analog communications lab manual

RRS College of Engineering and TechnologyRRS College of Engineering and TechnologyRRS College of Engineering and TechnologyRRS College of Engineering and Technology

Analog Communications Lab ManualAnalog Communications Lab ManualAnalog Communications Lab ManualAnalog Communications Lab Manual

1

List of the experiments:

S. No Name of the experiment Page No

1. Amplitude modulation and Demodulation

2

2. Frequency modulator and demodulator

5

3. Balanced modulator

8

4. Synchronous detector 11

5. SSB modulation and Demodulation

14

6. Mixer characteristics

20

7. Pre-Emphasis & De- Emphasis

22

8. Phase Locked Loop(PLL)

25

9. Fibre Optic Analog Link

28



2

1. AMPLITUDE MODULATION AND DEMODULATION

AIM:

APPARATUS REQUIRED:

1. AM trainer kit

2. Dual trace CRO

3. CRO Probes & patch chords

BLOCK DIAGRAM:

CH1 CH2

THEORY:

In Amplitude modulation the amplitude of the carrier signal is varied by the modulating

voltage whose is invariably, lower than that of the carrier frequency. In practice, the

carrier frequency (HF), while the modulating frequency (AF).

Message

signal

source

Carrier

source

AM

Modulator

AM

Demodulator



3

Formally, AM is defined as a system of modulation in which the amplitude of the carrier

is made proportion to the instantaneous amplitude of the modulating voltage. Let the

modulating and carrier signal can be represented as

m(t)= Am Cos 2πfmt

c(t) = Ac Cos 2πfct respectively

.

The ultimate AM equation is = Ac(1+ µ Cos 2πfmt ) Cos 2πfct , where µ is the

modulation index. If µ=1 the AM is perfectly modulated, if µ <1 the AM is under

modulated and finally, if µ >1 the AM is over modulated.

PROCEDURE:

1. Connect the circuit as shown in the block diagram.

2. Apply the 1Vp-p message signal from the message source and 1Vp-p carrier

from the carrier source to the modulator.

3. Connect the message signal on CH1 of CRO and connect the modulated

output on the CH2 of the CRO.

4. Observe the perfectly modulated AM wave on the CRO and calculate the

modulation index (µ), amplitude of the modulated waveform.

5. Now varying the amplitudes of the message and carrier and observe the both

over and under modulated AM waves, and calculate the modulation index (µ)

values.

6. Apply the modulated output to the demodulator and observe the demodulated

signal and calculate the amplitude and phase difference w.r.t to the

modulating signal.

OBSERVATIONS:

Modulation Vmax Vmin µ=(Vmax-Vmin)/(Vmax+ Vmin)

Perfect

Under

Over



4

WAVEFORMS:

RESULT:

Amplitude modulated signal is generated and original signal is

demodulated from the AM signal. Depth of modulation is calculated for the

various amplitude levels of the modulating signals.



5

2. FREQUENCY MODULATION AND DEMODULATION

AIM: 1. To generate frequency modulated signal and determine the modulation index

and bandwidth for various values of amplitude and frequency of modulating signal.

2. To demodulate a Frequency Modulated signal using FM detector.

APPARATUS REQUIRED:

1. FM trainer kit

2. Dual Trace CRO

3. CRO Probes & Patch chords

BLOCK DIAGRAM:

CH1 CH2

THEORY:

In Frequency modulation the instantaneous Frequency of the carrier

signal is varied by the modulating voltage. Basically, FM is the continuous time

angle modulation technique and also it is a non-linear modulation process, which

having the constant envelope.

The bandwidth required for the FM is more compare then the AM. The

FM is divided into two types according to β is NBFM (β<1) & WBFM (β>1).

Where β is the modulation index=∆f (max freq. deviation)/ fm (message freq)

Message signal

source

FM Modulator with

in -built Carrier

generator FM

Demodulator



6

PROCEDURE:


2. First without applying modulating signal to the modulator observe the

output and measure amplitude and frequency of the signal, and ensure

that is the carrier.

3. Apply the 2Vp-p message signal from the message source to the

modulator.Connect the message signal on CH1 of CRO and connect

the modulated output on the CH2 of the CRO.

4. Observe the Frequency modulated wave on the CRO and calculate the

modulation index (β), amplitude of the modulated waveform.

5. Now varying the amplitudes of the message and observe the

modulated FM wave, and calculate the modulation index (β) values.

6. Apply the modulated output to the FM demodulator and observe the

demodulated signal and calculate the amplitude and phase difference

w.r.t to the modulating signal.

OBSERVATIONS:

Amplitude

(volts

Fmax (Hz) Fmin (Hz) ∆f =(Fmax-

Fmin)

β = ∆f /fm



7

WAVEFORMS:

RESULT: Phase reversal in DSB-SC Signal is occur at the zero crossing of modulating

signal is observed.



