button switches have an internal springmechanism returning the button to its “out” or “un pressed position”, for momentaryoperation. Some push button switches will latch alternatively ON or OFF withevery push of the button. Othe push button is pulled back out. This last type ofpush button switches usually have a mushroom shaped button for easy push pullaction.

Fig 1.23 Push button switch

Selector Switch

Selector switches are actuated with a rotary knob or level of some sortto select one of two or more positions like the toggle switch, selector switchescan either rest in any of their positions or contain spring- return mechanisms formomentary operation.

Fig 1.24 Selector switch


Joystick Switch

A joystick switch is actuated by a level free to move in more than oneaxis of motion. One or more of several switch contact mechanisms are actuateddepending on which way the level is pushe, and sometimes represents the directof joystick level motion required to actuate the contact. Joystick hand switchesare commonly used for crane & robot control.

Fig 1.25 Joystick Switch

Rocker Switch

In many ways rocker switches are similar to toggle switches. They arewidely used for mains ON-OFF functions & have a two position capability.Some include an integral neon lamp to indicate when the circuit is on In view oftheir intended use, these switches are often able to switch voltages of around250V AC and current levels of around lamp.

Rotary Switch

As the name implies, rotary switches are operated by turning a knobselecting the correct position enables the positions, they enable a particular pointto be connected to one of a number of other points in the electronics circuits.

The purpose of a switch is to make and break electrical circuits. Toachieve this a switch comprises two main sections namely contacts & the actuator.The contacts are the fixed part and the actuator moves over them to make orbreak the constant.

Normally Closed (NC)

This type of switch has contacts that in the normal position, or biasedposition of the switch are closed, i.e. the contacts have made contact. Utilizingthe switch then open the contacts.

Normally Open (NO)

This type of switch has contacts that in the normal position, or biasedposition of the switch are open, i.e. the contacts have made contact. Utilizing theswitch then close the contacts.


Change Over (CO or C/O)

These types of switches have no form of bias and may have severalcontacts. Rotary switches are generally of the change over type.

Relay

Relay is an electrical operated switch consisting of mechanism to makeor break the connection in an electric circuit. Relay consists of three componentsbasic coil, armature (level) and yoke. In electromagnetic relay when currentpasses through coil it generates electromagnetic field that attracts the armature.

Fig 1.26 Symbol of relay

Types of Relays

Voltage suppression Relays

As relays are used in industrial purposes very often, they are mostlycontrolled with the help of computers. But when relays are controlled with suchdevices, there will surely be the presence of semiconductors like transistors.This will in turn cause the presence of voltage spikes. As a result, it is reallynecessary to introduce voltage suppression devices, otherwise they will clearlydestroy the transistors.

Fig 1.27 Voltage suppression relay using diode

This voltage suppression can be introduced in two ways either thecomputer provides the suppression or the relay provides the suppression. If therelay provides the suppression they are called voltage suppression relays. Inrelays voltage suppression is provided with the help of the resistors of high valueand even diodes and capacitors out of these diodes and resistors are morecommonly used whatever device is used, it will be clearly stated in the relay.

No : Normal openCom : CommonNC: Normal Closed

No

Com

Nc

12

34


De spiking diode Relays

A diode in the reverse biased position is connected in parallel with therelay coil. As there is no flow of current due to such a connection, an opencircuit of the relay will cause the current to stop flowing through the coil. Thiswill have effect on the magnetic field the magnetic field will be decreased instantly.This will cause the rise of an opposite voltage with very high reverse polarity tobe induced this mainly caused because of the magnetic lines of force that cut thearmature coil due to the open circuit. Thus the opposite voltage rises until thediode reaches 0.7Volts, as soon as their cut off voltage is achieved, the diodebecomes forward biased. This causes a closed circuit in the relay, causing theentire voltage to pass through the load. The current thus produced will be flowingthrough the circuit for a very long time as soon as the voltage is completelydrained this current flow will also stop.

Fig 1.28 De spiking diode relay

De- Spiking resistor relays

A resistor is almost efficient as that of a diode. It can not only suppressthe voltage spikes efficiently, but also allows the entire current to flow through itwhen the relay is in the ON position. Thus the current flow through it sill also bevery high. To reduce this, the value of the resistance should be as high as 1 kiloohm. But as the value of the resistor increases the voltage spiking capability ofthe relay decreases.

Fig 1.29 De- Spiking resistor relays

On - Off

RL

On - Off

RL


How to test a Relay

A relay will usually have a coil, pole terminal and a set of contacts. Theset of contacts that are open when the relay is not energized are called normallyopen (N/O) contacts and the set of contacts that are closed when the relay isnot energized are called normally closed (N/C) contacts.

• Keep the multimeter in the continuity check mode.

• Check for continuity between N/C contacts and pole.

• Check for discontinuity between N/O contacts and pole.

• Now energize the relay using the rated voltage.

• For example use a 9V battery for energizing a 9V relay. The relay will engage with clicking sound.

• Now check for continuity between N/O contacts & pole

• Also check for discontinuity between N/O contacts & pole

• As a final test, measure the resistance of the relay coil using a multimeter & check it is matching to the value stated by the manufacturer.

Connectors

An electrical connector is a conductive device for joining electrical circuittogether. The connection may be temporary, as for portable equipment, or mayrequire a tool for assembly and removal, or may be a permanent electrical jointbetween two wires or devices. The reliability and operation of connector dependson mechanical strength voltage & current carrying capacity, number of contacts& spacing between them.

There are different types of connectors

Audio connector, Video connector, RF connector, Printer connector,PCB connector.

Audio connector and video connectors are electrical connectors forcarrying audio signal and video signal, of either analog or digital format. AnalogA/V connectors often use shielded cables to inhibit radio freq interference &noise.

A Co-axial RF connector is an electrical connector designed to work atradio frequencies in the multi megahertz range. RF connectors are typically usedwith Co-axial cables and are designed to maintain the shielding that the currentdesign.


Cables

A cable is most often two or more wires reusing side by side and bonded,twisted or braided together to form a single assembly, but can also refer to aheavy strong rope. In mechanic cables, otherwise known as wire ropes, areused for lifting, halving and towing or conveying force through tension. Cablesare used to carry electrical currents.

Types of Cables

Straight through cable

Four pair, eight wire, straight through cables, which means that the colorof wire on pin1 on one end of the cable is same on that of pin1 on the otherbend. Pin2 is same as pin2, so on.

Cross over Cables

A cross over cable means that the second & third pairs on one end ofthe cable will be reversed on the other end. All 8 conductors should be terminatedwith RJ-45 modular connectors cross over cable is used between switches, it’sconsidered to be part of the “vertical” cabling. Vertical cabling is also calledbackbone cabling. A cross over cable can be used as a backbone cable toconnect two or more switches in a LAN, or to connect two isolated host tocreate a mini LAN.

Roll over Cables

A 4-pair roll over cable. This type of cable is typically 3.05m long butcan be as long as 7.62m. A roll over cable can be used to connect a host ordumb terminal to the console port on the back of a router or switch.

Both the ends of theRJ-45 connectors on them. One end plugs directlyinto the RJ-45 console management port on the back of the router or switch.Plug the other end into an RJ-45 to DB9 terminal adapter. This adapter convertsthe RJ-45 to a 9 pin female D connector for attachment to the PC or dumbterminal serial.

Short Answer Type Questions1. Define Voltage, current, resistance.

2. Write applications of analague/Digital multimeters.

3. Write applications of shunts and multipliers.

4. Write applications of DC prone Supplies.


5. Write advantages of digital meter.

6. What are applications of transformers?

7. Mention types of switches

8. What is a relay?

9. Write names of connectors.

10. Write applications of cables.

Long Answer Type Questions1. Write procedure to measure Voltage, current and resistance with multimeter.

2. Explain extension of range of given Ammeters.

3. Explain extension of range of given voltmeters.

4. Explain DC power supply with neat diagram.

5. Explain working of Digital LCR meter.

6. Explain measuring Procedure of DC resistance of a transformer.

7. Explain Coil resistance of Loud Speaker.

8. How do you test switchers, Relays, Connectors, Cables using DMM.

Structure2.1 Study & use of CRO (Single trace &dual trace) for measuring frequency & amplitude.

2.2 Study & use of AF/RF Signal generators

2.3 Study and use DSO for measuring frequency, amplitude, phase, modulation index of A.M.

2.4 Identification of Diodes and Transistors

2.5 Data sheets of Diodes and Transistors


• Application of CRO to measure frequency, phase, amplitude and modulation index.

• Applications of AF/RF signal generator

• Applications of Dual Storage Oscilloscope

• Measurement of frequency, Amplitude, Phase, Modulation index in A.M.

• Identification of leads of diodes and transistors.

• Study of data sheets of diodes and transistors.

2UNIT

CRO and FrequencyGenerators


Voltage

Amplitude Peak to peak voltage

Time period

Time

2.1 Study & use of CRO (Single trace &dual trace) for measuring frequency & amplitude

Oscilloscope

An oscilloscope is a test instrument which allows you to look at the‘shape’ of the electrical signals by displaying a graph of voltage against time onits screen.

The graph usually called the trace is drawn by beam electrons strikingthe phosphor coating of the screen making it emit light, usually green or blue.

‘Oscilloscope contains a vacuum tube with a cathode (negative electrode)at one end to emit electrons and an anode (positive electrode) to acceleratethem Ø so they move rapidly down the tube to the screen. This arrangement iscalled an electron gun. The tube also contains electrodes to deflect the electronbeam up/down & left/right.

The electrons are called cathode rays because they are emitted by thecathode and this gives the oscilloscope a name of cathode ray oscilloscope. Asingle trace oscilloscope can display one trace on the screen.

Measuring Voltage and Time Period

The trace on an oscilloscope screen is a graph of voltage against time.The shape of this graph is determined by the nature of the i/p signal.

Fig 2.1 Sinusoidal Wave form

In addition to the properties labeled on the graph there is a frequencywhich is the number of cycles per second. The diagrams shows a sine wave butthese properties apply to any signal with a constant shape.


4 V

2 V

- 2 V

- 4 V

05 10 15 20 25

30

Amplitude

Is the maximum voltage reached by the signal. It is measured in volts’V’.Peak voltage is another name for amplitude. Peak-Peak voltage is twice thepeak voltage (amplitude) Time period is the time taken for the signal to comp0leteone cycle.It is measured in seconds(s), but time periods tend to be short somilliseconds(ms) and microseconds(µs)are often used.

1ms=0.001s

1µs=0.000001s

Frequency is the number of cycle per second. It is measured in Hertz(Hz), but frequencies tend to be high, so kilohertz (KHz) &Mega Hertz (MHZ).

Frequency=1/Time Period

Time Period = 1/Frequency

Voltage

Voltage is shown on the vertical axis and the scale is determined by theY amplifier (volts/cm) control. Usually Peak-Peak voltage is measured becauseit can be read correctly even if the position of 0V is not known.

The Amplitude is half the Peak-Peak voltage.

Fig 2.2 Wave form shows amplitude vs time

Example: Peak-Peak voltage = 4 × 2V/cm = 8V

Amplitude (peak voltage) = ½ ×Peak-Peak voltage = 4V


Channel A

Channel B

Delay line

TriggerExt

Line Triggerswitch

Time Period

Time is shown the horizontal X-Axis and the scale is determined by theTIMEBASE (TIME/CM) control. The time period (often just called period) isthe time for one cycle of the signal. The frequency is the number of cycles persecond, frequency=1/Time Period.

