09-com104

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SIGMA TRAINERS

AHMEDABAD (INDIA)

FREQUENCY MODULATION/

DEMODULATION SYSTEM

TRAINER

MODEL-COM104

More

than2000

Trainers

Since

23

Years

TRAINERS

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INTRODUCTION

This trainer has been designed with a view to provide practical and experimental Knowledge of Frequency

Modulation/Demodulation technique as practically implemented in Analog Communication systems on a SINGLEP.C.B of size 24 "x 15".

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

2. Built in IC based power supply.

3. On Board AF Modulating signal generator - Sine waveFrequency Range : 300 Hz to 3.4 KHz

Amplitude : 0 to 5 Vpp.

4. VCO Frequency Range : 50 KHz to 150KHz.

5. PLL Frequency Range : 50KHz to 150KHz

6. Modulator Type : VCO (Voltage controlled oscillator) type.7. Demodulator Type : - Ratio detector.

- Foster-Seeley detector.

- Phase lock loop (PLL) detector.8. All parts are soldered on single pin TAGS on single PCB of size 12" x9" with complete circuit diagram

Screen-printed.

9. Standard Accessories : 1. A Training Manual.

2. Connecting Patch cords.

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CIRCIUT DETAILS FREQUENCY MODULATION & DEMODULATION

The FM Modulation and Demodulation System consist of following sections.

1. Modulating Audio Signal Generator 2. VCO FM Modulator section

3. Varactor FM Modulator section4. FM demodulator consisting of

a. FM demodulation by PLL

b. FM demodulation by Foster-sally circuit

c. FM demodulation by Ratio Detector

5. Power supply.

(1) Modulating Audio Signal Generator section:

IC 1 (ICL 8038) waveform generator IC is used generates sine wave siganl. Pot P2 (22K) is used to vary its

frequency. The frequency range is 300Hz to 3.4Khz. Two 100K presets are adjusted for proper peaks of sine

wave signal. 1K presets is used to adjust duty cycle. The sine wave output signal is available at pin 2 of IC1 andit is then amplified by IC4 (LM356). The amplified output is available Sine wave output terminal. Pot P2 is usedto vary the amplitude of Sine wave signal. The switch S1 is used for course variation of frequency. The output

amplitude vary from 0 to 5Vpp.

(2). FM modulator using VCO:

A voltage-controlled oscillator (VCO) produces the frequency, which is proportional to the voltages applied to

it. IC 8038 is used here as VCO. The frequency of oscillation is determined by resistance and capacitance at pin

11-10 and by a control voltage V at pin 8. 356 IC is used as amplifier.

The basic free running frequency can be set between 100KHz to 200KHz by varying 10K Pot and preset 22K.

This frequency is FM modulated by applying modulating signal at Modulating I/P terminal. Here two seperate

FM modulators are provided.

(3). FM demodulators:

(a) FM demodulation by PLL: -

An IC 565 PLL IC is used here for FM demodulation. It contains voltage-controlled oscillator (VCO) which

Produces the frequency, which is proportional to the voltages applied to it. The frequency of oscillation isdetermined by resistance and capacitance at pin 8 and 9 and by a 4K7 preset. 308 Ic is used as amplifier.

The basic free running frequency can be set between 200KHz to 1MHz by varying 4K7 preset.

(b). FM Demodulator using Ratio Detector:-

In this section two diodes OA79, two resistors (100K), two capacitors (10nPPC) and coils L1 and L2 areconnected as ratio detector circuit. The FM modulated signal form FM modulator is applied to the base of

transistor Q1 through coupling capacitor (0.1uDC). This signal is amplified and amplified output is then FMdemodulated by ratio detector circuit. The demodulated output available at junction of Capacitors is then filtered

by resistor 4K7 and capacitor 0.47 PPC. The recovered signal is available at DEMOD O/P terminal.

