Oscilloscope 0002

8/9/2019 Oscilloscope 0002

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When the square wave generator switching frequency is much higher than the either

signal frequency, bits of each signal are alternately presented to the oscilloscopes vertical

input to reproduce the two signals on the screen.

R, controls the position. The signals on the screen can be overlapped, for the e a s ) ,

co IIIpa rison.

Dual trace CRO Dual beam CRO

1. One electron beam is used to generate two Two electron beams are used.

traces.

2. One vertical amplifier is used. Two vertical amplifiers are used.

3. The two signals are not displayed The two signals are displayed simultaneously.

simultaneously in real time but appears to bedisplayed simultaneously.

4. Same beam is shared between the two Signals Two separate beam~ are used hence easy to

hence difficult to switch quickly between the switch between the traces.

traces.

5. As two signals are displayed separately, the The two signals must have same frequency or

signals may have different frequencies. their frequencies must be integer multiples of

each other.

6. The size and weight is less. The size and weight is more.

7. Cannot be operated at fast speeds hence two Can be operated at very high speed hence two

separate fast transient signals can not be separate fast transient signals can be easilygrabbed. grabbed.

8. The cost is less due to single beam. The cost is more due to two beams.

9. The two different modes of oeration are The two different types are using double gun

alternate and chop. tube or split beam using single electron gun.

1. It is used to measure a.c. as well as d.c. voltages and currents. It is useful to

"/ calculate the parameters of the voltages as peak to peak value, r.m.s. value, duty

cycle etc.

2. In laboratory to measure the f requency, period, phase relationships between the

~ signals and to study periodic as well as nonperiodic signals.

:Y-1n radar, it is used for giving the visual representation of target such as aeroplane,

ship etc.

4/ln radio applications, it is used to trace and measure a signal through out the RF, IF

and AF channels of radio and television receivers. It provides the only effective


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is useful to

value, duty

the RF, IFy effective

way of adjusting FM receivers, broadband high frequency RF amplifiers and

automatic frequency control circuits.

5. In medical applications, it is used to display the cardiograms which are useful for

,/ the diagnosis of heart of the patient. Similarly electromyograms are useful t,o study

muscle condition of the patient.

6. In industry, it is used for many purposes. It is used to observe B-H curves, P-V

diagrams and other effects. Used to study the response of various transducers

which measure strain, pressure, temperature etc. It is used to observe the radiation

pattern generated by transmitting antenna.

7. I t is used to determine the modulation characteristics and to detect the standing

waves in transmiss.ion lines.

8. Curve tracers use the oscilloscope technique for testing the acti ve devices such as

\'acuum tubes, transistors and integrated circuits.

9. It is used to measure capacitance, inductance and also used to check the dIodes. It

can be used to check the faulty components in the various circuits.

1. 01 '(11 (1IIlld expllllll the s tructu re I Il1 d mlllll compOII ' ll ts of (o1l1'ell t lol ll ll cothode ral / tul lC.

2. W i lY phos pho r s cr ee ll Is pro vi de d w ith 111 1 I I lumll llum layer 7

3. st ll te the l 'I Irlous phosphors I I IIUlllg di fferel l t perslstellce I I ll d co lours . S tat e t h"l r appl /ca tio ll a reas.

4 . S ta te the un rio us charac ter is ti cs o f P31 phosphor .

.J. Draw the b lock d iagram o f ge ll e ra l purpose CR.a. Explal l l the fUl lctlolls of ( ' ( IrIOIIS /Jlocks.