Time = Distance in cm × Time/cm

Example

Time period = 40cm × 5ms/cm = 20ms

And Frequency = 1/Time Period

= 1/20ms =50Hz

Dual Trace Oscilloscope

The Comparison of two or more voltages is very much necessary in theanalysis of many electronic circuits and systems. This is possible by using morethan one oscilloscope but in such a case it is difficult to trigger the sweep of eachoscilloscope precisely at the same time. A common and less costly method tosolve this problem is to use dual trace or multi trace oscilloscopes. In this method,the same this problem is to use dual trace or multi trace oscilloscopes .In thismethod , the same electron beam is used to generate two traces which can bedefected from two independent vertical sources. The two methods are used togenerate two independent traces which are alternate sweep method and other ischop method.

The Block Diagram of dual trace oscilloscope is shown in fig.

Fig 2.3 Block Diagram of Dual Trace Oscilloscope


There are two separate vertical i/p channels A & B .A separatespreamplifier and attenuator stage exits for each channel. Hence amplitude ofeach i/p can be individually controlled. After preamplifier stage, both the signalsare fed to an electronic switch.

The switch has an ability to pass one channel at a time via delay line tothe vertical amplifier. The time base circuit uses a trigger selector switch S2which allows the circuit to be triggered on either A or B channel, on line frequencyor on an external signal. The horizontal amplifier is fed from the sweep generatoror the B channel via switch S1&S3.The X-Y mode means, the oscilloscopeoperates from channel A as the vertical signal and the channel B as the horizontalsignal. Thus in this mode very accurate X-Y measurements can be done.

Depending on the selection of front controls several modes of operationcan be selected such as channel A only, channel B only. Channel A&B as separatetraces, signals A+B, A-B, B-A or – (A+B) as single trace.

Let us study the two modes of operation i.e. alternate sweep & chopmode.

1. Alternate mode

When the display mode selector is in the alternate mode the electronicswitch alternatively connects the vertical amplifier to channel A & to channel B.Initially each vertical amplifier is adjusted with the help of attenuator and positioncontrol such that the two images are positioned separately on the screen. Anelectronic switch is controlled by using a toggle flip-flop. The switching takesplace at the start of each newsweep.

Fig 2.4 Traingular Wave form


The switching rate of an electronic switch is synchronized to the sweeprate so that CRT spot traces channel A signal on one sweep and channel Bsignal on the next succeeding sweep. Thus two channel are alternatively connectedto the vertical amplifier. The change once of an electronic switch takes placeduring the fly back period of the sweep. During the fly back, the electron beamis invisible and the change once is also invisible i.e. without flicker.

Thus the alternate mode displays one vertical channel for a full sweep &the vertical channel for next sweep. The time relationship in alternate mode ofdual trace C.R.O is shown in fig.

The sweep trigger signal is available from channel A or B and the triggerpick-takes place before the electronic switch. This technique maintains the correctphase relationship between the A & B signals.

The main limitation of this method is that the display is not the actualrepresentation of two events taking place simultaneously. The signals are displayedas if they were existing at two different times. Similarly, the alternate mode cannotbe used for displaying very low frequency signals.

Chop Mode

In this method, there is a switching from one vertical channel to other,many times during a single sweep. This switching from one vertical channel toofficer is at such a rapid rate that the display is created from small segments ofthe actual waveform.

The electronic switch is free running oscillator at a rate of 100 to 500KHZ entirely independent of the sweep generator frequency. Thus the switchsuccessively connects the small segments of the channel A & B waveforms tothe main amplifier. At the chopping rate of 500KHZ.For example 1µsec segmentsof each waveform are fed to the display.

If the chopping rate is faster than horizontal sweep rate, then the individuallittle segments fed to the vertical amplifier together reconstitute the original Aand B waveforms on the CRT screen, without any visible interruption. The littlechopped segments merge to appear continuous to the eye.

The time relationship of dual trace CRO in chop mode is shown in fig2.5.


Fig 2.5 CRO wave forms in chop mode

These are advantages and disadvantages of both the methods hencemost oscilloscopes have a switch which is capable of selecting either of themodes.

Electronic Switch

The electronic switch used in dual trace oscilloscope is a device whichenables two signals to be displayed simultaneously on the screen by a single gunCRT. The circuit diagram of an electronic switch shown in fig 2.6

Fig 2.6 Electronic switch output of the CRO


Each signal is applied to separate control & gate stage. The resistanceR1&R2 adjust the amplitudes of channel A & B signals.Q1 & Q2 are theamplifiers while Q3 & Q4 are the switches.

Input from channel A is applied to Q1 after proper gain control byR1.Input from channel B is applied to Q2 after proper gain control by R2.

The square wave generator provides alternate biasing signals to Q3 &Q4, alternatively when Q3 is conducting; Q4 is cut-off and vice-versa. WhenQ3 is cut-off, the Q3 sends the channel A signal at the o/p.

When the square wave generator switching frequency is much higherthan the either signal frequency, bits of each signal are alternatively presented tothe oscilloscopes vertical I/p to reproduce the two signals on the screen.

R5 controls the position. The signals on the screen can be overlapped,for the easy comparison.

2.2 Study & Use of AF/RF Signal GeneratorsSignal generators have a variety of applications, such as checking the

stage gain frequency response, and alignment in receivers and in a wide range ofother electronic equipment.

There are various types of signal generators but several requirementsare common to all types.

1. The frequency of the signal should be known and stable.

2. The amplitude should be controllable from very small to relatively large values.

3. Finally the signal should be distortion free.

AF Sine & Square Wave Generators

The block diagram of an AF sine-square wave audio oscillator is illustratedin fig.

Fig.2.7 Block Diagram of AF signal generator


The signal generator is called an oscillator. A Wien Bridge oscillator isused in this generator. The Wien Bridge oscillator is the best for the audiofrequency range. The frequency of oscillations can be changed by varying thecapacitance in the oscillator. The frequency can be changed in steps by switchingin resistors of different values.

The output of the Wien Bridge oscillator goes to the function switch.The function switch directs the oscillator o/p either to the sine wave amplifier orto the square wave shaper. At the o/p, we get either a square or sine wave. Inthe sine wave mode, the signal is amplified and given to the o/p terminal throughthe attenuator. The amplitude can be set to any desired value by means ofattenuator and the magnitude control. When the function switch is in squarewave position, the oscillator o/p is given to a shaping circuit that converts thesinusoidal signal into square wave signal.

The square wave signal is amplified and through an attenuator given tothe o/p terminals, the attenuator is used for varying the amplitude of the squarewave o/p.

The instrument generates a frequency ranging from10Hz to 1MHz,continuosly variable in 5 decades with overlapping ranges. The o/p sine waveamplitude can be varied from 5mv to 5v (rms).The o/p is taken through a pushpull amplifier. For low o/p, the impedance is 100&!.The square wave amplitudescan be varies from 0-20v (peak).The instrument requires only 7w of power at220V-50HZ.

Front Panel Controls of AF Oscillator

The front panel of a AFO consists of the following

Function switch : It selects either sine wave or square wave o/p

Amplitude control: Amplitude adjustment for waveforms

Frequency selector: It selects the frequency in different ranges andvaries it continuously in a Ratio of 1:11 .The scale is non-linear.

Frequency multiplier: It selects the frequency range over 5 decadesfrom 10Hz to 1MHz.

Amplitude selector: It alternates the signal in 3decades, 1, X0.1 &X0.01.

Symmetry control : It varies the symmetry of the square wave from30% to 70 %.


Output Variable : This provides sine wave or square wave o/p ON-OFF switch.

Applications of AFO

Frequency Range : 10Hz to 1MHz (in five sub ranges)

Frequency accuracy : ±2% under normal conditions.

Output waveforms : sine & square waves

Frequency Response: within ±1dh (of a 1 KHz reference over theentire frequency range)

Output voltage

Sine : continuously variable 0 to 10 Vrms

Square: continuously variable 0 to 20 Vp-p

Output impedance: 600&!

Distortion: Less than 0.5% below 500 KHz (less than 1% above 500KHz), independent of Load impedance.

Rise and Fall Time: Less than 100 nano seconds (square wave o/p)

Power Requirement: 230V±10%AC,50/60Hz,10VA

Data cycle: 49% to 51 % (square wave)

Function Generator

A function generator is a versatile instrument that produces a choice ofdifferent waveforms whose frequencies are adjustable over a wide range. Thecommon o/p waveforms are the sine, square, triangular and saw tooth waves.The frequency may be adjustable from a fraction of a hertz to several hundredKHz.

The various o/p’s of the generator can be available at the same time.For example, the generator can provide a square wave to test linearity of anamplifier and simultaneously provide a sawtooth to drive the horizontal deflectionamplifier of the CRO to provide a visual display.

Usually the frequency is controlled by varying the capacitor in the LCand RC circuit. In this instrument the frequency is controlled by varying themagnitude of current which drives the integrator. This instrument delivers sine,triangular and square waves a frequency range of 0.01Hz to 100KHz.


The frequency control n/w is grounded by the frequency dial onthe front panel of the instrument or by an externally applied control voltage thefrequency control voltage regulates two current sources. The upper currentsource supplies a constant current to the integrator whose o/p voltage increaseslinearly with time.

The o/p voltage is given by e out =

Fig 2.8 Function Generator Block Diagram

An increase or decrease in the current supplied by the upper currentsource increases or decreases the slope of the o/p voltage. The voltagecomparator multivibrator changes state at a predetermined level on the positiveslope of the integrator o/p voltage. This change of state cuts off the upper currentsupply to the integrator and switches on the lower current supply.

The lower current source supplies a reverse current to the integrator, sothat its o/p decreases linearly with time. When the o/p voltage reaches apredetermined level on the negative slope to the o/p waveform, the voltagecomparator again changes state and cuts off the lower current source while atthe same time switching on the upper current source again.

The o/p of integrator is a triangular waveform whose frequency isdetermined by the magnitude of the current supplied by the constant currentsources.

The comparator o/p delivers a square wave o/p voltage of the samefrequency. The resistance diode network alters the slope of the triangular waveas its amplitude changes and produces a sine wave with less than 1% distortion.


The o/p circuitry of the function generator consists of two o/pamplifiers that provide two simultaneous, individually selected outputs of any ofthe waveform functions.

Applications of AF oscillator and Function Generator

The AF oscillators are useful for service shops, production and testingdepartments, educational institutions laboratories.

1. Amplifier frequency response.

2. Tone control test

3. Amplifier performance evaluation using square wave

4. Amplifier overload characteristics

5. Speaker system testing

6. FM receiver alignment

7. Preset frequency selection

8. Communication receiver signal

RF Signal Generator

The block diagram of a R.F signal generator is shown in fig it consistsof a single master oscillator, designed for the highest frequency range andfrequency dividers are switched into produce lower range.

Fig 2.9 RF signal generator


The highest frequency range of 34-80 MHz is passed through B1 anuntuned buffer amplifier, B2 & B3 are additional amplifiers and A is the mainamplifier. The lowest frequency range produced by the cascade frequency divideris the highest frequency range divided by 512 or 29, or 67-156 KHz thus, thefrequency stability of the highest range is imported to the low frequency ranges.

The use of buffer amplifiers provides a very high degree of isolationbetween the master oscillator and the power amplifier almost eliminates thefrequency effects between the i/p and o/p circuits caused by loading.

The master oscillator is tuned by a motor driven variable capacitor forfast coarse tuning a rocker switch is provided, which sends the indicator glidingalong the slide rule scale of the main frequency dial at approx 7% frequencychanges per second. The oscillator can then be fine tuned by means of a largerotary switch, with each division corresponding to 0.01% of the main dial setting.

The master oscillator has both automatic & manual controller. Theavailability of the motor driven frequency control is employed for programmableautomatic frequency control devices. Internal calibration is provided by the 1MHzcrystal oscillator. The supply voltage of the master oscillator is regulated by atemperature compensated reference circuit.

