AB101 DSB Modulator and Demodulator Operating … DSB Modulator and Demodulator Operating Manual Ver.1.1 An ISO 9001 : 2000 company 94-101, Electronic Complex Pardesipura, Indore-

AB101 DSB Modulator and Demodulator

Operating Manual Ver.1.1

An ISO 9001 : 2000 company

94-101, Electronic Complex Pardesipura, Indore- 452010, India Tel : 91-731- 2570301/02, 4211100 Fax: 91- 731- 2555643 e mail : [email protected] Website : www.scientech.bz Toll free : 1800-103-5050

mailto:[email protected]

http://www.scientech.bz

AB101

Scientech Technologies Pvt. Ltd. 2

AB101


RoHS Compliance

Scientech Products are RoHS Complied. RoHS Directive concerns with the restrictive use of Hazardous substances (Pb, Cd, Cr, Hg, Br compounds) in electric and electronic equipments. Scientech products are “Lead Free” and “Environment Friendly”. It is mandatory that service engineers use lead free solder wire and use the soldering irons upto (25 W) that reach a temperature of 450°C at the tip as the melting temperature of the unleaded solder is higher than the leaded solder.

DSB modulator and demodulator AB101

Table of Contents

1. Introduction 4

2. Theory 6

3. Experiments

• Experiment 1 13 Study of double Sideband AM Generation

• Experiment 2 17 Study of Double sideband AM Reception by Envelope Detector

4. Warranty 19 5. List of Accessories 19

AB101


Introduction AB101 is a compact, ready to use DSB Modulator and Demodulator experiment board. This is useful for students to understand process of modulation and demodulation. Test points are provided in board to understand waveforms at inputs and outputs of each block. It can be used with Scientech Analog Lab ST2612 which has built in DC power supply, AC power supply, function generator, modulation generator, continuity tester, toggle switches, and potentiometer.

List of Boards : Model Name AB01 Diode characteristics (Si, Zener, LED) AB02 Transistor characteristics (CB NPN) AB03 Transistor characteristics (CB PNP) AB04 Transistor characteristics (CE NPN) AB05 Transistor characteristics (CE PNP) AB06 Transistor characteristics (CC NPN) AB07 Transistor characteristics (CC PNP) AB08 FET characteristics AB09 Rectifier Circuits AB10 Wheatstone Bridge AB11 Maxwell’s Bridge AB12 De Sauty’s Bridge AB13 Schering Bridge AB15 Common Emitter Amplifier AB14 Darlington Pair AB16 Common Collector Amplifier AB17 Common Base Amplifier AB18 Cascode Amplifier AB19 RC-Coupled Amplifier AB20 Direct Coupled Amplifier AB21 Class A Amplifier AB22 Class B Amplifier (push pull emitter follower) AB23 Class C Tuned Amplifier AB25 Phase Locked Loop (FM Demodulator & Frequency Divider /

Multiplier) AB28 Multivibrator ( Mono stable / Astable) AB29 F-V and V-F Converter AB30 V-I and I-V Converter AB31 Zener Voltage Regulator AB32 Transistor Series Voltage Regulator AB33 Transistor Shunt Voltage Regulator AB35 DC Ammeter AB39 Instrumentation Amplifier AB41 Differential Amplifier (Transistorized)

AB101


AB42 Operational Amplifier (Inverting / Non-inverting / Differentiator) AB43 Operational Amplifier (Adder/Scalar) AB44 Operational Amplifier (Integrator/ Differentiator) AB45 Schmitt Trigger and Comparator AB49 K Derived Filter AB51 Active filters (Low Pass and High Pass) AB52 Active Band Pass Filter AB54 Tschebyscheff Filter AB56 Fiber Optic Analog Link AB57 Owen’s Bridge AB58 Anderson’s Bridge AB59 Maxwell’s Inductance Bridge AB64 RC – Coupled Amplifier with Feedback AB65 Phase Shift Oscillator AB66 Wien Bridge Oscillators AB67 Colpitt Oscillator AB68 Hartley Oscillator AB80 RLC Series and RLC Parallel Resonance AB82 Thevenin’s and Maximum power Transfer Theorem AB83 Reciprocity and Superposition Theorem AB84 Tellegen’s Theorem AB85 Norton’s theorem AB88 Diode Clipper AB89 Diode Clampers AB90 Two port network parameter AB91 Optical Transducer (Photovoltaic cell) AB92 Optical Transducer (Photoconductive cell/LDR) AB93 Optical Transducer (Phototransistor) AB96 Temperature Transducer (RTD & IC335) AB97 Temperature Transducer (Thermocouple) AB101 DSB Modulator and Demodulator AB102 SSB Modulator and Demodulator AB106 FM Modulator and Demodulator

………… and many more

AB101


Theory Amplitude Modulation (AM) Amplitude modulation is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage. In short information signal (modulating signal) is used to control the amplitude of the carrier wave. As the information signal increases in amplitude, the carrier wave is also made to increase in amplitude. Likewise, as the information signal decreases, then the carrier amplitude decreases.

