16-17-Com411-com412

8/3/2019 16-17-Com411-com412

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www.sigmatrainers.com

AM DSB/SSB TRANSMITTER

& RECEIVER TRAINER

MODEL-COM411-COM412

Since

23

Years

TRAINERS

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INTRODUCTION

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

Modulation techniques on a SINGLE P.C.B.

SPECIFICATIONS (COM411)

1. Builtin Sine wave Generator : With ' adjustable Amplitude & Frequency (300 Hz - 3.4 KHz)

2 Modulators : 3 Nos Balanced Modulators

3. Carrier Frequency : 455KHz & 1 MHz

4. Audio Output : Amplifier with speaker

5. Transmitter Output : Amplifier (Gain adjustabl e )

6. Standard Accessories : 1. A Training Manual

2. Connecting Patch cords.

SPECIFICATIONS (COM412)

1. Construction : Super heterodyne

2. Frequency Range : 520 KHz to 1620 KHz

3. Intermediate Freq. (IF) : 455 KHz

4. Input Circuits : 1. RF Amplifier,

2. Mixer

3. Local Oscillator

4. Beat Frequency Oscillator

5. IF Amplifier 1

6. 1F amplifier 2

5. Tuning : With Variable Capacitor (ganged)

6. Receiving media : Telescopic Antenna

7. Detector Circuits : 1) Diode Detector

2) Product Detector 8. Audio Output : Amplifier with speaker

9. Standard Accessories : 1. A Training Manual

2. Connecting Patch cords

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CIRCUIT DESCRIPTION OF AM DSB/SSB RECEIVER TRAINER

The AM Demodulation System consists of following sections.

1. Local Oscillator and Mixer (Frequency Converter)

2. 1 st IF amplifier

3. 2nd IF Amplifier

4. Diode detector.

5. AGC6. Product detector

7. Beat Frequency Local Oscillator

8. Power supply.

(1) FREQUENCY CONVERTER STAGE. This stage consists of following circuits.

(a) Antenna Circuit:

The signal picked by the ferrite rod antenna is tuned by the gang capacitor, which gets connected across

the primary of Antenna coil L1. L1 acts as autotransformer and signal across the secondary of L1 is fed to the

base of the converter transistor Q1 (BF194) through C1 (10n). C22 is the trimmer across L1 for tuning of L1-C22tank circuit.

(b) Oscillator Circuit:

Gang Capacitor gets connected across the primary of L4 to form the oscillator tuned circuit and capacitor

C20 (470pf). C2 is the oscillator trimmer. The secondary of L4 gets connected to the collector of Q1. The part of

primary of L4 feed oscillations into the emitter circuit through C2 (10n), coupling capacitor.

(c) Converter transistor:

Q1 (BF 194) is a self-oscillating mixer. R3 (470E), C3 (47n) is decoupling filter circuit. C3 (47n)

bypasses high frequency noise from +6 supply. R1 (390K) is a base-biasing resistor for Q1. The mean current is

limited by R2 (1K). The output of the converter is available across the secondary of L7 (IFT1), the primary of

which is tuned to IF frequency (455KHz) by capacitor C17 (2n7). This output is given to the base of Q2.

IF AMPLIFIER STAGES

This circuit incorporates two-stage amplifier to provide sufficient gain.

(2) 1st IF amplifier:

Q2 (BF 195C) acts as 1st IF amplifier. The base of Q2 is connected through R5 (68K) to the detector output. R6 (100E) and C4 (47n) is decoupling filter for +6 supply. The base potential depends on R4 (220K) base

biasing resistor and the detector current supplied by R5. The detector current is proportional to the signal strengthreceived. This controls the bias of Q2 (BF 195C) automatically to the signal received. This is called A.G.C. C6

(4.7/16) is used as base bias and AGC decoupling capacitor. The output of Q2 is available across the secondary of L8 (IFT2), the primary of which is tuned to IF by the Capacitor C18 (2n7). This output is given to the base of Q3

(BF 195D).

(3) 2nd IF amplifier:

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to AGC circuit made of R5 (68K) and C6 (4.7/16).

(5) AGC

The sound received from the LS will depend on the strength of the signals received at the antenna. Thestrength of the received signals can vary widely due to fading. This will cause variations in sound which can be

annoying. Moreover, the strength of signals can also be too large in close vicinity of MW transmitters causing

overloading of the 2nd IF amplifier.

Automatic gain control (AGC) is used to minimize the variations in sound with changes in signal strengthand to prevent overloading. The operation of AGC depends on the fact that the gain obtained from any transistor

depends on its collector current and becomes less when the collector current is reduced to cut off (or increased to

saturation).For AGC, DC Voltage obtained from the detection of IF signals is applied to the 1st amplifier transistor

base in such a way that an increase in this voltages reduces the gain of the transistor. The result is that when the

strength of the incoming signal increases, the DC voltage also increase and this tends to reduce the gain of the

amplifier thus not permitting the output to change much. Here R5 (68K) and C6 (4.7/16) performs this function.C6 (4.7/16) is the AGC decoupling capacitor to by pass any AF signals and keep the bias steady.