6. D ra w a lld ex p la l ll t he b lock d iag ra m o f th e v er tical ampl if ie r u sed III oscilloscopes.

7. E xp la i ll t he fUllc ti on o f d elay lille i ll o sc il lo scopes . W l lI ch are t he two t ypes o f delay l il ie s 7

8. Sta te a nd I!xplain v or lo us fr on t pane l con tro ls o f a S imp le CR.a.

9. Wllllt I~ z -ax is I n tensi ty cOll tro l ?

10 . ll lfhat I s t he r ol e o f a t i1l1e base generator 7 I ii /hat are the basic t l ll1e base requfr( 'mel l ts 7

11. D ra w alld expla il l t he b lock diagram o f t ml e v as e u se d III

Ilo rm al CR.a.12 . Explaill the follo w ll 1g m o de s o f o p er at io ll o f time base generator

I) Free run mod e

I I) A u to mode

Iii) sIngl1 ! sw ee p 1Ilod e

13. Draw the b lock d iagram of a trigg er ge l/eratoI'. E xp la in t he uar lous cont ro l s a ssoc ia ted W i th I t .

14 . What is th e us e of A Cs a nd A CF c ontrols?

15 . W h ic h a re t he h ipic al t ri gg er s OI/rce s 7 E x pl ai n t he ir sigl1lficol/cl'.

16 . E x pl ai n t he u s e o ff ol lo w in g c on tr ol s

I) INT

11 ') EX T

III) LINE


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------------------------------

I

Special Oscilloscopes III

In many ,1pplications it is necessary to investigate the wilveforms hilving verv high

trt'quL'nC\' or the sigrh1ls which Me nonrepetitive and single event. In sonw ,1ppllclllon'>,

tht' dat.l I~, required to be stored and to be used later whencver neces~cilln"cope'>

,1 I"L ' necessary to perform such specia I functions. The various specie11 oscilloscope'>


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on the normal time base generator output. Due to this additinoal time base, any part of

the waveform can be brightened when oscilloscope is running on a normal time base. The

delayed time base then can be used to fill the screen completely with the brightened part

of the waveform. The user then can study that part of the waveform in great detail.

In c 1 delayed time base oscilloscope, a variable time delay circuit is used in the basIC

time base circuit. This allows the triggering of sweep time after the delay time. Thus the

delay time is variable. This time is denoted as td. After this, the sweep is triggered for the

time t,. Then the portion of the waveform for the time t x gets expanded on the complete

()scillo~cope screen, for the detail study.

If inpu,t is pulse waveform and leading edge is used to trigger the delay time, then

bgging edge can be displayed to fill the entire oscilloscope screen. This is shown in the

Fig 5.2(a). Similarly jf the lagging edge is used to trigger the delay time then leading edge

Gln be displayed on the entire screen for the time tx. This is shown in the Fig. 5.2 (b). I f

the time delay is perfectly adjusted, then any portion of the waveform can be extended to

fill the entire screen. This is shown in the Fig. 5.2 (c).

Delay A ,- Sweep . Atime is is triggeredtriggered I

I

I

I

I BI

Input ~I t..-I I I

pulse ,td I tx'

Portion A-B

is enlargedon the screen

(a) Studyof lagging

edge

(b) Studyof leading

edge

0----- tx

Fig. 5.2 Use of additional time delay

5.2.1 Block Diagram of Delayed Time Base Oscilloscope

The Fig. ').3 shows the block diagram of the delayed time base oscilloscope which use

Ill'lin time base (MTB) and delayed time base (DTB).

I I

I I

I I

I I

I I

I I

I I'--I t I I. d tx

B (c) Studyof any portion

of waveform


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MTB

unblanking

circuit

Main time

base

MTB

Ramp output

of MTB

Summation

of

unblanklng

pulses

Potentiometer

for trigger

level control

Summing

circuit

To CRT

grid

Horizontal

deflection

amplifier

Fig. 5.3 Delayed time base oscilloscope

The normal time base circuit is main time base (MTB) circuit which functions same as

c 1 conventional oscilloscope. The function of MTB blanking circuit is to produce an

unblanking pulse which is applied to CRT grid to turn on an electron beam in the CRT,during the display sweep time. The ramp output of MTB is given to the horizontal

deflection amplifier via switch S. It is also given as one input to the voltage comparator.

