20-Com208

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www.sigmatrainers.com

SIGMA TRAINERS

AHMEDABAD (INDIA)

PAM/PWM/PPM

MODULATION/DEMODULATION

TRAINER

MODEL-COM208

More

than2000

Trainers

Since

23

Years

TRAINERS

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CHAPTERS

1. To study complete theory & circuit details of PAM/PWM/PPM Modulation / Demodulation.

EXPERIMENTS

1. To generate PAM signal y modulating with Audio signal generator

2. To demodulate PAM signal using Low Pass Filter

3. To generate PWM signal y modulating with Audio signal generator

4. To demodulate PWM signal

5. To generate PPM signal y modulating with Audio signal generator

6. To demodulate PPM signal

7. To generate Voice signal PAM modulation and demodulate it

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INTRODUCTION

This trainer has been designed with a view to provide practical and experimental knowledge of a general

Circuit of PAM/PWM/PPM on SINGLE P.C.B of size 14" x 11".

1. Power supply requirement : 230V AC, 50 Hz.

2. Built in IC based power supply.

3. On Board AF Modulating signal generator - Sine wave

Frequency Range : 300 Hz to 3.4 KHz

Amplitude : 0 to 5 Vpp.

4. On Board Sampling Pulse signal generator.Frequency Range : 2 KHz to 40 KHz.

Pulse width : Variable.

5. On Board variable DC power supply to see the effect of DC on theoutput waveform : 0 to + 5 VDC)

6. On Board Input Audio amplifier with Volume control for modulating external signal from Mike or Tape

recorder.

7. On Board Output Audio amplifier with speaker & Volume Control.

8. Modulator Sections : Sample and Hold using multiplexer.

9. Demodulator Sections : Comparator &

Active Low Pass Filter -Cut off fre. 3.4 KHz.

10. Standard Accessories : 1. A Training Manual.

2. Connecting Patch cords.

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CHAPTER -1

THEORY & CIRCUIT DETAILS OF PAM / PWM / PPM MODULATION & DEMODULATION

The PAM / PWM / PPM Modulation and Demodulation System consists of following sections:

1. Input Audio Amplifier section2. Modulating Audio Signal Generator section

3. Sampling Pulse Generator section

4. Pulse Amplitude Modulator section

5. Pulse Amplitude Demodulator section

6. Pulse Width Modulator section

7. Pulse Width Demodulator section8. Pulse Position Modulator section

9. Pulse Position Demodulator section

10. Output Audio amplifier section11. Power supply section

(1) Input Audio Amplifier section: -

The Input Audio amplifier circuit consists of Pre-amplifier and Output amplifier stage. This section is used toamplify low level audio signal coming from Mike and give it to A.M. Modulator section for live A.M.

modulation.

The pre-amplifier consists of one transistor BC 548B is connected in C-E configuration. The input signal frommike is connected to the base of BC548 through coupling capacitor. The amplified audio signal obtained at the

collector of BC548 is given to the output driver amplifier consisting of IC TBA810 at pin 10 through volume

control 10K Preset/ Pot.

The IC TBA810 performs the functions of the audio amplifier, driver and the output stage. The amplifiedoutput signal available at the Pin 16 of this IC is connected to AUDIO terminal via coupling capacitor.

(2) Modulating Audio Signal Generator section: -

IC 8038 - waveform generator - is used generate sine wave signal. 10K Pot is used to vary its frequency. The

frequency range is 300 Hz to 3.4Khz. Two 100K presets are adjusted for proper peaks of sine wave signal. 1K preset is used to adjust duty cycle. The sine wave output signal available at pin 2 of IC 8038 is given to IC 356

through Amplitude pot for amplification. The amplified sinewave signal from pin 6 of IC 356 is then available atSINE O/P terminal. 22k Pot is used to vary the amplitude of Sine wave signal. The output amplitude varies

from 0 to 5Vpp.

(3) Sampling Pulse Generator section :-

To generate PAM signal Sample and Hold circuit is used. The sampling pulse generator is required to provide

sampling signal to S/H circuit.

This section is based on voltage-controlled oscillator (VCO) IC CD4046B. The pulse frequency is determinedby varying control voltage of VCO at pin 9 of this IC. This control voltage is varied by frequency pot (10K Pot).

The pulse frequency vary from 2 to 40 KHz. The saw tooth pulse signal is generated from this pulse signalusing integrator circuit. The IC 356 is used as integrator.

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(4) Pulse Amplitude Modulator section:-

To generate PAM signal Sample and Hold circuit or Natural sampling is used. Here IC CD 4053B is used as

multiplexer. The sample/Hold and Natural sampling is selected by sampling switch. The modulating input signalis given to one input of multiplexer IC CD 4053B.