A double sideband transmission was the first method of modulation developed and, for broadcast stations, is still the most popular.

By looking at figure 1 below, two inputs are coming in amplitude modulator block one is the information signal i.e. modulating signal and the other is high frequency carrier signal we can see that the modulated carrier wave does appear to 'contain' in same way the information as well as the carrier.

Figure 1

AB101


Depth of Modulation : The amount by which the amplitude of the carrier wave increases and decreases depends on the amplitude of the information signal and is called the 'depth of modulation'.

The depth of modulation can be quoted as a fraction or as a percentage.

Percentage modulation = max min

max min

V V

V V

−

+ x 100%

Here is an example:

Figure 2

In above figure 2 we can see that the modulated carrier wave varies from a maximum peak-to-peak value of 10 volts, down to a minimum value of 6 volts. Inserting these figures in the above formula, we get:

Percentage modulation = 610

610

+

− x 100%

= 16

4 x 100 %

= 25% or 0.25

AB101


Frequency Spectrum : Assume a carrier frequency (fc) of 1 MHz and amplitude of, say 5 volts peak-to-peak.

The carrier could be shown as :

Figure 3

If we also have a 1 KHz information signal, or modulating frequency (fm), with amplitude of 2V peak-to-peak it would look like this:

Figure 4

When both signals have passed through the amplitude modulator they are combined to produce an amplitude modulated wave.

The resultant AM signal has a new frequency spectrum as shown in figure 5 inserting changes that occurs as a result of the modulation process :

1. The original 1 KHz information frequency has disappeared. 2. The 1 MHz carrier is still present and is unaltered.

AB101


There are two new components :

Figure 5 Carrier frequency (fc) plus the information frequency, called the upper side frequency (fc +fm). And Carrier frequency (fc) minus the information frequency, called the lower side frequency (fc - fm). The resulting signal in this example has a maximum frequency of 1001 KHz and a minimum frequency of 999 KHz and so it occupies a range of 2 KHz. This is called the bandwidth of the signal. Notice how the bandwidth is twice the highest frequency contained in the information signal. If the information signal consisted of range of frequencies, each separate frequency will create its own upper side frequency and lower side frequency. The Power in Sidebands : The modulated carrier wave i.e. the DSB output contains the original carrier and the sidebands. The carrier wave is unaltered by the modulation process and contains at least two thirds of the total transmitted power. The remaining power is shared between the two sidebands. The power distribution depends on the depth of modulation used and is given by :

2)2N(1

power)(carrier power Total+

=

Where N is the depth of modulation, the greater the depth of modulation, the greater is the contained within the sidebands. The highest usable depth of modulation is 100% (above this the distortion becomes excessive). Since, at least twice as much power is wasted as is used; this form of modulation is not very efficient when considered on a power basis.

AB101


Double sideband (DSB) Modulator : We are using balanced modulator, which is based on IC MC1496, to modulate our information signal. In figure 6 we can see that the amplitude of the carrier is increased and decreased in sympathy with the incoming information signal.

AM Modulation Process

Figure 6 To emphasize the connection between the information and the final waveform, a line is sometimes drawn to follow the peaks of the carrier wave as shown in figure 7. This shape, enclosed by a dashed line in out diagram, is referred to as an 'envelope', or a 'modulation envelope'.

Figure 7

The modulated signal is now nearly ready for transmission. If the modulation process has given rise to any unwanted frequency components then a band pass filter can be employed to remove them.

AB101


Double sideband (DSB) Demodulator : We are using envelope detector i.e. diode detector to demodulate the modulated signal.

Diode Detector : The function of the diode detector is to extract the audio signal from the modulated signal. It performs this task in a very similar way to a half wave rectifier converting an AC input to a DC output. Figure 8 shows a simple circuit diagram of the diode detector.

Figure 8 In figure 8, the diode conducts every time the input signal applied to its anode is more positive than the voltage on the top plate of the capacitor.

When the voltage falls below the capacitor voltage, the diode ceases to conduct and the voltage across the capacitor leaks away until the next time the input signal is able to switch it on again. See figure 9.

Figure 9

AB101


The result is an output which contains three components : 1. The wanted audio information signal.

2. Some ripple of the high frequency. 3. A positive DC voltage level.

A low pass filter followed by the envelope detector on the board is used to remove the high frequency ripple and a capacitor blocks the DC voltage level. Figure 10 shows the result of the information signal passing through the diode detector and low pass filter.

The input to the diode detector

Output of diode detector includes: a DC level, the audio signal and ripples.

Output after filtering Figure 10

Again the information signal is getting at the output of the filter block. If you varied frequency of the information signal corresponding changes occurs in the output frequency of the filter.

AB101


Experiment 1 Objective : Study of double Sideband AM Generation Equipments Needed : 1. Analog board AB101 2. DC power supplies +12V and -12V from external source or ST2612 Analog

Lab. 3. Oscilloscope

4. Function generator (ST4063) 5. 2 mm patch cords

Figure 11

AB101


Procedure : This experiment investigates the generation of double sideband amplitude modulated (AM) waveforms, using the AB101 board.