The other input to the voltage comparator is derived from the potentiometer whose level is

adjustable.

Key Point : Whcl / the levels of ramp ou tput o f M TB a nd trigger lev el set by

potel/tio m ete rs a re e qual, th en tire vo lt ag e c om p a mto r p ro du ccs a ncga tiv e o r positiv e output

sp ik e a t t h at i nstan t.

This spike is used to trigger the delayed time base (DTB) circuit. The time required by

the ramp output to reach the level set by potentiometer is the delay time Td, which is

adjustable.

Similar to MTB, DTB also has a blanking circuit which produces an unblanking pulse,

during the ramp time of DTB.


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The unblanking pulses from MTB and DTB a re added by summing circuit and given

to the CRT grid. The unblanking pulse of MTB produces a trace of uniform intensity. But

during ramp time of DTB, the addition of two pulses decides the intensity of the trace on

the screen. Hence during DTB time, the voltage applied to CRT grid is almost twice than

the voltage corresponding to MTB time. This increases the brightness of the displayed

waveform for t he DTB t ime.

This brightened part of the displayed waveform can be moved across as per the

requirement, by adjusting td with the help of the potentiometer.

When the part of the waveform to be brightened is identified, then the DTB ramp

output is connected to the input of the horizontal deflection amplifier through switch S.

The DTB ramp time is much smaller than MTB period but its amplitude (- V to + V) is

same as MTB ramp. 1Ience it causes the oscilloscope electron beam to be deflected from

one side of the screen to the other, during short DTB time. By adjusting DTB time/ div

control, the brightened portion can be extended, so as to fill the entire screen of the

oscilloscope. The horizontal deflection starts only after the delay time t d from the

beginning of the MTB sweep. Thus very small part of the waveform can be extended on

the entire screen.

5.2.2 Waveforms of the Delayed Time Base Oscilloscope

The waveforms of the delayed time base oscilloscope are shown in the Fig. 5.4.

~--r-LI I

I I

.*= 1 c MTB r o m p ' " 'p "I I

. I I-v.- - - ,@ -~------~---l D.C. v~~~~~:~~ltrigger

Comparison of I + V I

C D and ( 3 ) - - - - - + - ! 1 - i - - - l DTB ramp outputdecides triggering : I I : _ V - = -ofDTB I I I I

JJiL~ : , , :

I I I I

I I I I

. U iJ Addition of unblanking pulses + () ~ i ~ Voltage during DTB time IS very high

I I I I

I I I I

JTllLI :: :I I II

I I I

;tditx~

t

DTB unblanking pulse

(only for DTB ramp period)

The displayed waveform witha portion dUring

DTB period brightened

DTBtime


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By using the alternate mode, selection of MTB und DTB is possible and hence entire

w,wE'formas well c1Sintensified portion of the waveform can be simultimeously displayed

01 1 the screen as two sepc1rate waveforms. This is shown in the Fig. 5.5.

Brightenedportion

Entire waveform with':,,:,,(.1 portion for

time tx is itensified

""\:',,:1 Brightened portion displayed--~ during DTB time

Fig. 5.5 Both the waveforms displayed simultaneously

5.2.3Application

The basic applicution of delayed time base oscilloscope is to extend any part of the

w,weform on the entire screen of the oscilloscope and make it bright; so thut detail

,1I1cllysisof that portion can be done. The lagging edge and leading edge of the pulse are

best eXclmples which can be investigated thoroughly with the help of delayed time base

technique.

The conventional cclthode ray tube has the persistence of the phosphor ranging from a

felYmillisecond to several seconds. But some times it is necessary to retain the image for

much 'longer periods, upto several hours. It requires storing of a waveform for a certainduration,' independent of phosphor persistence. Such a retention property helps to display

the waveforms of very low frequency.