During sample and hold process, the capacitor 0.1/100 PPC is charged to the current value of modulating signal

by sampling pulse signal during ON period (Sampling period). Then this capacitor hold this current voltage until

next sample time comes. It slightly discharges during off period (Hold time). Then it once again charges to the

next value of modulating signal. Thus the voltage on this capacitor approximates to modulating signal provided

the modulating signal frequency is far less than sampling pulse frequency.

(5) Pulse Amplitude Demodulator section :-

This section is based on low pass filter. It passes only low frequencies up to 3.4 KHz and reduces all otherfrequencies. Thus this removes high frequency quantization noise of PAM signal. This section is based on two

741 ICs and R-C circuits. By removing high frequency we recover original modulating signal.

(6) Pulse Width Modulator section:-

To generate PWM signal comparator circuit is used. Here IC LM 311 is used as comparator. The modulatinginput signal is given to one input of comparator. The sampling saw tooth waveform is given to second input of

comparator. The output of comparator is a PWM signal.

(7) Pulse Width Demodulator section :-

This section is based on comparator and low pass filter.

(a) Comparator:-The PAM signal is given to one input of comparator. The 1-volt DC reference signal is given to second input

of comparator. This 1 volt reference signal is generated by IC 741 by setting 4K7 preset. Then output ofcomparator is pulse width demodulated signal.

(b) Low pass filter:-

This pulse width demodulated signal is then passed through Low pass filter made of three 741 IC. The Low

pass filter passes only low frequencies up to 3.4 KHz and reduces all other frequencies. Thus this removes high

frequency quantization noise of PWM signal. By removing high frequency we recover original modulating signal.

(8) Pulse Position Modulator section:-

To generate PPM signal comparator and Multiplexer circuit is used. Here IC LM311 is used as comparator andIC CD 4053B is used as Multiplexer. 10K Pot is used to vary its output.

The modulating input signal is given to one input of comparator. The sampling saw tooth waveform is given tosecond input of comparator. The output signal of comparator is given to IC 4013 & 4528 in sampling generator

section to produce 1 and 1 signals. The output of comparator is also given to Multiplexer. The Multiplexer

produces PPM signal with the help of 1 and 1 signals.

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(9) Pulse Position Demodulator section: -

This section is based on comparator, devider and low pass filter.

The PPM signal is given to one input of comparator. The 1-volt DC reference signal is given to second input of

comparator. This 1 volt reference signal is generated by IC 741 by setting 4K7 preset. The comparator generate

SAW tooth signal at its output. The frequency of this signal is then divided by two by IC 4013.

Then the output of divider is given to Low pass filter made of two 741 ICs. The Low pass filter passes only

low frequencies up to 3.4 KHz and reduces all other frequencies. Thus this removes high frequency quantizationnoise of PPM signal. By removing high frequency we recover original modulating signal.

(10) Output Audio Amplifier Section: -

This section is same as input audio amplifier except pre-amplifier section of transistor.

(11) Power supply section:-

The regulated power supply is used for different supply voltages.Following output D.C. Voltages are required to operate this trainer.

+15V, 250mA,

-15V, 250mA,

+ 5V, 250mA+ 5V, 250mA

Three terminal regulators are used for different output voltages i.e. IC 7805 for + 5V, IC 7815 for +15V, IC

7915 for 15V, 7905 for 5V

These ICs are supplied different dc input voltages by two Bridge rectifiers consisting of D1-D4 and D5-D8 &

two 1000/25 EC and 1000/10 EC. The capacitors at each input & each output are for filtering purpose.

**************

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EXPERIMENT PROCEDURE: -

Exp. 1: - Pulse Amplitude Modulation / Demodulation

(See connection diagram CN1)

1. Connect links as per connection diagram CN1.

2. Keep Frequency pot of Audio generator in mid position i.e. apply 1KHz sine wave signalConnect CRO channel 1 at Sine terminals. Adjust amplitude of sine wave to 1 Vpp.

_________Waveform (W1)3. Keep Frequency pot of Sampling pulse generator in mid position.

_________Waveform (W2)

4. Connect CRO Channel 2 at PAM O/P signal and observe PAM signal _________Waveform (W3)

5. Connect CRO Channel 2 at demodulated output of Low pass filter section.

Observe recovered sine wave signal. _________Wavefor m (W4)

6. Now vary amplitude of sine wave modulating signal and observe its effect on PAM output as well as

on recovered signal.

7. Vary frequency of sine wave modulating signal and observe its effect on PAM output as well as on

recovered signal.