(6). PRODUCT DETECTOR : -

This section is similar to Balanced Modulator section. The difference is only that input pin 8 is given RFcarrier oscillator signal from RF carrier oscillator and pin 1 is given AM modulated signal from 2 nd IF amplifier.

The output is product of these two signals which contains basic audio modulating signal, which can be filtered by

low pass filter made of R-C (2K preset and 4n7 capacitor) circuit.

(7). BEAT FREQUENCY LOCAL OSCILLATOR: -The Local oscillator is required at demodulator side for Product detector because in DSB-SC signal, carrier is

suppressed and hence carrier is required to be reinserted at demodulator side.

This oscillator is made of single transistor & tuned coil based on Hartley oscillator. It generates RF frequencysignal from 400KHz to 500 KHz. The output frequency can be varied by tuning IFT coil.

(8) Power supply section: -

The regulated power supply is used for different supply voltages.

Following output D.C. Voltages are required to operate AM Modulation demodulation system.

+12V, 250mA,

-12V, 250mA,

+ 6V, 250mA

Three terminal regulators are used for different output voltages i.e. IC 7806 for + 5V, IC 7815 for +15V, IC7915 for -15V,

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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BLOCK DIAGRAM OF AMPLITUDE MODULATION & DEMODULATION

Modulating Balanced Diode

Audio Signal Modulator Amplitude Detector Detected

Modulated Signal Output

Variable-RCAudio Time ConstantSignal

RF Carrier

Modulator Demodulator

BLOCK DIAGRAM OF DSB-SC AMPLITUDE MODULATION & DEMODULATION

Modulating BalancedAudio Signal Modulator (without carrier insertion) Detected

Output

DSB-SC Amplitude

Modulated Signal

Audio

Signal

RF Carrier


AF

RF

Product

Detector AF

RF

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BLOCK DIAGRAM OF SSB-SC AMPLITUDE MODULATION & DEMODULATION

Modulating Balanced

Audio Signal Modulator (without carrier insertion) Detected

Output

SSB-SC Amplitude

Modulated Signal

Audio

Signal

RF Carrier


Product

Detector AF

RF

Band passFilter

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EXPERIMENTS

AMPLITUDE MODULATION & DEMODULATION EXPERIMENTS

EXP.1. AM TRANSMISSION AND RECEPTION:

(See connection diagram CN1)

1. Connect links as shown in connection diagram CN1.

2. Connect CRO channel-1 at Sine O/P terminal. Adjust amplitude of sine wave to 1Vpp and audio

frequency to 1 KHz. _______________________________________________________Waveform (W1)

3. Connect CRO channel-2 at 1MHz o/p of 1MHz Oscillator. Connect this carrier at RF carrier input of

balanced maodulator-1. _________________________________________________Waveform (W2)

4. Now Connect CRO Channel 2 at o/p terminal of balanced modulator-1. Trigger CRO by channel-1 . The

Amplitude Modulated wave will be observed. _________________________________Waveform (W3)

5. Connect Amplitude modulated output of Transmitter trainer board to Mixer input of Receiver trainer

board. Connect ground link between two boards.

6. Vary tuning frequency in receiver by tuning control. Tune OSC O/P frequency near 1455KHz and

observe demodulator sine wave at DET O/P terminal of Diode Detector. If required vary gain of output

amplifier in transmitter board. ______________________________________________Waveform (W4)

7. Vary the input Oscillator Frequency and Amplitude. Observe Diagonal clipping and negative clipping.

8. Connect DET O/P of diode detector to the I/ P input terminal of output audio amplifier. Connect

Loudspeaker to LS output RCA socket of Output Audio Amplifier.

9. Here audio tone from LS. Adjust volume control as required. Change frequency of Audio Oscillator and

here different tones.

10. Now remove both links between Transmitter and Receiver. Observe received signal from air

transmission. If required change gain of power amplifier of transmitter and tuning of receiver.

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EXP. 2. DSB TRANSMISSION AND RECEPTION:



2. Connect CRO channel-1 at Sine O/P terminal.

Adjust amplitude of sine wave to 1Vpp and audio frequency to 1 KHz. _________Waveform (W5)

3. Connect CRO channel-2 at 1MHz o/p of 1MHz Oscillator.

Connect this carrier at RF carrier input of balanced maodulator-2. _____________Waveform (W6)

4. Now Connect CRO Channel 2 at o/p terminal of balanced modulator-2.

Trigger CRO by channel-1.