The modulation is done at the power amplifier stage. For modulation,two internally generated signals are used, that is 400Hz and 1 KHz. Themodulation level may be adjusted up to 95% by a control device. Flip Flops canbe used as frequency dividers to get a ratio of 2:1.

Applications of RF Signal Generator

1. RF oscillator is an instrument which can generate various RF voltages required for alignment and servicing of radio equipments.

2. It can be used for the measurement of gain of each stage at RF frequency.

3. It is used for accurate alignment and measurement of all tuned circuits in radio receiver or transmitter.

4. It can be used to determine the distortion characteristic of an amplifier by connecting the o/p of the amplifier to a CRO when the i/p at the amplifier is given through a signal generator.

Specifications of RF Signal Generators

Frequency ranges: 50 KHz to 150KHz

150 KHz to 420 KHz

420 KHz to 500 KHz


500 KHz to 1500 KHz

1.5 MHz to 5 MHz

5 MHz to 15 MHz

15 MHz to 80 MHz

RF output : variable for 1 v to 100 v 6db

Output impedance: 1 v to 10 v : 20 Omega

100 v : 40 Omega

Modulated level : 30% 10%

AF modulation : 400Hz or 1KHz

Power supply : 230V, 1- 50Hz a.c

2.3 Study and Use DSO for measuring frequency,Amplitude, Phase, Modulation Index of A.M.2.3.1 Frequency Measurement

If the two frequencies are the same, we will obtain a simple figure on thescreen of the CRT, the shape of that figure being dependent upon the phase shiftbetween the two AC signals. Here is a sampling of Lissajous figures for twosine-wave signals of equal frequency, shown as they would appear on the faceof an oscilloscope (an AC voltage-measuring instrument using a CRT as its“movement”). The first picture is of the Lissajous figure formed by two ACvoltages perfectly in phase with each other.

Fig 2.10 Lissajous pattern for Zero Phase difference

+


If the two AC voltages are not in phase with each other, a straight linewill not be formed. Rather, the Lissajous figure will take on the appearance ofan oval, becoming perfectly circular if the phase shift is exactly 90o between thetwo signals, and if their amplitudes are equal: (Figure below).

Fig 2.11 Lissajous figures for phase 900 or 2700 phase shift

Lissajous figure: same frequency, 900 or 2700 phase shift.

Finally, if the two AC signals are directly opposing one another in phase(180o shift), we will end up with a line again, only this time it will be oriented inthe opposite direction: (Figure below).

Fig 2.12 Lissajous figure: same frequency, 1800 phase shift

Lissajous figure: same frequency, 1800 phase shift.


When we are faced with signal frequencies that are not the same,Lissajous figures get quite a bit more complex. Consider the following examplesand their given vertical/horizontal frequency ratios: (Figure below)

Fig 2.13 Lissajous figure: Horizontal frequency is twice that of vertical

Lissajous figure: Horizontal frequency is twice that of vertical.

The more complex the ratio between horizontal and vertical frequencies,the more complex the Lissajous figure. Consider the following illustration of a3:1 frequency ratio between horizontal and vertical: (Figure below)

Fig 2.14(a) Lissajous figure: Horizontal frequency is three times that of vertical

Lissajous figure: Horizontal frequency is three times that of vertical.


. . . and a 3:2 frequency ratio (horizontal = 3, vertical = 2) in Figurebelow.

Fig2.14 (b) Lissajous figure: Horizontal/vertical frequency ratio is 3:2

Lissajous figure: Horizontal/vertical frequency ratio is 3:2.

In cases where the frequencies of the two AC signals are not exactly asimple ratio of each other (but close), the Lissajous figure will appear to “move,”slowly changing orientation as the phase angle between the two waveforms rollsbetween 0o and 180o. If the two frequencies are locked in an exact integerratio between each other, the Lissajous figure will be stable on the view screenof the CRT.

2.3.2 Amplitude Measurement with CRO

Amplitude modulation (AM) is a technique used in electroniccommunication, most commonly for transmitting information via a radio carrierwave. AM works by varying the strength of the transmitted signal in relation tothe information being sent. As originally developed for the electric telephone,amplitude modulation was used to add audio information to the low-powereddirect current flowing from a telephone transmitter to a receiver. As a simplifiedexplanation, at the transmitting end, a telephone microphone was used to varythe strength of the transmitted current, according to the frequency and loudnessof the sounds received. Then, at the receiving end of the telephone line, thetransmitted electrical current affected an electromagnet, which strengthenedand weakened in response to the strength of the current. In turn, the electromagnetproduced vibrations in the receiver diaphragm, thus closely reproducing the


frequency and loudness of the sounds originally heard at the transmitter. In contrastto the telephone, in radio communication what is modulated is a continuouswave radio signal (carrier wave) produced by a radio transmitter. In its basicform, amplitude modulation produces a signal with power concentrated at thecarrier frequency and in two adjacent side bands. This process is known asheterodyning. Each sideband is equal in bandwidth to that of the modulatingsignal and is a mirror image of the other. Amplitude modulation that results intwo side bands and a carrier is often called double sideband amplitude modulation(DSB-AM). This is the process taking place at the transmitting end.

2.3.3 Phase Measurement with CRO

The connections are made as shown in the circuit and as said in thedescription. The time base (X-plates) band switch is kept in external mode. Thegain band switch of Y-plates is kept in desired range, so as to get completemaximum size ellipse on the screen. The maximum deflection (B) from the meanposition and the deflection (A) at t = 0, from the mean position are measuredusing the divisions on the screen. The experiment is repeated by varying thefrequency (f) of the signal generator in equal steps. The values of f , A and B arenoted in the table. The values of resistance and capacitance are also noted.

Fig 2.15 Phase Measurement with CRO

2.3.4 Modulation Index Measurement

It is the measure of extent of amplitude variation about an demodulatedmaximum carrier. This quantity is also called as modulation depth and it indicatesby how much the modulated variable varies around its ‘original’ level. For AM,it relates to the variations in the carrier amplitude. We compare the modulationindices both at the in put level and out put level as shown in the above equations.


Fig 2.16 Modulation Index Measurement

The circuit is connected for producing amplitude modulated wave. Thefrequency of the carrier wave is in MHz. First measure the peak to peak (2a)vertical voltage of the carrier wave of the Y2- plates on CR

O screen, find the peak voltage (a). Note this value in the table-1.(Thiscan be measured by connecting the CRO Y2-plates to the transformer secondarycoil or directly to the carrier in.) Set the frequency of the audio signal to nearly 1KHz and apply it to the base of T2 and adjust the time base of the CRO toobserve at least two audio waves on the screen of the CRO. Also adjust theamplitude of the audio signal such that the audio wave in the modulated wave iscompletely observed on Y2-plates. Now the audio signal peak to peak voltage(2b) and the peak voltage (a) are measured from the Y1-plates.


Fig 2.17 Diode Symbols

2.4 Identification of Diodes and Transistors

Transistor and diode numbering the numbering or code systems usedfor transistors, diodes and FETs.

There are many thousands of different types of diode and transistor.These have different characteristics according to the way they are designed andmade. Some may be intended for high power applications, like those used inpower amplifiers of power supplies, whereas others may be intended for smallsignal applications where low current consumption is an issue.

Other types of transistor may be required for radio frequencyapplications.




Fig 2.18 Transistor and Structural Views

Numbering schemes

There are many different ways of having numbering systems. The first isthat each manufacturer gives each type of transistor that the companymanufactures a type number. This would lead to a huge number of different typenumbers. There would also be a huge overlap as different manufacturers madetransistors that were virtually the same. To overcome this problem, and to allowelectronic equipment manufacturers to be able to buy the same part from anumber of different manufacturers, there are international numbering schemesthat have been developed. One is known as the Pro-electron scheme and wasoriginated in Europe. The other is known as the JEDEC scheme and originatedin the USA.

By looking at the transistor of diode type number, or code, to is possibleto identify elements about it. The Pro-electron scheme makes it possible tobroadly identify the capabilities of the transistor. For example parameters suchas the transistor being intended for low frequency power, RF, etc can bedetermined.

The JEDEC system details far less, being intended to be purely anumbering system. From the number it can be determined how many PN junctionsare in the device.


Pro-Electron Numbering or Coding System

This a BC107 is a low power audio transistor and a BBY10 is variablecapacitance diode for industrial or commercial use.

A Diode - low power or signal

B Diode - variable capacitance

C Transistor - audio frequency, lowpower

D Transistor - audio frequency,power

E Tunnel diode

F Transistor - high frequency, lowpower

G Miscellaneous devices

H Diode - sensitive to magnetism

L Transistor - high frequency, power

N Photocoupler

P Light detector

Q Light emitter

R Switching device, low power, e.g.thyristor, diac, unijunction

S Transistor - switching low power

T Switching device, low power, e.g.thyristor, triac

U Transistor - switching, power

W Surface acoustic wave device

X Diode multiplier

Y Diode rectifying

Z Diode - voltage reference

A Germanium

B Silicon

C GalliumArsenide

R Compoundmaterials

First Letter

S p e c i f i e ssemiconductormaterial

Second Letter

Specifies type of device Su bse que ntCharacters

The charactersfollowing the firsttwo letters form theserial number of thedevice. Thoseintended fordomestic use havethree numbers, butthose intended forcommercial orindustrial use haveletter followed bytwo numbers, i.e.A10 - Z99.


JEDEC Numbering or Coding System

Thus a 1N914 is a diode and a 2N3866 is a transistor.

2.5 Data sheets of Diodes and TransistorsDiodes

• 1N4001 - 1N4007

• 1N4148

• 1N47XXA - Series Zeners - National Semiconductor Datasheet

• 1n47XXA - Series Diodes - Diode Incorporated Datasheet

• 1N5400 - 1N5408

• 1N914

• MR750

• MUR405-MUR460 Ultrafast Diode

• MBR350, MBR360 - Schottky Diode

• Miscellaneous Diode Selector Guide

• Ultrafast Diode Selector Guide

• Fast Diode Selector Guide

• General Purpose Diode Selector Guide

• Schottky Diode Selector Guide

• Small Signal Diodes Selector Guide

• Zener Selector Guide

• LIR204x - Infrared Emitting Diode.

First Number Second Letter Subsequent numbers

1 = Diode

2 = Bipolar transistor

3 = FET

N

S e r i a lNumber ofDevice


Transistors

• ZTX449 Zetex NPN Transistor



• ZTX549 Zetex PNP Transistor

• ZVN3306A Zetex NMOS Transistor

• ZVP3306A Zetex PMOS Transistor

• BS250P Zetex PMOS Transistor

• 2N3904 NPN Small Signal Transistor

• 2N3906 PNP Small Signal Transistor

• 2N2222A NPN Small Signal Transistor (On Semiconductor)

• 2N2222A NPN Small Signal Transistor (Motorola)

• 2N2222A NPN Small Signal Transistor (Phillips)

• 2N956 NPN Small Signal Transistor

• VN2106 N-type MOSFET Array

• VP2106 P-Type MOSFET Array

• VP0109 P-Type MOSFET

• VN0109 P-Type MOSFET

• 2N5951 jFET Small Signal Transistor

• TIP31& TIP32 NPN & PNP Power BJT

• TIP142 Power NPN Darlington

• TIP147 Power PNP Darlington

• TIP102 Power NPN Darlington

• TIP107 Power PNP Darlington

• LM3046 Transistor Array


• MPF102 - N-type jFET

• IRF 530 N-type Power MOSFET

• IRF150 N-type Power MOSFET

• IRF9140 P-type Power MOSFET

• NTE2321 NPN Transistor Array (MPQ3904 equivalent)

• NTE2322 PNP Transistor Array (MPQ3906 equivalent)

• Small Signal BJT and FET Selector Guide\

• LPT2023 - Infrared Photo Transistor

• PNP Silicon Planar

• Medium Power Transistors.