8. Vary Pulse frequency of Sampling pulse and see the effect on PAM output as well as on recoveredsignal.

Exp. 2: - Voice signal Pulse Amplitude Modulation / Demodulation(See connection diagram CN2)

9. Connect output of Low pass filter to the input terminals of output audio amplifier section.

10. Here audio tone from LS. Adjust volume control as required.

11. Now disconnect signal input terminal of PAM modulator from sine wave signal and connect it to audio

output signal of Input audio amplifier.

12. Connect mike to mike- socket.

13. Keep volume controls of input audio amplifier and output audio amplifier in mid position.

14. Speak to mike and hear recovered audio in LS of output audio amplifier.

15. To verify Nyquist's Sampling Theorem keep modulating sine wave frequency to 2 KHz & amplitude 2Vpp.

Keep Sampling pulse width pot in mid position. Now vary sampling frequency slowly from 0 KHz to 2 KHz

by observing original signal and recovered demodulated signal. Measure the sampling frequency for which

original signal and recovered demodulated signal are nearly same i.e error is less. It will be more than4 KHz, which proves Nyqust's Sampling Theorem.

Conclusion: -

The error in recovered signal:increases with increase in signal amplitude,

increases with increase in signal frequency,

decreases with increase in sampling pulse frequency.increases with decrease in sampling pulse width.

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Exp. 3 - Pulse Width Modulation / Demodulation(See connection diagram CN3)




3. Keep Frequency pot of Sampling pulse generator in mid position.

________Waveform (W2a)

4. Connect CRO Channel 2 at PWM O/P signal and observe PWM signal _________Wavefor m (W5)



6. Now vary amplitude of sine wave modulating signal and observe its effect on PWM output as well ason recovered signal.

7. Vary frequency of sine wave modulating signal and observe its effect on PWM output as well as onrecovered signal.

8. Vary Pulse frequency of Sampling pulse and see the effect on PWM output as well as on recovered

signal.

Conclusion:-

The error in recovered signal:

increases with increase in signal amplitude,


decreases with increase in sampling pulse frequency.

increases with decrease in sampling pulse width.

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Exp. 4 - Pulse Position Modulation / Demodulation(See connection diagram CN4)




3. Keep Frequency pot of Sampling pulse generator in mid position.

________Waveform (W2a)

4. Connect CRO Channel 2 at PPM O/P signal and observe PPM signal _________Wavefor m (W7)



6. Now vary amplitude of sine wave modulating signal and observe its effect on PPM output as well ason recovered signal.

7. Vary frequency of sine wave modulating signal and observe its effect on PPM output as well as onrecovered signal.

8. Vary Pulse frequency of Sampling pulse and see the effect on PPM output as well as on recovered

signal.

Conclusion:-

The error in recovered signal:

increases with increase in signal amplitude,


decreases with increase in sampling pulse frequency.increases with decrease in sampling pulse width.

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TEST POINT WAVEFORMS

W1. Modulating Sinewave Signal :- (at sine wave connector)

+1 V

-1V

W2. Sampling Pulse signal: - (at Sampling Pulse generator O/P)

10KHz 4V-TTL

100us

W3. PAM modulated output signal: - (at Sampling Circuit section O/P)

+1 V

-1V

At NT position of switch

At S/H position of switch

H = 1 msV = 1 VPP

Trig = CRO-1

H = 70 usV = 4 VPPTrig = CRO-1

H = 0.5 msV = 1.0 V

Trig = CRO-2

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W4. PAM demodulated output: - (at o/p of LPF section)

+1 V

-1 V

W2a. Sampling Pulse signal (SAW tooth signal): -

2VPP

100us

W5. PWM modulated output signal :- (at Sampling Circuit section o/p)

4V-TTL

W6. PWM demodulated output :- (at o/p of Demodulator section)

+1 V

-1V

H = 40 us

V = 4.8 VPPTrig = CRO-1

H = 20 usV = 2.0 V

Trig = CRO-2

H = 0.5 ms

V = 1.0 V

Trig = CRO-1

H = 1 ms

V = 1.0 VPP

Trig = CRO-1

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W7. PPM modulated output signal :- (at Sampling Circuit section o/p)

4V-TTL

W8. PPM demodulated output :- (at o/p of Demodulator section)

100 mv

-100 mv

**************

H = 20 usV = 2 V

Trig = CRO-1

H = 0.5 msV = 100mv

Trig = CRO-2

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Pulse Amplitude Modulation / Demodulation

Connection Diagram CN-1

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Voice Signal Pulse Amplitude Modulation / Demodulation


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Pulse Width Modulation / Demodulation


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Pulse Position Modulation / Demodulation


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