The DSB-Amplitude Modulated wave will be observed. ______________________Waveform (W7)

5. Connect Amplitude modulated output of Transmitter trainer board to Product Detector input of Receiver

trainer board. Connect ground link between two boards.

6. Observe demodulator sine wave at O/P terminal of Product Detector. If required vary gain of output

amplifier in transmitter board. ___________________________________________Waveform (W8)

7. Vary the input Oscillator Frequency and Amplitude. Observe its effect on output.

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EXP. 3. SSB TRANSMISSION AND RECEPTION:



2. Connect CRO channel-1 at Sine O/P terminal.

Adjust amplitude of sine wave to 1Vpp and audio frequency to 1 KHz. ____Waveform (W9)

3. Connect CRO channel-2 at 455KHz o/p of 455KHz Oscillator.

Connect this carrier at RF carrier input of balanced maodulator-3. ______Waveform (W10)

4. Now Connect CRO Channel 2 at o/p terminal of bandpass filter.

Trigger CRO by channel-1.

The SSB-Amplitude Modulated wave will be observed. _______Wa veform (W11)

5. Connect Amplitude modulated output of Transmitter trainer board to Product Detector input of Receiver

trainer board. Connect ground link between two boards.

6. Observe demodula tor sine wave at O/P terminal of Product Detector. If required vary gain of output amplifier

in transmitter board. --------------------------------------------------------------------Waveform (W12)

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TEST POINT WAVEFORMS (AM)

W1. Modulating Sinewave signal: - (at Sine O/P terminal of Audio Oscillator) : 1 KHz, 1Vpp-

+ 0.5V

-0.5V

1ms

W2. R. F. Carrier Signal: - (at RF O/P terminal of RF Oscillator) fc = 1MHz, 4Vpp

+2 V

-2 V 1us

W3. AM modulated Signal: - (at AM MOD O/P terminal of Balanced Modulator)

Vmax

+1V

-1V

Vmin

H = 1 msV = 1 V

Trig = CRO-1

H = 1usV = 1V

Trig = CRO-1

H = 0.5 msV = 1.0 V

Trig = CRO-1

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W4. AM Demodulated signal:- (at DET O/P terminal of Diode Detector):-

+0.1V

-0.1V

1ms

Over modulated AM modulated signal :- (at AM MOD O/P terminal of Balanced Modulator)

fm=1Khz, 5Vpp

Conclusion: This completes experiment of simple Amplitude modulation and demodulation.

H = 1 ms

V = 0.1 V

Trig = CRO-2

H = 0.5 ms

V = 2.0 V

Trig = CRO-2

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TEST POINT WAVEFORMS – DSB-SC

W5. Modulating Sinewave signal: - (at Sine O/P terminal of Audio Oscillator) fm=1KHz, 1Vpp

+0.5V

-0.5V1ms

W6. R. F. Carrier Signal: - (at RF O/P terminal of RF Oscillator) fc = 1MHz, 4Vpp

+2 V

-2 V 1us

W7. DSB-SC Modulated signal: - (at AM MOD O/P terminal of Balanced Modulator)

I/ P +0.5V

fc -0.5V

H = 1 ms

V = 0.5 V

Trig = CRO-2

H = 1 ms

V = 1 V

Trig = CRO-1

H = 1us

V = 1VTrig = CRO-1

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W8. Demodulated DSB-SC Signal: - (at O/P terminal of Product detector)

+0.1V

-0.1V

Conclusion: This completes experiment of DSB-SC Amplitude modulation and demodulation

H = 1 ms

V = 0.1V

Trig = CRO-2

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TEST POINT WAVEFORMS – SSB-SC

W9. Modulating Sinewave signal: - (at Sine O/P terminal of Audio Oscillator) fm=1Khz, 1Vpp

+0.5V

-0.5V

1ms

W10. R. F. Carrier Signal: - (at RF O/P terminal of RF Oscillator) fc = 455KHz, 4Vpp

+2 V

-2 V 2us

W11. SSB-SC Modulated signal: -

+0.5V

-0.50V

H = 1 msV = 0.5V

Trig = CRO-2

H = 1 msV = 1 V

Trig = CRO-1

H = 1usV = 1V

Trig = CRO-1

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W12. Demodulated SSB-SC Signal: - (at DEMD O/P terminal of Product detector)

+0.1V

-0.1V

Conclusion: This completes experiment of SSB-SC Amplitude modulation and demodulation.

H = 1 ms

V = 0.1 V

Trig = CRO-2

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CN-1: AM TRANSMISSION AND RECEPTION:

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CN-2: DSB/SC TRANSMISSION AND RECEPTION:

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CN-3 SSB/SC TRANSMISSION AND RECEPTION:

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