Features

• 60 Volt VCEO

• 1 Amp continuous current

• Ptot = 1 Watt

Fig 19 Transistor data sheet


Applications of Diodes and Transistors

Diodes

• Protect circuits by limiting the voltage (clipping and clamping).

• Turn AC into DC (voltage rectifier).

• Voltage multipliers (e.g. double input voltage).

• Non-linear mixing of two voltages (e.g. amplitude modulation.

Transistors

• Transistors are the heart of modern electronics (replaced vacuum tubes).

• Voltage and current amplifier circuits

• High frequency switching (computers)

• Impedance matching

• Low power

• Small size, can pack thousands of transistors in mm2.

Short Answer Type Questions1. Write operating controls of CRO.

2. Write applications of CRO.

3. Write applications of Spiral Generator.

4. How do you test diode.

5. How do you test Transistor.

6. Write use of Data sheets.


Long Answer Type Questions1. How do you measure frequency, Amptitude using CRO?

2. Write working of PF/RF signal generator with diagram.

3. Mention any 3 types diode, transistors. How do you identify the leads.

4. Write data sheet of transistors.


Structure 3.1 Classification of Inverters

3.2 Working and Single-phase Inverters using MOSFET

3.3 Working of Voltage Source Inverter

3.4 Need for Uninterrupted Power Supply

3.5 Working of Three-phase UPS

3.6 Types of UPS

3.7 Block diagram of Off-line UPS

3.8 Working of Online UPS

3.9 Classification of UPS

3.10 UPS ICs used Version and Servicing Procedures


• Study of Online and offline UPS.

• Types of Inverters

• Working of Single-phase bridge inverters using MOSFET

• Working of Voltage source inverter

3UNIT

UPS and Inverters


• Study of need for uninterrupted power supply.

• Working three-phase inverter.

• Types of UPS

• Working of Offline UPS

• Working of Online UPS

• Study of Online and Offline UPS and its application

3.1.Classification of InvertersThe dc-ac converter, also known as the inverter, converts dc power to

ac power at desired output voltage and frequency. The dc power input to theinverter is obtained from an existing power supply network or from a rotatingalternator through a rectifier or a battery, fuel cell, photovoltaic array or magnetohydrodynam ic generator. The filter capacitor across the input terminals of theinverter provides a constant dc link voltage. The inverter therefore is an adjustable-frequency voltage source. The configuration of ac to dc converter and dc to acinverter is called a dc-link converter.

Inverters can be broadly classified into two types, voltage source andcurrent source inverters. A voltage–fed inverter (VFI) or more generally avoltage–source inverter (VSI) is one in which the dc source has small or negligibleimpedance. The voltage at the input terminals is constant. A current–sourceinverter (CSI) is fed with adjustable current fromthe dc source of high impedancethat is from a constant dc source. A voltage source inverter employing thyristorsas switches, some type of forced commutation is required, while the VSIsmadeup of using GTOs, power transistors, power MOSFETs or IGBTs, selfcommutation with base or gate drive signals for their controlled turn-on andturn-off.

Types of Inverters

1. Voltage control in Single - Phase inverters

2. Pulse width modulation control

3. Sinusoidal-Pulse Width Modulation (SPWM)

4. Single-phase inverters

5. SPWM with Bi-polar switching

6. SPWM with Uni-polar switching


7. SPWM With Modified Bipolar Switching Scheme (MBPWM)

8. Generalized Carrier-based PWM

9. Bipolar and Modified Bipolar PWM Schemes with Zero Sequence Voltage.

10. Implementation of the Bipolar and the Modified bipolar PWM Schemes for an RL load

3.2 Working of Single-Phase inverters using MOSFETPower MOSFET

Metal oxide semiconductor field effect transistor (MOSFET) is a powerof transistor. The switching speed of the modern transistors is much higher thanthat of thyristors and are extensively employed in dc-dc and dc-ac converters .However, their voltage ratings and current ratings are lower than those of thyristorsand are used in low to medium-power applications. A power MOSFET is avoltage-controlled device and requires only a small input current.

Fig 3.1 Power MOSFET using Inverter

The switching speed is very high and the switching times are of theorder of nanoseconds. As the MOSFETs conduct in the duration for which thegate pulse is present and it doesn’t conduct when the gate pulse is removed,there is no need for an external commutation circuitry. Power MOSFETs findincreasing applications in low-power high frequency converters. The inputimpendence is very high, 10^9 to 10^11 oh ms. They require very low low gateenergy and low switching and low conduction losses. However MOSFETs have


the problem of electrostatic discharge and also its difficult to protect them undershort circuited fault conditions.

The two types of MOSFETs are

1. Depletion MOSFETs, and

2. Enhancement MOSFETs.

A depletion type MOSFET remains on at zero gate voltage where as anenhancement type of MOSFET remains off at zero gate voltage, theenhancement type MOSFETs are generally used as switching devices in powerelectronics. In this project we have used n-channel enhancement MOSFETs.

Choice of MOSFETs over other Power Transistors

The other types of power transistors are BJTs (Bipolar JunctionTransistors), SITs (static induction transistors), IGBTs (insulated gate bipolartransistors) and COULUMBS.

MOSFETs do not have the problem of second breakdown phenomenaas do BJT. A BJT is a current-controlled device and its current gain is highlydependent on the junction temperature. The high on-state drops in SITs limit itsapplications for general power conversions. The switching speed of IGBTs isinferior to that of MOSFETs. IGBTs are costlier than the MOSFETs.COULUMBS is a new technology for high voltage power MOSFETs, expectfor switching losses (same as the conventional MOSFETs) COULUMBS isadvanced and improved version of power MOSFET. Our hardwareimplementation is limited to three level inverters which doesn’t needCOULUMBS technology which is much costlier than MOSFETs.

A practical MOSFET consists of three pins namely G-gate, D-drain,and S-source. Gate signal is a given between G and S. Supply is a given betweenD and S. it’s called the common source connection.

3.3 Working of Voltage Source InverterVoltage source inverters are used to regulate the speed of three-phase

squirrel cage motors by changes the frequency and the voltage and consist ofinput rectifier, DC link and output converter. They are available for low voltagerange and medium voltage range. Low Voltage Inverter The three-phase lowvoltage air cooled frequency inverter is a cabinet built single or multi drive designedfor industrial applications and for customised solutions too and is available in 1-quadrant and 4-quadrant operation for 6-pulse and 12-pulse mains supplyconnection. The used semi-conductors are diodes and IGBT’s.


Voltage and power range

1-quadrant operation, 6 -/ 12-pulse

3 AC 400 V 3 – 1845 / 609 - 1845 kVA

3 AC 500 V 4 – 2312 / 765 - 2312 kVA

3 AC 690 V 15 – 3310 / 750 - 3310 kVA

4-quadrant operation

3 AC 400 V 123 – 1125 kVA

3 AC 500 V 97 – 1380 kVA

3 AC 690 V 210 – 1382 kVA

Fig 3.2 voltage source inverter

Characteristic features

• Cabinet ready for connection

• Alphanumeric multilingual control panel

• Direct torque control (DTC)

• Adaptive programming with 15 function blocks without additional hardware.

• Easy and fast commissioning procedure with Start-up Assistant


• Process interface for field bus control

• Control solutions for specific drive applications Cabinet ready for connection.

3.4 Need for Uninterrupted Power supplyAn uninterruptible power supply, also uninterruptible power

source, UPS or battery/flywheel backup, is an electrical apparatus thatprovides emergency power to a load when the input power source, typicallymains power, fails. A UPS differs from an auxiliary or emergency power systemor standby generator in that it will provide near-instantaneous protection frominput power interruptions, by supplying energy stored in batteries or a flywheel.The on-battery runtime of most uninterruptible power sources is relatively short(only a few minutes) but sufficient to start a standby power source or properlyshut down the protected equipment.

Fig 3.3 UPS

A UPS is typically used to protect computers, data centers,telecommunication equipment or other electrical equipment where an unexpectedpower disruption could cause injuries, fatalities, serious business disruption ordata loss. UPS units range in size from units designed to protect a single computerwithout a video monitor (around 200 VA rating) to large units powering entiredata centers or buildings. The world’s largest UPS, the 46-megawatt, Battery


Electric Storage System (BESS), in Fairbanks, AK, powers the entire city andnearby rural communities during outages.

3.5 Working of Three-Phase Inverters

Fig 3.4 Three Phase UPS

The dc to ac converters more commonly known as inverters, dependingon the type of the supply source and the related topology of the power circuit,are classified as voltage source inverters (VSIs) and current source inverters(CSIs). The single-phase inverters and the switching patterns and so the threephase inverters are explained in detail here. Three-phase counter parts of thesingle-phase half and full bridge voltage source inverters. Single-phase VSIscover low-range power applications and three-phase VSIs cover medium tohigh power applications.


The main purpose of these topologies is to provide a three-phase voltagesource, where the amplitude, phase and frequency of the voltages can becontrolled. The three-phase dc/ac voltage source inverters are extensively beingused in motor drives, active filters and unified power flow controllers in powersystems and uninterrupted power supplies to generate controllable frequencyand ac voltage magnitudes using various pulse width modulation (PWM)strategies. The standard three-phase inverter has six switches the switching ofwhich depends on the modulation scheme. The input dc is usually obtained froma single-phase or three phase utility power supply through a diode-bridge rectifierand LC or C filter.

3.6 Types of UPS

Fig 3.5 UPS Technology

Types of UPS

The vast majority of UPS in use today store their energy in sealed valve-regulated lead-acid batteries (SVRLAs).

Two UPS technologies dominate, ‘on-line’ and ‘off-line’. Almost allunits described by their manufacturers as being ‘line interactive’ are off-line unitsbut with the addition of an Automatic Voltage Regulation (AVR) transformer.

The description ‘on-line / off-line’ simply indicates whether the inverter(that part of the UPS that converts the DC provided by the batteries to the AC


that we use from a wall socket) is on-line or off-line during normal use. Almostall UPS rated at 10kVA and over are on-line.

To understand the power quality issues that UPS are designed to resolve,and for help in selecting the most appropriate UPS technology, why not viewUnderstanding Standby Power. Generally, the following distinctions apply:

Off-line UPS

• Lower cost than other units

• Under mains power unit charges batteries and power passes directly to the load.

• During mains failure batteries provide power to DC/AC inverter to provide 230VAC power to the load.

• Square-wave or Sine-wave output

On-line UPS

• Constant duty inverter

• Design inherently improves power quality and reliability

• No break

• Sine-wave output

• Static bypass improves reliability

Line Interactive UPS

These UPS offer enhanced power protection over the basic off-linedesigns because they provide additional line conditioning. They can also copewith a wider range of input voltages without resorting to battery.

High Capacity off-line UPS for Data Centres

With an understanding of the need for power saving within Data Centres,and having tracked the latest developments in off-line UPS technology, UPSSystems have now become the sole UK distributor of PureWave UPS productsfrom S&C Electric in Canada.


Fig 3.6 High capacity Offline UPS

PureWave is a high-capacity, off-line, immediate-discharge UPS thatprovides power protection to entire facilities served by a single source. Its modularstructure supports Data Centre loads between 313kVA and 20MVA, protectingthe power sensitive equipment from voltage sags, surges, transients, momentarydisruptions and complete outages.

3.7 Block diagram of Off-line UPSThe offline / standby UPS (SPS) offers only the most basic features,

providing surge protection and battery backup. The protected equipment isnormally connected directly to incoming utility power. When the incoming voltagefalls below a predetermined level the SPS turns on its internal DC-AC invertercircuitry, which is powered from an internal storage battery.

