Analog Communications Lab Manual_asrao

Analog Communications Lab Manual

Dept. of ECE Balaji Institute of Engineering & Sciences

DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING

LABORATORY MANUAL

ANALOG COMMUNICATIONS (III B.Tech., - I Sem.)

Prepared by

A.SANYASI RAO , Assoc. Prof.

S.SRINIVAS , Asst. Prof

BALAJI INSTITUTE OF ENGINEERING & SCIENCE S Laknepally, Narsampet, Warangal



LIST OF EXPERIMENTS

1. Amplitude modulation and demodulation.

2. DSB-SC Modulator & Detector.

3. SSB-SC Modulator & Detector (Phase shift method).

4. Frequency modulation and demodulation.

5. Study of spectrum analyzer and analysis of AM & FM signals.

6. Pre emphasis & De emphasis.

7. Time division multiplexing & de-multiplexing.

8. Frequency division multiplexing & de-multiplexing.

9. Verification of sampling theorem.

10. Pulse Amplitude Modulation & De-modulation.

11. Pulse Width Modulation & De-modulation.

12. Pulse Position Modulation & De-modulation.

13. Frequency Synthesizer.

14. AGC Characteristics.

15. PLL as FM Demodulator.



LIST OF EXPERIMENTS

Cycle1

1. Amplitude modulation and demodulation.

2. DSB-SC Modulator & Detector.

3. SSB-SC Modulator & Detector (Phase shift method).

4. Frequency modulation and demodulation.

5. Pre emphasis & De emphasis.

6. Frequency Synthesizer.

Cycle2

7. Time division multiplexing & de-multiplexing.

8. Verification of sampling theorem.

9. Pulse Amplitude Modulation & De-modulation.

10. Pulse Width Modulation & De-modulation.

11. Pulse Position Modulation & De-modulation.

12. PLL as FM Demodulator.



Analog Communications

Hardware

Experiments



1. AMPLITUDE MODULATION AND DEMODULATION

Objective

1. To observe the process of amplitude modulation and demodulation and calculate depth of

modulation.

2. To study the process of over modulation.

Equipment

1. Amplitude modulation and demodulation trainer kit

2. CRO with probes

3. Patch cords

THEORY:

Amplitude Modulation is defined as a process in which the amplitude of the carrier

wave c(t) is varied linearly with the instantaneous amplitude of the message signal m(t).The

standard form of an amplitude modulated (AM) wave is defined by

Where Ka is constant called the amplitude sensitivity of the modulator. The demodulation

circuit is used to recover the message signal from the incoming AM wave at the receiver. An

envelope detector is a simple and yet highly effective device that is well suited for the

demodulation of AM wave, for which the percentage modulation is less than 100%.Ideally, an

envelope detector produces an output signal that follows the envelop of the input signal wave

form exactly; hence, the name. Some version of this circuit is used in almost all commercial

AM radio receivers. The Modulation Index is defined as m = (Vmax + Vmin) / (Vmax

Vmin) Where Vmax and Vmin are the maximum and minimum amplitudes of the modulated

wave.

Block Diagram



Procedure

Modulation

1. Switch on the trainer kit.

2. Measure the output voltages of regulated power supply +12V, -12V.

3. Observe outputs of RF and AF signal generators using CRO. Set RF voltage

approximately 10Vpp of 100 KHz frequency and AF voltage as 300 mVpp of 2KHz

frequency.

4. Now connect AF and RF signals to the respective inputs of modulator as shown in figure.

Initially set both the signals at zero level.

5. Observe both modulating and modulated signals simultaneously in CRO.

6. Adjust RF signal amplitude to have modulator output at 10 Vpp by keeping AF signal at

zero level.