Mainly two types of storage techniques are used in cathode ray tubes which are:

i) Mesh storage and ii) Phosphor storage

5.3.1 Mesh Storage

Basically mesh storage consists of a dielectric material deposited on a storage mesh.

This is caJled storage target. It i s placed between the deflecting plates and the phosphor

screen. The writing beam i.e. normally focused electron beam charges the dielectric

material of storage target positively where hit.

Now the low velocity electrons are bombarded on storage target from the flood gun.

The positively churged storage target material allows these electrons to pass through, to

the phosphor screen. Thus the image stored with the help of storage mesh gets reproduced


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on the screen. Thus the storage technique has both storage target and a phosphor display

target used for storing and displaying the image.

The construction of storage cathode ray tube is shown in the Fig. 5.6.

Phosphor

screen

Writing

gun

Storage mesh

Collector mesh

In addition to the standard CRT, this CRT consists of dielectric material deposited on

storage mesh, a collector mesh, a flood gun and a collimator. The dielectric material suchas magnesium fluoride is deposited in a thin layer on the storage mesh. This is called

storage target. This technique uses the principle of secondary emission. An electron gun

producing an electron beam is called the writing gun.

When the target is bombarded by the stream of primary electrons, an energy transfer

takes place. This separates other electrons from the surface of the target. This process is

called secondary emission. The number of secondary electrons depends on the velocity of

the primary electrons, the intensity of the electron beam, the chemical composition of the

target and the condition of its surface. The ratio of secondary emission current and

primM)' beam current is called the secondary emissionratio

denoted as,

The wntmg gun produces a beam of electrons which contains the information of

signal. This beam hit the storage surface, with secondary emission ratio much greater than

unity. Thus the areas where electron beam hit, loose the electrons due to secondary

emission. Thus the write beam deflection pattern is traced on the storage surface as a

positive charge pattern. Since the insulation of the dielectric material is adequate to

prevent charge migration for a considerable length of time and thus the pattern is

effectively stored.

Now to make this pattern visible, special electron gun known as flood gun is switched

on even after many hours. The collimator electrodes act like focusing electrodes and thus

,1djust the electron paths. The collimator electrodes constitute a low voltage electrostatic

I('n~ system.

Ylost of the electrons are stopped and collected by the collector mesh. But the electrons

can pass through the positively charged areas of the storage target while the areas where

tilt' imilge is not stored are negatively charged and electrons repel from those areas. Thus

thl' e

rl'gio

~crCt"

fl nnd

the F

5U rf,

bece

the

this


9/14

nergy transfer

his process is

he velocity of

osition of the

n current and

formation of

greater than

to secondary

surface as a

adequate to

e pattern is

is switched

es and thus

electrostatic

he electrons

areas where

areas. Thus

the electrons near stored positive charge only can pass through to the post


10/14

5.4.1 Sampling Time Base

The time base circuit of the sampling oscilloscope is different than the conventional

oscilloscope. The time base of sampling oscilloscope has two functions:

i) To move the dots across the screen

ii) To generate the sampling command pulses for the sampling circuit.

It consists of synchronous circuit, which determines the sampling rate and establishes a

reference point in time with respect to the input signal.

The time base generates a triggering pulse which activates the oscillator to generate a

ramp voltage. Similarly it generates a stair case waveform. The ramp generation is based

on the output of the synchronizing circuit.

Both the ramp as well as staircase waveforms are applied to a voltage comparator.

This comparator compares the two voltages and whenever these two voltages are equal, it

generates a samppng pulse. This pulse then momentarily bias the diodes of the sampling

gate in the forward direction and thus diode switch gets closed for short duration of time.

The capacitor charges but for short time hence, it can charge to only a small percentage of

the input signal value at that instant. This voltage is amplified by the vertical amplifier

and then applied to the vertical deflecting plates. This is nothing but a sample. At the

same time, the comparator gives a signal to the staircase generator to advance through one

step. This is applied to horizontal deflecting plates, thus during each step of the stair case

waveform, the spot moves across the screen.