Fig 3.7 Block diagram of Offline UPS


The SPS then mechanically switches the connected equipment on to itsDC-AC inverter output. The switchover time can be as long as 25 millisecondsdepending on the amount of time it takes the standby UPS to detect the lostutility voltage. The UPS will be designed to power certain equipment, such as apersonal computer, without any objectionable dip or brownout to that device.

3.8 Working of Online-UPSThe line-interactive UPS is similar in operation to a standby UPS, but

with the addition of a multi-tap variable-voltage autotransformer. This is a specialtype of transformer that can add or subtract powered coils of wire, therebyincreasing or decreasing the magnetic field and the output voltage of thetransformer. This is also known as a Buck–boost transformer.

This type of UPS is able to tolerate continuous undervoltage brownoutsand overvoltage surges without consuming the limited reserve battery power. Itinstead compensates by automatically selecting different power taps on theautotransformer. Depending on the design, changing the autotransformer tapcan cause a very brief output power disruption,[4] which may cause UPSsequipped with a power-loss alarm to “chirp” for a moment.

Fig 3.8 Block diagram of Online UPS

This has become popular even in the cheapest UPSs because it takesadvantage of components already included. The main 50/60 Hz transformerused to convert between line voltage and battery voltage needs to provide twoslightly different turns ratios: One to convert the battery output voltage (typicallya multiple of 12 V) to line voltage, and a second one to convert the line voltageto a slightly higher battery charging voltage (such as a multiple of 14 V). The


difference between the two voltages is because charging a battery requires adelta voltage (up to 13-14 volts for charging a 12 volt battery). Furthermore, itis easier to do the switching on the line-voltage side of the transformer becauseof the lower currents on that side.

To gain the buck/boost feature, all that is required is two separateswitches so that the AC input can be connected to one of the two primary taps,while the load is connected to the other, thus using the main transformer’s primarywindings as an autotransformer. The battery can still be charged while “bucking”an overvoltage, but while “boosting” an undervoltage, the transformer output istoo low to charge the batteries.

Autotransformers can be engineered to cover a wide range of varyinginput voltages, but this requires more taps and increases complexity, and expenseof the UPS. It is common for the autotransformer to cover a range only fromabout 90 V to 140 V for 120 V power, and then switch to battery if the voltagegoes much higher or lower than that range.

In low-voltage conditions the UPS will use more current than normal soit may need a higher current circuit than a normal device. For example to powera 1000-watt device at 120 volts, the UPS will draw 8.33 amperes. If a brownoutoccurs and the voltage drops to 100 volts, the UPS will draw 10 amperes tocompensate. This also works in reverse, so that in an overvoltage condition, theUPS will need less current.

3.9 Classification of UPSClassification (Types) of Uninterrupted Power Supply(UPS)

Fig 3.9 UPS


UPS is an acronym for Uninterrupted Power Supply which is analternative source of power supply which provides an interruption free supply tosensitive electronic equipment like Personal Computers and Super Computersetc. You can read this article to know more about U.P.S. :

Now we are going to discuss the types of UPS, that is classification ofUninterrupted Power Supply. Uninterrupted Power Supply can be classified as:

• On the Basis of their Output Power

• On the Basis of their Working

1. On the Basis of their O/P Power

On the basis of output power, the UPS system may be classified intotwo types:

1. Low Power Rating UPS and

2. High Power Rating UPS

The output power of a UPS is rated in KVA i.e. Killo Volt Ampere.The UPS system can be classified on the basis of power to be handled by it. Fora single personal Computer or any other single sensitive device, we can useUPS of small rating. But if we have to install a UPS system in a lab or for anetwork of computers then we have to select one having high power rating. Thepower rating simply determines the maximum power to be handled by UPS. We should choose power rating of the UPS according to our need. For exampleif we uses low power rating UPS for a number of Computers then it can’thandle the power flow through it and thus lead to destruction.

2. On the Basis Of Working

On the basis of working, the UPS System may be classified into threetypes, given as:

• On line UPS

• OFF line UPS

• Line Interactive UPS

Online UPS

In online UPS, the internal circuitry of UPS is sorted as:

AC Mains->Rectifier (Charger)->Battery->Inverter->Load

As shown above, we can see that there is no link between Mains ACSupply and Load in ON line UPS . The load get power supply from Inverter


whether the main supply is ON or OFF.(Although, we can draw an AC linedirectly from AC mains to load, in case our inverter stop working due to anyreason. Then we have to use transfer switch which will transfer control of powerfrom inverter to direct AC mains) In this type of UPS, the inverter workscontinuously therefore this type of UPS are also called Continuous mode UPS.

Working

When the Mains AC Supply is ON, the rectifier circuit converts incomingAC into DC, which is fed to battery. Battery gets charged by this DC supplyand the output of rectifier is also fed to the inverter. Inverter circuit do the invertof rectifier. That is it converts incoming DC signal to AC. Then this output ofinverter is fed to the load. In this type of UPS, the load is isolated from ACmains so there is no affect of interrupted (mains) power supply on the load.

When the Main supply is OFF. Then rectifier stop its operation as thereis no incoming AC signal to convert into DC. But the whole process takes placesimilarly. The bank or batteries provides power to inverter. As the output ofbatteries are in DC, So the inverter converts DC into AC and this inverted ACis fed to load for its operation. Now depending on the type and quantity ofbatteries, the UPS can give power to the load for a specific time. This time iscalled Back up time. The back up time of commonly used UPS is 15 to 40minutes.

Advantages of ON-line UPS: They gives very good continuity ofpower supply and protection.

Dis-Advantages of ON-line UPS: Following are the main dis-advantages of ON-line UPS:

• Heavy in Weight

• Requires Large Space

• More Costly

OFF-Line UPS

The only difference between ON Line and OFF line UPS is that inOffline UPS load has a direct link to ac mains via a static switch, while havingother same circuitry. In case of OFF line UPS, the inverter doesn’t remains itON state as ON line UPS, Instead it comes to On only when the Main supplygets OFF. When the main supply is ON the load derives power directly fromAC supply. When the Mains AC supply gets OFF, then inverter come to playand convert incoming DC supply from Batteries to AC. This inverted ac is then


fed to load. Here we make use of static switch which immediately switchesfrom one supply to another in case ac mains gets off. The switching time of OFFline UPS is almost equal to five mili seconds.

Advantages Of OFF-line UPS

• Less Expensive

• Light Weight

• Requires Less Space.

• This type of UPS is mostly used for our personal computers.

Line Interaction UPS

Line interaction UPS is similar to OFF line UPS. In this type of UPS,the inverter circuit works only when there is no AC Power supply. When ACMains are ON, the load derives current directly from Mains power supply. Incase when there is some failure in AC mains then the inverter goes ON andinverts incoming DC signal from battery to AC. This Inverted AC signal is thenfed to load. The switching time of this UPS is large than both previous UPS.This type of UPS is commonly used at our homes. The main advantage of thistype of UPS is that its back up time is very high(almost 1 hour to 4 hours). Sothis type of UPS are used where switching time does not matter for example:Fans, Lights, TV , DVD and other home appliances.

3.10 UPS ICs used Version and Servicing ProceduresChoose a charger that can supply enough current to charge the battery

and keep up with the inverter’s load. This will be a fairly heavy duty charger.

• Check RV suppliers for ‘Converters’, designed to run larger RVs if you are making a big system.

• Check solar power sources for “big” whole house chargers and inverters for very large systems.

• If an RV or home converter has an inverter built in, make sure it’s isolated (or can be isolated) from the input power.

• Make sure the charger handles the kinds of batteries you are going to buy.

Choose only deep cycle batteries. Do not use a car or truck battery,nor a ‘marine’ battery. If you will be using only one battery, a gel or ‘maintenancefree’ battery will work adequately. For larger systems composed of multipledeep-cycle batteries, select only wet cells or AGM cells.


• Make sure the batteries are ventilated for escaping hydrogen gas.

• If you buy wet cells, make sure the charger supports an ‘equalize’ charge.

• Lead acid batteries are sold in 6 volt and 12 volt sizes. You will need to connect them in series to raise the voltage, or in parallel to increase the amp-hours available.

• 12 volts = 2x6V volt batteries connected in series

• 24 volts = 4x6V or 2x12V batteries in series

• When connecting series-parallel, connect pairs of batteries in parallel and then connect those pairs in series, not chains of series batteries in parallel.

• Do not mix different kinds of batteries. Newer batteries added to existing sets of batteries will be as worn as the originals very quickly.

• In larger series-parallel setups it’s a good idea to swap batteries around every year or so.

• Batteries that are shallowly drained (cycled) will last a long time, while batteries that are deeply cycled will have shorter lives.

• A fully charged, new 12 volt battery is 12.6 volts at rest (each of six cells is 2.1 volts).

• A fully charged, new 6 volt battery will be at 6.3 volts at rest.

• When a 12 volt charger is operating on it, the voltage will be higher. A float charge (maintenance charge) for a 12 volt system is 13.5 to 13.8 volts; active charging requires at least 14.1 volts. You may see it go as high as 16 volts when charging, depending upon the charger. After a full charge, if the battery is not going to be float charged, the at-rest voltage will slowly return to the nominal full-charge voltage.

• A discharged 12 volt battery is 11.6 volts at rest. A discharged 6 volt battery is 5.8 volts at rest. The voltage may temporarily fall below these levels while powering a large load, but should return to a point within the nominal range after a 1-hour rest. Over-discharging to less than 1.93 volts per cell at rest will permanently damage your battery.

• Batteries can be measured with a voltmeter for an approximate state of charge, but many dead batteries can hold a ‘shallow charge’ which


drops off rapidly when current is drawn. You’ll need to test them with a ‘live’ load over a series of hours to verify them.

• A regulated 12 volt power supply can not fully charge a discharged 12 volt battery, but it makes a good float charger if the output voltage is correct (again, 13.5-13.8 volts for a 12 volt system). Check the water level in the cells often, and replenish as needed with distilled water.

Choose an inverter

• Rated for continuous duty at substantially more power than you think you’ll need.

• Enough ‘peak’ current to handle motor starting loads, which can be from 3 to as much as 7 times the rated running wattage.

• Inverters are available for input voltages of 12, 24, 36, 48, and 96 volts, and a few less common voltages. The higher the voltage the better, especially for large systems. 12 volts is the most common, but in no case should one consider 12 volts for a system of greater than 2400 watts output (The amount of current that has to be handled is simply too high).

• Some of the better inverters have a built-in 3-stage automatic battery charger and transfer relay, greatly simplifying the system. These inverters are well worth the extra money; if fact they save money overall, as the built-in charger is a bargain compared to the price of a comparable stand-alone charger.

Get cables and fuses and other hardware to interconnect batteries,charger and inverter.

• These should be very heavy gauge, well made, and as short as you can fit it all together with. This is to keep the cable resistance low.

• Consider spending just a bit more for a bus bar interconnect with big dividers, instead of just ‘wires everywhere’. It is tidy and helps prevent accidental shorts. It also makes it easier to remove defective batteries.

Wear protective gear and observe safety precautions.

• Don your eye protection to protect against acid splashes to the eye.

• Wear protective, non-conductive gloves if possible.

• Remove any jewelry and any metallic items you might be wearing.


Securely attach the charger cables to the deep cycle battery, notingpolarity.

Prepare the charging system. Plug the charger into the wall and powerit on. Make sure it begins a proper charge cycle, and make sure the inverter ispowered off.

Attach and test the inverter if it is separate from the charger.Hook up the cables to the batteries, noting polarity. Turn the inverter on and testit with some suitable AC load. You shouldn’t see sparks, smoke, or fire at anypoint. Leave the inverter on with a load similar to your planned load and allowthe battery to charge overnight. This will test that the charger and load are agood match. In the morning, the battery should be fully charged.