• Connect +12V and -12V DC power supplies at their indicated position from external source or ST2612 Analog Lab.

1. With the help of function generator (ST4063 Function generator) set sine wave of 1 MHz frequency and amplitude 800mV or 0.8Vpp approximately this is the carrier input to our balanced modulator.

2. Give carrier frequency input to carrier signal socket to the AB101 board.

3. With the help of function generator (ST4063 Function generator) set sine wave of frequency ranging to 300 Hz to 3.4 KHz and amplitude 2Vpp approximately this is the audio frequency sine wave which will be as our modulating signal.

4. Give information signal to modulating signal socket to the AB101 board.

5. Turn on power to the DSB modulator and demodulator (AB101) board. 6. Monitor, in turn, the two inputs to the balanced modulator block, at TP1 and

TP2 with the help of oscilloscope. 7. Turn the balance potentiometer P1, on the board. It is this block that we will use

to perform double-side band amplitude modulation. 8. Next, examine the output of the balanced modulator block at TP3, together with

the modulating signal at TP1, with the help of oscilloscope. Trigger the oscilloscope on the TP 3 signal. Check that the waveforms as shown in figure 12.

Figure 12

AB101


The output from the balanced modulator block (at TP3) is a double-sideband. AM waveform, which has been formed by amplitude-modulating the 1MHz carrier sine wave with the audio-frequency sine wave. The frequency spectrum of this AM waveform is as shown below in figure 13, where fm is the frequency of the audio modulating signal.

Figure 13

9. To determine the depth of modulation, measure the maximum amplitude (V max) and the minimum amplitude (Vmin) of the AM waveform at t.p.3, and use the following formula :

Percentage Modulation = Vmax Vmin

Vmax Vmin

−

+

Where V max and V min are the maximum and minimum amplitudes shown in figure 13.

10. Now vary the amplitude and frequency of the modulating signal, by adjusting the amplitude and frequency knob of the function generator. Note the effect that varying each potentiometer has on the amplitude modulated waveform (TP3). The amplitude and frequency of the two sidebands can be reduced to zero by reducing the amplitude of the modulating audio signal to zero. Do this by turning the amplitude potentiometer to its MIN position, and note that the signal at t.p. 3 becomes an un-modulated sine wave of frequency 1 MHz, indicating that only the carrier component now remains. Return the amplitude potentiometer to its maximum position.

11. Now turn the balanced potentiometer P1 on the board, until the signal at TP3 is as shown in figure 14.

AB101


Figure 14

The balance potentiometer varies the amount of the 1 MHz carrier component, which is passed from the modulator's output.

By adjusting the potentiometer until the peaks of the waveform (A, B, C and so on) have the same amplitude, we are removing the carrier component altogether.

We say that the carrier has been 'balanced out' (or 'suppressed') to leave only the two sidebands. This is called Double sideband suppressed carrier (DSBSC) modulation.

Note also that the amplitude of the modulating signal can be reduced to zero, by turning the Amplitude Var. knob on the function generator.

AB101


Experiment 2 Objective : To Study Double Sideband AM Reception by Envelope Detector Procedure : This experiment investigates the reception and demodulation of AM waveforms. Now the next step is to extract audio information from the amplitude modulated wave. This operation is performed by the diode detector block, whose output follows the changes in the amplitude of the signal at its input.

1. Connect cord between DSB output and DSB input sockets. By this connection, we give modulated input to the envelope detector.

Figure 16

2. Next set the output at TP3 and at TP1 according to the figure 3. Observe the signal flow from the input of envelope detector to anode of diode at

TP5 and on its cathode at TP6. Note that the signal at the diode detector's output follows the amplitude variations of the incoming signal as required.

4. Vary the potentiometer P2 in the envelope detector block while observing the output of envelope detector.

5. You can see variations in the detected output when you change the RC time constant of the filter formed by potentiometer P2 and capacitor C.

AB101


6. Next examine the output of filter at TP7 is changing when you change the frequency of information signal in the range 300 Hz to 3.4 KHz.

Figure 17

AB101


Warranty 1. We guarantee the product against all manufacturing defects for 24 months from

the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

2. The guarantee will become void, if

a) The product is not operated as per the instruction given in the operating manual.

b) The agreed payment terms and other conditions of sale are not followed.

c) The customer resells the instrument to another party. d) Any attempt is made to service and modify the instrument.

3. The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

4. The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

For any Technical Problem Please Contact us at [email protected]

List of Accessories

1. 2mm Patch Cord (Red) 16” ................................................................... 2 Nos. 2. 2mm Patch Cord (Black) 16”................................................................. 4 Nos. 3. 2mm Patch Cord (Blue) 16” .................................................................. 4 Nos.

4. e-Manual.................................................................................................1 No. Updated 26-06-2009

mailto:[email protected]

Documents

AB101 DSB Modulator and Demodulator Operating … DSB Modulator and Demodulator Operating Manual Ver.1.1 An ISO 9001 : 2000 company 94-101, Electronic Complex Pardesipura, Indore-