7. Now vary the amplitude of AF signal and observe the minimum and maximum voltages

of the modulated signal for different values of modulating voltages and calculate the

percentage of modulation.

minmax

minmax

VV

VVIndexModulation

8. Observe 100% modulation and over modulation by varying amplitude of AF signal.

Demodulation

1. Now connect modulator output to demodulator input pin.

2. Observe demodulated signal at output of demodulator using oscilloscope.

3. Compare it with the original AF signal.



Calculations & Observations

minmax

minmax

VV

VVIndexModulation

Modulati ng Signal Generator

Amplitude = Time Period = Frequency =

Carrier Signal Generator:


Demodulated Output:


Tabular Form:

Modulating

signal

amplitude (V)

Vmax Vmin

Modulation index

minmax

minmax

VV

VVma



Model Wave Forms

Precautions 1. Avoid loose connections.

2. Avoid parallax error while taking observations.

Result



2. DSB SC MODULAT OR AND DETECTOR

Objective

1. To observe the generation and detection processes of DSB-SC signal using Balanced

Modulator.

2. To observe the modulated and demodulated outputs.

Equipment

1. Balanced modulator and demodulator trainer kit

2. CRO with probes

3. Patch cards

Theory

Balanced modulator is used for generating DSB-SC signal. A balanced modulator

consists of two standard amplitude modulators arranged in a balanced configuration so as to

suppress the carrier wave. The two modulators are identical except the reversal of sign of the

modulating signal applied to them.

1. RF Generator:

Colpitts oscillator using FET is used here to generate RF signal of approximately 100

KHz Frequency to use as carrier signal in this experiment. Adjustments for Amplitude and

Frequency are provided in panel for ease of operation.

2. AF Generator:

Low Frequency signal of approximately 5 KHz is generated using OP-AMP based

Wein Bridge Oscillator. IC TL 084 is used as an active component; TL 084 is FET input

general purpose quad OP-AMP integrated circuit. One of the OP-AMP has been used as

amplifier to improve signal level. Facility is provided to change output voltage.

3. Regulated Power Supply:

This consists of bridge rectifier, capacitor filters and three terminal regulators to

provide required dc Voltage in the circuit i.e. +12v, -8v @ 150 ma each.

4. Modulator:

The IC MC 1496 is used as Modulator in this experiment. MC 1496 is a monolithic

integrated circuit Balanced modulator/Demodulator, is versatile and can be used up to 200

MHz.



5. Multiplier:

A balanced modulator is essentially a multiplier. The output of the MC 1496 balanced

modulator is proportional to the product of the two input signals. If you apply the same

sinusoidal signal to both inputs of a ballooned modulator, the output will be the square of the

input signal AM-DSB/SC: If you use two sinusoidal signals with deferent frequencies at the

two inputs of a balanced modulator (multiplier) you can produce AMDSB/SC modulation.

This is generally accomplished using a high- and a lower

m (such as an audio signal from microphone).

Circuit Diagram

Procedure

1. Switch on the trainer kit.

2. Measure the output voltages of regulated power supply +12V, -12V & -8V.

3. Observe the output of RF generator using CRO. Set carrier wave in RF generator to 100

KHz frequency and 300 mV peak to peak.

4. Observe the output of AF generator using CRO. Set modulating wave in AF generator to

5 KHz frequency and 100 mV peak to peak.



5. Connect the respective AF and RF outputs to the Balanced modulator input pins and

observe the DSB-SC output. (Here you can clearly observe the phase reversal at zero

crossings).

6. Connect the Balanced modulator output and RF generator output to the Synchronous

detector and observe the demodulator output.

7. Compare demodulated output with original AF signal.

Observations

Message Signal:


Carrier Signal :


Demodulated Signal:

Amplitude = Time Period = Frequency

Model Wave Forms



Precautions

1. Avoid loose connections

2. Avoid parallax error while taking observations.

Result



3. SSB SC MODULATOR AND DETECTOR

Aim

To observe process of single side band signal generation using phase shift method and to

demodulate the same using synchronous detector.