Thus the sampling time base is called a staircase-ramp generator in case of a sampling

oscillosope.

The input signal is

applied to the diode

sampling gate. At the start

of each sampling cycle atrigger inpu t pulse is

generated which activates

the blocking oscillator. The

oscillator output is given to

the ramp generator which

generates the linear ramp

signal.

Since the sampling must be synchronized with the input signal freq\,lency, the signal is

delayed in the vertical amplifier.

Input

signal To vertical

deflecting plates

To horizontal

deflecting plates

Trigger

input

The block diagram of

sampling oscilloscope is

shown in the Fig. 5.14.


11/14

The staircase generator produces a staircase waveform which is applied to an

attenuator. The attenuator controls the magnitude of the staircase signal and then it is

applied to a voltage comparator. Another input to the voltage comparator is the output o f

the ramp generator. The voltage comparator compares the two signals and produces the

output pulse when the two voltages are equal. This is nothing but a sampling pulse whichis applied to sampling gate through the gate control circuitry.

This pulse opens the diode gate and sample is taken in. This sampled signal is then

applied to the vertical amplifier and the vertical deflecting plates.

The output of the staircase generator is also applied to the horizontal deflecting plates.

During each step of staircase the spot moves on the screen. The comparator output

advances the staircase output through one step.

After certain number of p\llses about thousand or so, the staircase generator resets. The

sm,lIler the size of the steps of the staircase generator, larger is the number of samples and

higher is the resolution of the image.

The waveforms of sampling oscilloscope are shown in the Fig. 5.15. In sampling

oscilloscope, a staircase generator is used as input to horizontal section instead of ramp.

The sampling of Ithe waveform is done at the beginning of each step of the staircase

waveform. The sampled output is used for the vertical section. When this sampled output

is combined with the unblanking pulses, a dot waveform is obtained on the screen.

Triggering pulses tostaircase and ramp generators

1

I

I

I J 1 I

: : : I +V

J - - - - L J - - - f -VI I I I

I I I I

I I I I

I +V

I

II

h/:VjJ\i/1iViVVV/

- I I I II I I

l - - - - = - - - - - n L- I

I

I

Output of comparatorto sampling gate andunblanking circuit


12/14

v) The storage cathode ray tube is very much expensive than conventional cathode

ray tube.

vi) The storage cathode ray tube requires additional power supplies.

vii) Only one waveform can be stored in storage tube. If two traces are to be

compared, they are required to be superimposed on the same screen and must

be displayed together.

viii) The stored waveform cannot be reproduced on the external device like computer.

The digital storage oscilloscope eliminates the disadvantages of the analog storage

oscilloscope.It replaces the unreliable storage method used in analog storage scopes with

the digital storage with the help of memory. The memory can store data as long as

required without degradation. It also allows the complex processing of the signal by the

highspeed digital signal processing circuits.

In this digital storage oscilloscope, the waveform to be stored is digitised ,md then

stored in a digital memory. The conventional cathode ray tube is used in this oscilloscope

hencethe cost is less. The power to be applied to memory is small and can be supplied by

small battery. Due to this the stored image can be displayed indefinitely as long ,15 power

is supplied to memory. Once the waveform is digitised then it can be further loaded into

thecomputer and can be analysed in detail.

5.6.1 Block Diagram

The block diagram of digital storage oscilloscope is shown in the Fig. 5.21.Vertical

amplifier

Control

logic

Vertical deflection

amplifier

Cathode

ray tube


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As done in all the oscilloscopes, the input signal is applied to the amplifier and

attenuator section. The oscilloscope uses same type of amplifier and attenuator circuitry as

used in the conventional oscilloscopes. The attenuated signal is then applied to the vertical

amplifier.The vertical input, after passing through the vertical amplifier, is digitised by an

analog to digital converter to create a data set that is stored in the memory. The data setis

processed by the microprocessor and then sent to the display.