Dismantle the Test Rig

Design a tidy enclosure. This could be shelves in a shed, or a verylarge container. This will hold the batteries, charger, and inverter. Generally thecharger and inverter should not be right next to the batteries where escaping gascan get to them. If so, it can shorten the life of the electronics, or ignite gasesfrom sparking if vents are blocked. Some partition should be installed andseparate air circulation should be provided for the charger and inverter.Alternatively, mount the charger/inverter outside the battery box. Once ready,install the components into it.

Make the connections. Runs of cable should be kept fairly short. Youneed easy access to every battery to check, so clean and tighten cables. Forwet cells, you need to be able to easily take every top off to check fluid levelsand get distilled water into them. Make sure the inverter is grounded. You mayground it to the ground wire on the charger’s input AC, or use a grounding roddriven into the soil.

Supplement alternatives where beneficial or necessary. You maysupplement or replace the charger with solar, wind, etc., connected to their ownapplicable charge controller. This can keep the power running far longer than itotherwise would, even indefinitely. Also, you may supplement the charger with agenerator. Attach a truck alternator to a small internal combustion engine, use agenerator with 12 volt charging output, or unplug the charger from its AC outletand then use a ‘regular’ AC generator to power the charger.

The UPS can be located outside

• Install an inside and outside outlet through a wall connected only to each other. You can plug the UPS inverter into the outside outlet (with a ‘gender bender’ extension cable) to power the inside outlet.


• Disconnect and isolate an indoor circuit from the main circuit breaker panel. Route the wire out of that box through one of the punch-outs or remove it, and connect it to the inverter, providing conduit to shield as applicable. All plugs/lights/smoke detectors/etc. on that circuit will be powered by the UPS, so test and make sure nothing ‘extra’ is connected to it.

• Run conduit and/or get fancy as you see fit, relative the permanence of your solution.

Short Answer Type Questions1. What are applications of UPS?

2. Mention the types of UPS.

3. Mention the types of invertors.

4. What are advantages of using MOSFET in invertors?

5. What is Voltage Somse inverter?

6. Write applications of three phase inverters.

7. What is the difference between on line/off-line UPS?

8. Write IC member used in inverter.

Long Answer Type Questions1. Explain working of Single phase bridge inverter using MOSFET.

2. Explain working of voltage source inverter.

3. Explain working of three-phase inverter.

4. Draw the block diagram of off-line UPS. Explain working.

5. Draw and explain on line UPS

6. Explain servicing procedure of IC’S Used in inverter.

Structure4.1 Telephone Communication

4.2 Operation of Basic Telephone Equipment

4.3 Working of Digital Dailing instrument

4.4 Intercom System working

4.5 Concepts of Mobile Communicating System

4.6 Intelligent Network Concept

4.7 Cellular Concept

4.8 Celllular System Operation

4.9 Significance Frequency reused Handoff features

4.10 Concepts of Digital Cellular Mobile System

4.11 GSM Standards and Services

4.12 Radio Characteristics of GSM

4.13 Concepts of CDMS system used in Mobile Communication.


• Study of basic telephone system.

4UNIT

Telephone and CellularCommunication


• Working of telephone equipment.

• Working of Digital dialing instrument.

• Working of Simple Intercom system.

• Concepts of Mobile communication system.

• Concept of Intelligent network.

• Working of cellular concept

• Working of Cellular operation system.

• Significance of frequency re-use and hand off features.

• Concepts of Digital cellular mobile system

• GSM standards and service aspects

• Radio characteristics of GSM

• Basic concepts of CDMA system use in mobile communication.

4.1 Telephone CommunicationIntroduction

Telephone communication is a routine, but important, component ofevery healthcare practice. Everyone in your office should approach telephonecommunications as an opportunity to provide patients with good service and toobtain important information. A patient’s first and lasting impression of yourpractice is often from a telephone call.

Fig 4.1 Diagram of Telephone


Fig 4.2 Diagram of Telephone

Healthcare providers also must be mindful of the potential liability risksassociated with telephone communication. Areas of increased telephone officeliability may include allegations of failure to diagnose, delay of treatment, impropertreatment, failure to follow-up, and breach of confidentiality.

Absent or improper documentation of telephone contacts and messagescan negatively impact the defense of a malpractice claim. Therefore, it is essentialthat telephone calls be documented with the same detail as an office visit. Havepolicies and procedures to enhance andmonitor the quality of your practice’stelephone communications with patients. All staff share in the responsibility toprovide patients with courteous and efficient telephone communication.

Tips for Effective Telephone Communication

1. Train all office staff in telephone etiquette, including handling an angry or dissatisfied patient. The attitude of the person who answers the telephone will set the first impression of your office.

2. A caller should always have the option of speaking with a person.

3. Try to answer the telephone by the third ring and monitor calls that are put on hold. Allow callers to speak first, and ask for and get their permission to place them on hold.


4. If your office is equipped with an automatic call distribution system, limit the menu selections to four or five at most. The first message should always be, “If this is an emergency, dial 911 or go to the emergency room immediately.”

5. Conduct telephone conversations out of the hearing of patients to protect the caller’s privacy.

6. Obtain the caller’s phone number and confirm identifying patient information.

7. When a return call is required, ask the caller what time he or she will be available, and give an approximate time for the return call. Then, make return calls as promised. This conveys a message to patients that you care and are respectful of their time and concerns.

8. Develop a Telephone Advice Protocol Manual for nursing and other staff authorized to give telephone advice that addresses areas such as handling routine questions and doing telephone assessments and triage. Monitor staff compliance with the protocol.

9. Instruct staff to consult a physician or other designated clinician whenever they have concerns or questions regarding their telephone assessment or advice. Respond promptly and positively when staff seek guidance.

10. Appointment books should be written in black or blue ink. Entries should never be erased or obliterated with erasure fluid. When making a change, simply mark a line through the information already entered and record the change below. Keep old appointment books and telephone logs for as long as you maintain medical records.

11. Develop a policy and procedures for handling phoned-in lab reports that include how “panic values” are to be relayed to the physician.

12. The practitioner who orders a test should be the person who calls the patient to communicate sensitive results.

4.2 Operation of Basic Telephone EquipmentInset Receiver Type 1L

This receiver uses short cobalt-steel permanent magnet which provideshigher flux density than the tungsten-steel magnet used in the Bell receiver. Thecase is either aluminium or moulded bakelite which is threaded so that a bakeliteearpiece (No 18) can be screwed to hold the diaphragm in place.


The resistance of each coil is 40 ohms (total 80 ohms). the impedancevaries from about 110 to 710 ohms over the voice frequency range and is about350 ohms at 1000 cps.

The magnet and pole piece assembly are mounted on either a die-castaluminium case or a case of moulded phenolic material which is threaded so thatthe bakelite earpiece (No 23) can be screwed to hold the diaphragm in position.

The DC resistance is about 55 ohms. The impedance varies from about100 to 640 ohms over the voice frequency range and is about 290 ohms at1000 cps.The type 1L and 2P receivers do not have Bessemer steel pieces as inthe Bell receiver (1A). However, the wide pole pieces provide an alternative airpath of large cross-section at the base of the coils which shunts the high reluctanceof the permanent magnet for the changes of flux in the magnetic circuit.

Fig 4.3 Cross section of later type bell receiver

4.3 Working of Digital Dialling InstrumentM2616

• Handsfree

• On-Hook Dialing

• Message Waiting Indicator

• Wall Mount Capability

• 13 configurable Feature Keys


Fig 4.4 Digital Dialling Instrument

Volume Control Bar for

• Ringing Tone

• Buzz Tone

• Speaker

• Handset/Headset

• Handsfree

Support for the following set options

• 2 x 24 Character Display

• MCA data option to provide integrated voice and data transmission

• External Alerter Interface for high ambient noise environments

• Add-on 22 configurable Feature Key Expansion Modules (2 maximum)

• Analog Terminal Adapter (ATA) for simultaneous use of a fax, modemor other analog device through the ATA’s RJ-11 connection.

M2008/M2008HF

• Handsfree (on the M2008HF only)

• On-Hook Dialing




• 8 configurable Feature Keys (7 on the M2008HF)



• Ringing Tone

• Buzz Tone

• Speaker

• Handset/Headset

• Handsfree (M2008HF)


• 2 x 24 Character Display



• Analog Terminal Adapter (ATA) for simultaneous use of a fax, modemor other analog device through the ATA’s RJ-11 connection

M2006

• On-Hook Dialing



• 6 configurable Feature Keys




• Ringing Tone

• Buzz Tone

• Speaker

• Handset/Headset




• Analog Terminal Adapter (ATA) for simultaneous use of a fax, modem or other analog device through the ATA’s RJ-11 connection.

4.4 Intercom System Working

An intercom (intercommunication device), talkback or doorphone isa stand-alone voice communications system for use within a building or smallcollection of buildings, functioning independently of the public telephone network.Intercoms are generally mounted permanently in buildings and vehicles.

Intercoms can incorporate connections to public address loudspeakersystems, walkie talkies, telephones, and to other intercom systems. Someintercom systems incorporate control of devices such as signal lights and doorlatches.


Basic Intercom System terms

Fig 4.7 Intercom Instrument

Sub-station by Bolinder’s_teleradio (50’s)

Master Station or Base Station – These are units that can control thesystem, i.e., initiate a call with any of the stations and make announcements overthe whole system.

Sub-station – Units that are capable of only initiating a call with a MasterStation but not capable of initiating calls with any other stations (sometimescalled slave units).

Door Station – Like sub-stations, these units are only capable ofinitiating a call to a Master Station. They are typically weather-proof.

Intercom Station – Full-featured remote unit that is capable of initiatingand receiving party-line conversation, individual conversation and signalling. Maybe rack-mounted, wall-mounted or portable.

Wall Mount Station – fixed-position intercom station with built-inloudspeaker. May have flush-mounted microphone, hand-held push to talkmicrophone or telephone-style handset.

Belt Pack – portable intercom station worn on the belt such as aninterruptible feedback (IFB) with an earpiece worn by talent.

Handset – permanent or portable telephone-style connection to anintercom station. Holds both an earpiece and a push to talk microphone.


Headset – portable intercom connection from a belt pack to one orboth ears via headphones with integrated microphone on a boom arm. Connectsto a belt pack.

Paging Signal – An audible and/or visual alert at an intercom station,indicating that someone at another station wants to initiate a conversation.

Power Supply – Used to feed power to all units. Often incorporatedinto the design of the base station.

4.5 Concepts of Mobile Communicating SystemCellular Telephone Systems

Radio telephone system should be structured to achieve high capacitywith limited radio spectrum while at the same time covering very large areas.

Older System

Achieve a large coverage area by using a simple, high poweredtransmitter. Put BS on top of mountains or tall towers, so that it could providecoverage for a large area. The next BS was so far away that interference wasnot an issue. Severely limit the number of users that could communicatesimultaneously. Noise-limited system with few users. The Bell mobile system inNew York City in the 1970s could only support a maximum of twelve simultaneouscalls over a thousand square miles.

The number of simultaneous calls a mobile wireless system canaccommodate is essentially determined by the total spectral allocation for thatsystem and the bandwidth required for transmitting signals used in handling acall.

Cellular systems accommodate a large number of users over a largegeographic area, within a limited frequency spectrum. High capacity is achievedby limiting the coverage of each base station transmitter to a small geographicarea called a cell so that the same radio channels may be reused by anotherbase station located some distance away. The coverage area is divided intomany cells.

Replace a single, high power transmitter (large cell) with many low powertransmitters (small cells) each providing coverage to only one cell area (a smallportion of the service area). A sophisticated switching technique called a handoenables a call to proceed un-interrupted when the user moves from one cell toanother.


The concept of cells was first proposed as early as 1947 by BellLaboratories in the US, with a detailed proposal for a \High-Capacity MobileTelephone System” incorporating the cellular concept submitted by BellLaboratories to the FCC in 1971. The first AMPS system was deployed inChicago in 1983.