(c). FM Demodulator using Foster Seeley Detector:-In this section two diodes OA79, two resistors (100K), two capacitors (10nPPC) and coils L1 and L2 are

connected as Foster Seeley Detector circuit. The FM modulated signal form FM modulator is applied to the baseof transistor Q1 through coupling capacitor (0.1uDC). This signal is amplified and amplified output is then FM

demodulated by Foster seeley detector circuit. The demodulated output available at junction of upper resistor and

capacitor is then filtered by resistor 4K7 and capacitor 0.47 PPC. The recovered signal is available at DEMOD

O/P terminal. (a) FM demodulation by PLL: -

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An IC 565 PLL IC is used here for FM demodulation. It contains voltage-controlled oscillator (VCO) which

Produces the frequency, which is proportional to the voltages applied to it. The frequency of oscillation isdetermined by resistance and capacitance at pin 8 and 9 and by a 4K7 preset. 308 Ic is used as amplifier.

The basic free running frequency can be set between 200KHz to 1MHz by varying 4K7 preset.

(4). Low pass filter: -

This section passes only low frequencies up to 3.4 KHz & reduces all other frequencies. This filter is requiredto remove RF carrier signal components from demodulated signal. This section is based on IC 741 & R-C circuits

(5) Power supply section:

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

to operate FM Modulation demodulation system.

+15V, 250mA ,- 15V, 250mA ,

+ 5V, 250mA

Three terminal regulators are used for different output voltages i.e. IC 7805 for + 5V, IC 7815 for +15V, IC7915 for -15V,

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

and C1, C2, C3, C4. The capacitors at each input and each output are for filtering purpose. SW1 is main ACON/OFF Switch.

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

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EXPERIMENTS

Exp. (1). To generate FM signal using VCO Modulator: -

1. Connect CRO channel-1 at O/P of VCO Modulator-1. Vary Carrier frequency Pot of VCO modulator to produce 100KHz frequency carrier signal.

_______Waveform (T2)

2. Now connect Sine wave output of Audio Generator to VCO Modulator input terminals.

Connect CRO channel-1 at Sine wave output signal. Adjust amplitude of

sine wave to 1 Vpp and audio frequency to 1 KHz. _______Waveform (T1)

3. Connect CRO Channel-2 FM output of VCO FM modulator section. Trigger CRO by channel-1. The

Frequency Modulated wave will be observed.


Exp. (2). To demodulate FM signal using Phase lock loop (PLL) detector: -

1. Connect the FM modulated output of VCO FM modulator to input of PLL detector.

2. Connect DEMOD O/P of PLL demodulator to input of Low pass filter. Observe O/P of Low pass filter.This demodulated sine wave.


Exp. (3). To demodulate FM signal using Ratio detector:-

3. Connect the FM modulated output of VCO FM modulator to input of Ratio detector.

4. Connect DEMOD O/P of Ratio detector to input of Low pass filter. Observe O/P of Low pass filter.

This demodulated sine wave. Also tune gang condenser C5 for proper output signal.


Exp. (4). To demodulate FM signal using Foster-Seeley detector: -

5. Connect the FM modulated output of VCO FM modulator to input of Foster-Seeley detector.

6. Connect DEMOD O/P of Foster-Seeley detector to input of Low pass filter. Observe O/P of Low pass

filter. This demodulated sine wave. Also tune gang condenser C6 for proper output signal.


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TEST POINT WAVEFORMS

T1. Modulating sinewave signal: (at sine wave connector)

1ms

0.5

0.5

1

V P P

T2. R.F. Carrier Signal: (at F.M. modulator o/p) connector

2VPP

0.01ms

T3. FM modulated output signal: at FM modulator o/p connector)

T4. FM demodulated output: (at o/p of PLL detector)

0.5V

1 V P P

H = 1.0 us

V = 2.0 V

Trig = CRO-2

O/P = 1 VPP

Freq. = 1 KHz

I/P = 1 Volt P.P

Freq. = 1 KHz

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T5. FM demodulated output: (at o/p of Ratio detector)

0.5V

1KHz -0.5V

T6. FM demodulated output: (at o/p of Foster-Seely detector)

0.5V

-0.5V

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

H = 0.5 msV = 0.5 V

Trig = CRO-2

H = 0.5 ms

V = 0.5 V

Trig = CRO-1

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09-com104

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