To digitise the analog signal, analog to digital (AID) converter is used. The output of

the vertical amplifier is applied to the AID converter section. The main requirement of

AID converter in the digital storage oscilloscope is its speed, while in digital voltmeters

accuracy and resolution were the main requirements. The digitised output needed only in

the binary form and not in BCD. The successive approximation type of AID converter is

most oftenly used in the digital storage oscilloscopes. .

The digitising the analog input signal means taking samples at periodic intervals of the

input signal. The rate of sampling should be at least twice as fast as the highest frequency

present in the input signal, according to sampling theorem. This ensures no loss of

information. The sampling rates as high as 100,000 samples per second is used. This

requires very fast conversion rate ofAID converter.

Key Point : Hence, gener al ly f la sh a n al og t o d igital converters are u sed, whose reso lutio n

decreases as th e sa m p li ng r ate increase s.

If a 12 bit converter is used, 0.025 % resolution is obtained while if 10 bit AIDconverter is used then resolution of 0.1 % (1 part in 1024) is obtained. Similarly with 10 bit

AID converter, the frequency response of 25 kHz is obtained. The total digital memory

storage capacity is 4096 for a single channel, 2048 for two channels each and 1024 for four

channels each.

The sampling rate and memory size are selected depending upon the duration and the

waveform to be recorded.

Once the input signal is sampled, the AID converter digitises it. The signal is then

captured in the memory. Once it is stored in the memory, many manipulations are

possible as memory can be read out without being erased.

~Odes of Operation

The digital storage oscilloscope has three modes of operation:

i) Roll mode ii) Store mode iii) Hold or save mode.

5 .6 . 2. 1 R o l l M o d e

---- This mode is used to display very fast varying signals, clearly on the screen.

The fast varying signal is displayed as if it is changing slowly, on the screen. In thismode, the input signal is not triggered at all.


14/14

1. Explain why tim e delay is necessary in oscilloscopes?

2. Sketch an d explain the block diagram of delayed t ime base oscilloscope.

3. Explain with the help of waveforms, how a port ion to be studied is brightened in d elay ed tim e baseoscilloscope.

4. Dl 'scribe the following s torage techniques used in storage oscilloscopes:

i) Mesh storngl'

ii) Ph osphor storage

5. Compare the mesh a n d p hosphor s torage techniques.

6. What is secondary emission ? Ho w it is useful in a storage oscil/oscope ?

7. Explain with suitable dia gra m th e er ase cycle in a mesh storage oscil/oscopl'.

8. State the limitations of analog storage oscilloscopes.9. Dr aw and explain th e block diagram o f digital storage oscilloscope.

10 . E xplain the mode s o f operation of digital storage oscilloscope.

11. State the advan tages of digital storage oscilloscope.

12. Write a note on sampling oscilloscope.

13. Explain thl ' sampling tim e b ase used in the sampling oscilloscopes.

14. Dr aw and explain the block diagram of the sampling oscilloscope.

15. Sta te the advantages of sampling oscilloscope.

16 . Dr aw and e x plain s taircase generator cirCliit used in sampling oscilloscope.17. Draw and explain voltage comparator circuit used in sampling oscilloscope.

18. Draw and explain sampling gate circuit used in sampling oscilloscope.

19. Explain the expanded mode operation of sampling oscilloscope.

20. Ho w sOlnp lin:! rate affects the accurate measurement of rise tim e? E xp la in w ith waveforms.

21. Explain the various modes o f operation of digital storage oscil/oscope.

22. Explain the acquisition methods used in the digital storage oscil/oscope.

23. Writl' about sample rate and bandwidth related to digital storage oscilloscope.

24. Explain the special f unction which can be performed by digital storage oscilloscope.

25 Explain the applications of digital storage oscil/oscope.

Documents

Oscilloscope 0002