Basic cellular system : Consist of mobile stations, base stations, anda mobile switching center (MSC).

Mobile switching center (MSC) : Sometimes called a mobiletelephone switching oce (MTSO)

• Coordinates the activities of all of the base stations

• Connect the entire cellular system to the PSTN.

• Accommodates all billing and system maintenance functions

Each mobile communicates via radio with one of the base stations andmay be handed-off to any number of base stations throughout the duration of acall.

Mobile station : Contains a transceiver, an antenna, and control circuitry.

Base stations : Serve as a bridge between all mobile users in the celland connects the simultaneous mobile calls via telephone lines or microwavelinks to the MSC.Consist of several transmitters and receivers whichsimultaneously handle full duplex communications.

Generally have towers which support several transmitting and receivingantennas.

Communication between the base station and the mobiles is defined bya standard “common air interface (CAl)” that species four different channels.

a. Forward voice channels (FVC): voice transmission from the basestation to mobiles

b. Reverse voice channels (RVC): voice transmission from mobiles tothe base station

c. Forward control channels (FCC) and reverse control channels (RCC).

Often called setup channels. Involve in setting up a call and moving it toan unused voice channel. Initiate mobile calls.


• Transmit and receive data messages that carry call initiation and service requests, and are monitored by mobiles when they do not have a call in progress.

• FCCs also serve as beacons which continually broadcast all of the traffic requests for all mobiles in the system.

• Supervisory and data messages are sent in a number of ways to facilitate automatic channel changes and handoff instructions for the mobiles before and during a call.

• Typically, about 5% of the entire mobile spectrum is devoted to control channels, which carry data messages that are very brief and bursty in nature, while the remaining 95% of the spectrum is dedicated to voice channels.

4.6 Intelligent Network conceptThe Intelligent Network (IN) is the standard network architecture

specified in the ITU-T Q.1200 series recommendations. It is intended for fixedas well as mobile telecom networks. It allows operators to differentiate themselvesby providing value-added services in addition to the standard telecom servicessuch as PSTN, ISDN and GSM services on mobile phones.

The intelligence is provided by network nodes on the service layer, distinctfrom the switching layer of the core network, as opposed to solutions based onintelligence in the core switches or telephone equipment. The IN nodes aretypically owned by telecommunications operators (telecommunications serviceproviders).

Intelligence Network Services

• Televoting

• Call screening

• Telephone number portability

• Toll free calls/Freephone

• Prepaid calling

• Account card calling

• Virtual private networks (such as family group calling)


• Centrex service (Virtual PBX)

• Private-number plans (with numbers remaining unpublished in directories).

• Universal Personal Telecommunication service (a universal personal telephone number)

• Mass-calling service

• Prefix free dialing from cellphones abroad

• Seamless MMS message access from abroad.

• Reverse charging

• Home Area Discount

• Premium Rate calls

• Call distribution based on various criteria associated with the call.

1. Location Based Routing

2. Time-based routing

3. Proportional call distribution (such as between two or more call centres or offices).

• Call queueing

• Call transfer

4.7 Cellular Concept1. Cellular Telephone Systems

2. Frequency Reuse

3. Cell planning with hexagonal cells

4. Co-Channel Interference

5. Trunking

6. Improving Coverage and Capacity

Cellular Telephone systems

Radio telephone system should be structured to achieve high capacitywith limited radio spectrum while at the same time covering very large areas.


Fig 4.8 Diagram of Cellular Phone

Frequency Reuse

Frequency reuse refers to the use of radio channels on the same carrierfrequency to cover different areas which are separated from one another bysufficient distances so that co-channel interference is not objectionable. Frequencyreuse is employed not only in mobile-telephone service but also in entertainmentbroadcasting and most other radio services.

Cell Planning with hexagonal Cells

There are only certain cluster sizes and cell layouts which are possible.The number of cells per cluster, N, can only have values which satisfy

N = i2 + i x j + j2


Where i and j are non-negative integers.

To locate the nearest co-channel neighbors of a particular cell, one mustdo the following: (1) move i cells along any chain of hexagons and then (2) turn60 degrees counter-clockwise and move j cells.

Co-Channel Interference

Interference is the major limiting factor in the performance of cellularradio systems. Interference has been recognized as a major bottleneck inincreasing capacity and is often responsible for dropped calls

Trunking

Allow a large number of users to share the relatively small number ofchannels in a cell by providing access to each user, on demand, from a pool ofavailable channels. Exploit the statistical behavior of users. Each user is allocateda channel on a per call basis, and upon termination of the call, the previouslyoccupied channel is immediately returned to the pool of available channels.

Fig 4.9 Trunking

Improving coverage and Capacity

As the demand for wireless service increases, the number of channelsassigned to a cell eventually becomes insufficient to support the required numberof users. At this point, cellular design techniques are needed to provide morechannels per unit coverage area. Techniques such as cell splitting, sectoring, andcoverage zone approaches are used in practice to expand the capacity of cellularsystems. Cell splitting allows an orderly growth of the cellular system.


4.8 Cellular system OperationThe cellular concept employs variable low-power levels, which allows

cells to be sized according to the subscriber density and demand of a givenarea. As the population grows, cells can be added to accommodate that growth.Frequencies used in one cell cluster can be reused in other cells. Conversationscan be handed off cell to cell to maintain constant phone service as the usermoves between cells.

Fig 4.10 Diagram of Cellular System

4.9 Significance Frequency reused Handoff FeaturesThe generic term channel is normally used to denote a frequency in

FDMA system, a time slot in TDMA system, and a code in CDMA system or acombination of these in a mixed system. Two channels are different if they usedifferent combinations of these at the same place. For example, two channels ina FDMA system use two different frequencies. Similarly, in TDMA system twoseparate time slots using the same frequency channel is considered two differentchannels. In that sense, for an allocated spectrum the number of channels in asystem is limited. This limits the capacity of the system to sustain simultaneouscalls and may only be increased by using each traffic channel to carry many callssimultaneously. Using the same channel again and again is one way of doing it.This is the concept of channel reuse. The concept of channel reuse can beunderstood a cluster of three cells. These cells use three separate sets of channels.This set is indicated by a letter. Thus, one cell uses set A, the other uses set B,and so on. This cluster of three cells is being repeated to indicate that three setsof channels are being reused in different cells. A similar arrangement with clustersize of seven cells. Now let us see how this helps to increase the system capacity.


Assume there are a total of F channels in a system to be used over a givengeographic area. Also assume that there are N cells in a cluster that use all theavailable channels. In the absence of channel reuse this cluster covers the wholearea and the capacity of the system to sustain simultaneous calls is F. Now if thecluster of N cells is repeated M times over the same area, then the systemcapacity increases to MF as each channel is used M times.

The number of cells in a cluster is referred to as the cluster size, theparameter 1/N is referred to as the frequency reuse factor, and a system using acluster size of N sometimes is also referred to as a system using N frequencyreuse plan. The cluster size is an important parameter. For a given cell size, asthe cluster size is decreased, more clusters are required to cover the given arealeading to more reuse of channels and hence the system capacity increases.Theoretically, the maximum capacity is attained when cluster size is one, that is,when all the available channels are reused in each cell. For hexagonal cellgeometry, the cluster size can only have certain values. These are given by

N =i2 +j2 +ij,

where i and j are nonnegative integers.

The cells using the same set of channels are known as co-channel cells.For example the cells using channels A are co-channel cells. The distance betweenco-channel cells is known as co-channel distance and the interference causedby the radiation from these cells is referred to as co-channel interference. Forproper functioning of the system, this needs to be minimized by decreasing thepower transmitted by mobiles and base stations in co-channel cells and increasingthe co-channel distance. Because the transmitted power normally depends onthe cell size, the minimization of co-channel interference requires a minimum co-channel distance; that is, the distance cannot be smaller than this minimumdistance.

Handoff

It is common for a mobile to move away from its servicing base stationwhile a call is in progress. As the mobile approaches the cell boundary, thestrength and quality of the signal it receives starts to deteriorate. At some stage,near the cell boundary, it receives a stronger signal from a neighboring basestation than it does from its serving base station. At this point the control of themobile is handed over to the new base station by assigning a channel belongingto the new cell. This process where a radio channel used by a mobile is changed,is referred to as handoff or handover [41, 44, 48–50]. When handoff is betweentwo base stations as described earlier, it is referred to as intercell handoff. Onthe other hand, when handoff is between two channels belonging to the same


base stations, it is referred to as intracell handoff. The situation arises when thenetwork, while monitoring its channels, finds a free channel of better quality thanthat used by a mobile and decides to move the mobile to this new channel toimprove the quality of channels in use. Sometimes, the network rearrangeschannels to avoid congestion and initiates intracell handoff. Handoff is alsonecessary between different layers of overlayed systems consisting of microcellsand macrocells. In these systems, the channels are divided into microcell channelsand macrocell channels. When a mobile moves from one microcell to anotherand there is no available channel for handoff, a macrocell channel is used tomeet the handoff request. This avoids the forced termination of a call. Later if achannel becomes available at an underlayed microcell, then the macrocell channelmay be released and a microcell channel is assigned to the call by initiating anew handoff.

Fig 4.11 Handoff system

Forced termination of a call in progress is undesirable and to minimize ita number of strategies are employed. These include reserving channels forhandoff, using channel assignment schemes that give priority to a handoff requestover new calls, and queuing the handoff request. The channel reservation andhandoff priority scheme reduce the probability of forced termination by increasingthe probability of blocking new calls. The queuing schemes are effective whenhandoff requests arrive in groups and there is a reasonable likelihood of channelavailability in the near future.

The handoff is initiated when the quality of current channels deterioratesbelow an acceptable threshold or a better channel is available. The channelquality is measured in terms of bit-error rate (BER), received signal strength, orsome other signal quality such as eye opening of radio signal that indicates signalto interference plus noise ratio.


For handoff initiation the signal strength is used as an indication of thedistance between the base and the mobile. For this reason, a drop in signalstrength resulting from Rayleigh fading is normally not used to initiate handoffand some kind of averaging is used to avoid the problem. In some systems theroundtrip delay between mobile and base is also used as an indication of thedistance. The measurement of various parameters may be carried out either atthe mobile or at the base. Depending on where the measurements are made andwho initiates the handoff, various handoff implementation schemes are possibleincluding network-controlled handoff, mobile-controlled handoff, and mobile-assisted handoff.

4.10 Concepts of Digital Cellular Mobile SystemIn contrast to the first-generation analog systems, second-generation

systems are designed to use digital transmission and to employ TDMA or CDMAas a multiple access scheme. These systems include North American dual-modecellular system IS-54, North American IS-95 systems, Japanese Personal DigitalCellular (PDC) systems, and European GSM and DCS 1800 systems. GSM,DCS 1800, IS-54, and PDC systems use TDMA and FDD whereas IS-95 is aCDMA system and also uses FDD for a duplexing technique. Other parametersfor these systems are shown in Table 1.2. In this section a brief description ofthese systems is presented.

Fig 4.12 Digital Cellular System


The PDC system, established in Japan, employs TDMA technique. Ituses three time slots per frequency channel and has a frame duration of 20 ms.It can support three users at full-rate speech and six halfrate speech users similarto IS-54. It has a channel spacing of 25 kHz and uses modulation. It supports afrequency-reuse plan with cluster size four and uses MAHO.