Equipment

1. SSB Trainer kit

2. CRO with probes

3. Frequency counter

4. Patch cords

Theory

An SSB signal is produced by passing the DSB signal through a highly selective band

pass filter. This filter selects either the upper or the lower sideband. Hence transmission

bandwidth can be cut by half if one sideband is entirely suppressed. This leads to single side

band modulation (SSB). In SSB modulation bandwidth saving is accompanied by a

considerable increase in equipment complexity.

Single Sideband Suppressed Carrier (SSB-SC) modulation was the basis for all long

distance telephone communications up until the last decade. It was called "L carrier." It

consisted of groups of telephone conversations modulated on upper and/or lower sidebands of

contiguous suppressed carriers. The groupings and sideband orientations (USB, LSB)

supported hundreds and thousands of individual telephone conversations.

Due to the nature of-SSB, in order to properly recover the fidelity of the original

audio, a pilot carrier was distributed to all locations (from a single very stable frequency

source), such that, the phase relationship of the demodulated (product detection) audio to the

original modulated audio was maintained.

Also, SSB was used by the U.S. Air force's Strategic Air Command (SAC) to insure

reliable communications between their nuclear bombers and NORAD. In fact, before satellite

communications SSB-was the only reliable form of communications with the bombers.

The main reason-SSB-is superior to-AM,-and most other forms of modulation are:



(1) Since the carrier is not transmitted in SSB, there is a reduction by 50% of the transmitted

power. In AM out of 100% modulation: 67% of the power is comprised of the carrier; with

the remaining 33% power in both sidebands.

(2) Because in SSB, only one sideband is transmitted, there is a further reduction by 50% in

transmitted power.

(3) Finally, because only one sideband is received, the receiver's needed bandwidth is reduced

by one half--thus effectively reducing the required power by the transmitter another 50%

Block Diagram:

Procedure

Modulation:

1. Study the circuit operation of SSB system thoroughly.

2. Observe the Output of the RF generator using CRO. There are 2 outputs from the RF

generator one is direct output and another is 90 phase shift of the direct output. Adjust the

RF signal frequency to 100 KHz and amplitude to 0.2 Vp-p (potentiometer is provided to

vary the output amplitude).

3. Observe the output of the AF generator using CRO. There are 2 outputs from the AF

generator. One is direct output and another is 90 phase shift to the direct output. A switch

is provided to adjust the required frequency of any one of 2 KHz, 4 KHz or 6 KHz. Set the



AF generator frequency to 2 KHz and amplitude of 10VP-P. AGC Potentiometer is provided

to adjust the gain of the oscillator (to set the output to good shape).

4. Connect the RF generator direct output (0o) and AF generator indirect (90) output to the

balanced modulator (A) and similarly RF generator indirect output (90) and AF generator

direct output (0) to the balance modulator (B).

5. Observe the outputs of both the balanced modulators simultaneously using dual trace

oscilloscope and adjust the balance control until you get the output waveforms properly.

6. To get SSB Lower Side Band (LSB) signal, connect the two balanced modulators outputs

to subtractor.

7. Measure and record the LSB signal frequency using frequency counter.

8. Calculate theoretical frequency of LSB and compare it with the practical value, LSB = RF

- AF

9. To get SSB Upper Side Band (USB) signal, connect the both outputs of balanced

modulators to the summer.

10. Measure and record the USB signal frequency using frequency counter.

11. Calculate the theoretical value of USB frequency and compare it with the practical value,

USB = RF + AF

Demodulation

1. Connect SSB-SC signal from the summer or subtractor to the SSB signal input pin of the

demodulator. Also connect RF direct output to the RF input pin of the demodulator.

2. Observe the demodulator output using CRO.

3. The output has to be the replica of the modulating signal. Compare the demodulated output

with the modulating signal.

Calculations & Observations

a. Theoretical frequency of LSB = RF AF

b. Theoretical frequency of USB = RF + AF

RF Generator (Carrier Waveform):


AF Generator (Message Waveform):




Subtractor Output (fc - fm) =

Adder Output (fc + fm) =

LSB Output


USB Output


Demodulated Output:


Model Graphs

DSB-Sc output

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Analog Communications Lab Manual_asrao