Code Division Multiple Access Digital Cellular System (InterimStandard-95)

This CDMA digital system uses CDMA as a multiple access techniqueand occupies the same frequency band as that occupied by AMPS; that is, theforward-link frequency band is from 869 to 894 MHz and the reverse-linkband is from 824 to 849 MHz. Forward-link and reverse-link carrier frequenciesare separated by 45 MHz. Each channel in IS-95 occupies a 1.25-MHzbandwidth and this is shared by many users. The users are separated from eachother by allocating 1 of 64 orthogonal spreading sequences (Walsh functions).The user data are grouped into 20-ms frames and are transmitted at a basic userrate of 9600 bps. This is spread to a channel chip rate of 1.2288 Mchip/s givinga spreading factor of 128. RAKE receivers are used at both base station andmobiles to resolve and combine multipath components. During handoff thestandard allows for base station diversity whereby a mobile keeps link withboth the base stations and combines.

4.11 GSM Standards and Services

The general packet radio service (GPRS), a data extension of the mobiletelephony standard GSM is emerging as the first true packet-switchedarchitecture to allow mobile subscribers to benefit from high-speed transmissionrates and run data applications from their mobile terminals. A high-level descriptionof the GPRS system is given with emphasis on services and architectural aspects.

Nomenclature

ANSI American National Standards Institute

APN Access point name

AuC Authentication center


BSCs Base station controllers

BSS Base station subservice

BTS Base transceiver stations

CDMA Code division multiple access

CLNS Connectionless network service

CONS Connection oriented network service

EDGE Enhanced data rates for GSM evolution

EIR Equipment identity register

ETSI European Telecommunications Standards Institute

GGSN Gateway GPRS support node

GMM/SM GPRS mobility management and session management

GPRS GPRS packet radio service

GSM global system for mobile communications

GSN GPRS support node

GTP GPRS tunnelling protocol

HLR Home location register

HSCSD High-speed circuit-switched data

IMSI International Mobile Subscriber Id

IMT 2000 International Mobile Telecommunications 2000

IP Internet protocol

IPsec Internet protocol security

LA location area

LLC logical link control

MAC medium access control


MO mobile originated

MS mobile station

MT mobile terminal, mobile terminated

MSC message switching centre.

NSS network switching subsystem

NSAP network service access point

NSAPI network service access point identifier

PCU packet control unit

PIN personal identification number

PDN packet data network

PDP packet data protocol

PDU protocol data unit, packet data units

PLMN public land mobile network

PTP point-to-point

PTM point-to-multipoint

RA routeing area

RF radio frequency

RLC radio link control

SAP service access point

SGSN serving GPRS support node

SNDCP subnetwork dependent convergence protocol

TDMA time division multiple access


Fig 4.13 GSM cellular system

In the past few years, fixed networks have witnessed a tremendousgrowth in data traffic due in good part to the increasing popularity of the Internet.Consequently new data applications are emerging and are reaching the generalpublic. At the same time the market is witnessing a remarkable explosion ofcellular and mobile technologies leading to demand that data applications becomeavailable to mobile users. Global system for mobile communications (GSM) [1]is the European standard for cellular communications developed by the European

Telecommunications Standards Institute (ETSI). Throughout Europe andthe rest of the world (including North America), GSM has been widely adopted.It has already been implemented in over 100 countries [2]. The most importantservice in GSM is voice telephony. Voice is digitally encoded and carried by theGSM network as a digital stream in a circuit-switched mode. GSM o€ers dataservices already but they have been constrained by the use of circuit-switcheddata channels over the air interface allowing a maximum bit rate of 14.4 kbit/s.For this reason, the GSM standard has continued its natural evolution toaccommodate the requirement for higher bit rates.

The high-speed circuit-switched data (HSCSD) are one solution thataddress this requirement by allocating more time slots per subscriber and thusbetter rates. It remains however insufficient for bursty data applications such asWeb browsing. Moreover, HSCSD rely on circuit switching techniques making


it unattractive for subscribers who want to be charged based on the volume ofthe data traffic they actually use rather than on the duration of the connection. Inturn, service providers need effective means to share the scarce radio resourcesbetween more subscribers. In a circuit-switched mode, a channel is allocated toa single user for the duration of the connection.

This exclusive access to radio resources is not necessary for dataapplications with the use of packet switched techniques. GPRS stands out asone major development in the GSM standard that benefits from packet switchedtechniques to provide mobile subscribers with the much needed high bit ratesfor bursty data transmissions.

It is possible theoretically for GPRS subscribers to use several timeslots (packet data channels) simultaneously reaching a bit rate of about 170kbit/s. Volume-based charging is possible because channels are allocated tousers only when packets are to be sent or received. Bursty data applicationsmake it possible to balance more efficiently the network resources betweenusers because the provider can use transmission gaps for other subscriberactivities.

4.12 Radio characteristics of GSMCharacteristics of GSM

Communication : mobile, wireless communication; support for voiceand data services

Total mobility : international access, chip-card enables use of accesspoints of different providers

Worldwide connectivity : one number, the network handles localization

High capacity : Better frequency efficiency, smaller cells, more customersper cell

High transmission quality : high audio quality and reliability for wireless,uninterrupted phone calls at higher speeds (e.g., from cars, trains)

Security functions : access control, authentication via chip-card andPIN

Mobile ServicesGSM services

• basic services

• voice services


• data services

• short message service

• additional services

• emergency number

• group 3 fax

• electronic mail

• supplementary services

• identification: forwarding of caller number

• suppression of number forwarding

• automatic call-back

• conferencing with up to 7 participants

Fig 4.14 Radio GSM system

Basic Services

• Services are supported by traffic channels

• full rate: 22.8 kbit/s (gross bit rate, unprotected transmission)

• half rate: 11.4 kbit/s (gross bit rate, unprotected transmission)


Voice services (speech coding with protection)

• full rate: 13 / 12.2 kbit/s (original coder / enhanced full rate coder)

• half rate: 5.6 kbit/s (enhanced half rate coder)

Data services (coding with different levels of protection)

• full rate: 9.6 / 4.8 / 2.4 kbit/s

• half rate: 4.8 / 2.4 kbit/s

• Enhanced data services

HSCSD (High Speed Circuit Switched Data)

• n X 14.4 / n X 9.6 / n X 4.8 kbit/s (n=1, 2, 3, 4)

• GPRS (General Packet Radio Service)

• Various rates (typically up to 53.6 kbit/s).

4.13 Concepts of CDMA systems used in MobilecommunicationSpreading: Chips and Symbols

• A chip is the shortest modulated signal in a DS-CDMA system

• Chip rate signal bandwidth

• A symbol is spread over multiple chips

• Spreading Factor (SF): # chips used to transmit one symbol spreading factor = SF = Rc/ Rs=chip rate / symbol rate

• SF is a spectrum spreading factor: bit rate Ò! chip rate

• Provides processing gain against noise and interference

• The Spreading Code is a sequence of SF chips

• Usually the chips are +-1

• Spreading code can be understood as SF × 1 vector c

• Normalization: cTc = SF

• Spreading

• Transmitted symbol x (any linear modulation).


Fig 4.15 CDMA System

De-spreading

• At receiver, de-spreading is performed

• Chips carrying information about symbol x are coherently combined.

• Noise and interference is de-spread and non-coherently combined.

• As long as interference is not transmitted with the same spreading code processing gain

Cross- and Autocorrelation

• A family of spreading codes is a set {cj}

• Different spreading codes define different code multiplexed channels

• Cross-correlation

• Correlation between two spreading codes; cTi cj

• Determines orthogonality between codes

• Auto-correlation

• Correlation between code and delayed copies of itself

• Determines performance in multipath environments.


Orthogonal Spreading Walsh-Hadamard Codes

• A family of spreading codes is orthogonal

• At most SF orthogonal codes with length SF

• Orthogonality lost if codes not received synchronously.

Walsh-Hadamard (WH) Spreading Codes

• Family of orthogonal spreading codes for SF = 2n

• SF orthogonal codes with spreading factor SF

• The four SF = 4 WH codes quoted on previous slide.

• Perfect cross-correlation properties between spreading codes with same timing (orthogonality)

WH OVSF Code Tree

• SF=2 user with code [+1 +1]

• Occupies all branches of code tree branching off from [+1 +1]

• Leaves room e.g. for 4 SF=8 users with codes from the other branch

• While 4 symbols Tx’d to SF=2 user, 1 symbol Tx’d to four SF=8 users.

• All these are orthogonal to each other: 8 symbols in 8 chip periods

Spreading in Cellular Systems

• Synchronous transmissions: orthogonal spreading

• Asynchronous transmissions: pseudo-random spreading

• Randomizes interference

• Downlink is intra-cell synchronous by definition

• Transmissions to all users have the same timing

• Orthogonal spreading between DL users

• Possible orthogonal multicode transmissions to a user

• To synchronize intra-cell UL, accurate Timing Advance is required

• Less than fraction of chip rate

• If UL synchronous, orthogonal spreading between users may be used


• If UL asynchronous, orthogonal spreading between users not possible

• Orthogonal spreading only for UL multicode transmission from a user.

• In CDMA systems, resource use in different cells not orthogonal

• Reuse 1

• Pseudo-random spreading (“scrambling”) to mitigate inter-cellinterference.

Spreading Partitioning

Functions of spreading in cellular system

1. provide immunity against multipath interference

2. whiten inter-cell interference (randomization)

3. provide orthogonal CDM for synchronized intra-cell channels

Partitioning of Spreading Code

• Scrambling code

• Long sequence of pseudo-random +-1:s generated by mathematically defined random sign generator, known at BS and MS

• Fulfils targets 1 and 2

• Channelization code

• Performs spreading from symbol rate to chip rate

• Length SF

• Family of SF orthogonal codes, provides multiple orthogonal channels, if needed

• Fulfils target 3

Spreading and Scrambling

• A symbol is spread by

• Multiplying with a channelization code of length SF chips

• Multiplying the chips with a changing set of SF consecutive chips of scrambling code


Spreading and Scrambling II

• If two synchronous transmissions are spread with orthogonalchannelization codes, and scrambled with the same scrambling code, thetransmissions remain orthogonal.

Advantages of CDMA

• All resources can be used in all cells

• Wideband channel and large SF produce sufficient SINR for cell edge users high system capacity

• Protection against multipath interference

• Based on good autocorrelation properties of the spreading codes

• If spreading factor is large

• Multipath diversity can be utilised by the RAKE-receiver

• It is easy to multiplex different channels in the code domain

• Control and transport channels, different users

• Silence periods in the transmitted signal do not consume resources.

Disatvantages of CDMA

• In UL, chip synchronisation between the users may be overwhelming.

• Multiple Access Interference (MAI) between users

• In DL, Inter-Path Interference (IPI) reduces orthogonality of users

• Accurate power control needed to avoid near-far problems which put distant users in an unfavourable situation

• CDMA is fundamentally an access scheme for low rates and many users.

• RAKE receiver works well in severe multipath channel only if significant fraction of the possible orthogonal codes are not used (DL)

• Despreading in UL works well only for large SF


• When striving for high data rates with high SINR requirements, IPI and MAI dominate performance

• More complex receivers (chip equalizers) are needed to mitigate IPI and MAI.

• Simplicity of DS-CDMA is lost.

Short Answer Type Questions1. Write applications of telephones

2. Write types of telephones

3. Write applications of intercom

4. What are the advantages of digital dialing?

5. Mention the cellular concepts.

6. What is re use frequency?

7. What is hand off features?

8. Mention the types of cell phone networks.

9. Expand GSM, CDMA.

10. Write any two Radio characteristics of GSM.

Long Answer Type Questions1. Explain basic telephone working system.

2. Explain working of basic telephone equipment.

3. Explain working of intercom working with neat diagram

4. Describe concepts of mobile communication system

5. Explain cellular concepts.

6. Explain significance of frequency re-use and handoff features.

7. Explain working of digital cellular system.

8. Write GSM standards.

9. Explain radio characteristic of GSM.

10. Explain basic concepts of CDMA systems.

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