8

3. BALANCED MODULATOR

AIM: To generate AM-Double Side Band Suppressed Carrier (DSB-SC) signal by using

balanced modulator.

APPARATUS REQUIRED:

1. Balanced modulator trainer kit

2. Dual trace CRO

3. CRO Probes & patch chords

BLOCK DIAGRAM:

CH1 CH2

THEORY:

The balanced modulator is used to generate the DSB signal. Balanced

modulator is also called as the product modulator. The IC required for the

balanced modulator is IC1496. The output of the balanced modulator is the

product of the two input signals. The two inputs of any modulator are the message

and carrier. The output but the DSB signal. When compared to the AM, in DSB

the carrier is suppressed and the lower side band and upper side bands are

transmitted. The power required to transmit the DSBSC signal less when

compared with the conventional AM. To demodulate the DSBSC signal we

should use the synchronous detector.

Message signal

source

Carrier

Source

Balanced /product

Modulator



9

PROCEDURE:


2. Apply the 1Vp-p message signal from the message source and 1Vp-p

carrier from the carrier source to the modulator.

3. Connect the message signal on CH1 of CRO and connect the balanced

modulator output on the CH2 of the CRO.

4. Observe the balanced modulator output on the CRO and calculate the

modulation index (µ), which must be 1, and note down the amplitude

of the balanced modulator output.

5. The balanced modulator output is same as that of perfect modulated

waveform of the AM.

OBSERVATIONS:

Modulation Vmax Vmin µ=(Vmax-Vmin)/(Vmax+ Vmin)

Perfect

WAVEFORMS:



10

DSB-SC waveform w. r. t modulating signal.

T

Time (s)

0.00 100.00u 200.00u 300.00u 400.00u 500.00u

Voltage (V)

-4.00

-2.00

0.00

2.00

4.00

RESULT: Amplitude modulated signal is generated and original signal is demodulated

from the AM signal. Depth of modulation is calculated for the various amplitude levels of

the modulating signals.



11

4. SYNCHRONOUS DETECTOR

AIM: To generate AM-Double Side Band Suppressed Carrier (DSB-SC) de modulator

signal by using synchronous detector.

APPARATUS REQUIRED:

1. Balanced modulator and demodulator trainer kit

2. Dual Trace CRO

3. CRO Probes Patch chords

BLOCK DIAGRAM: CH1 CH2

THEORY:

The balanced modulator is used to generate the DSB signal. Balanced

modulator is also called as the product modulator. The IC required for the

balanced modulator is IC1496. The output of the balanced modulator is the

product of the two input signals. The two inputs of any modulator are the message

and carrier.

The output but the DSB signal. When compared to the AM, in DSB the

carrier is suppressed and the lower side band and upper side bands are

transmitted. The power required to transmit the DSBSC signal less when

compared with the conventional AM. To demodulate the DSBSC signal we

should use the synchronous detector.

Message signal

source

Carrier source

Balanced

Modulator

Synchronous

detector



12

PROCEDURE:


2. Apply the 1Vp-p message signal from the message source and 1Vp-p

carrier from the carrier source to the modulator.

3. Connect the message signal on CH1 of CRO and connect the balanced

modulator output on the CH2 of the CRO.

4. Observe the balanced modulator output on the CRO and calculate the

modulation index (µ), which must be 1, and note down the amplitude of

the balanced modulator output. The balanced modulator output is same as

that of perfect modulated waveform of the AM.

5. Apply the balanced modulator output to the synchronous detector and

observe the demodulated signal and calculate the amplitude and phase

difference w.r.t to the modulating signal.

WAVEFORMS:



13

Balanced modulator o/p w.r.t message signal.

T

Time (s)

0.00 100.00u 200.00u 300.00u 400.00u 500.00u

Voltage (V)

-4.00

-2.00

0.00

2.00

4.00

Demodulation / output of synchronous detector

T

Time (s)

0.00 100.00u 200.00u 300.00u 400.00u 500.00u

Voltage (V)

-4.00

-2.00

0.00

2.00

4.00

RESULT:

The original signal is demodulated from the synchronous detector.



14

5.SSB MODULATION AND DEMODULATION

AIM: To generate the single side band modulation by using phase shift method and

demodulation by using synchronous detector.

Apparatus and components required:

1. SSB modulation and demodulation trainer kit

2. Dual Trace CRO

3. CRO probes and patch chords

BLOCK DIAGRAM:

THEORY:

The phase shift method avoids and some of their attendant disadvantages,

and instead makes use of the two balanced modulators and two phase –shifting

networks as shown in the block diagram. As indicated, one of the modulators,

M1, receives the carrier voltage (shifted by 90o) and the modulating voltage,

where as the other modulator M2, is fed the modulating voltage (shifted by 90o)

and the carrier voltage. Some times the modulating voltage phase shift is arranged

slightly different. It is made +45o for one of the balanced modulator and -45

o for

another, but result is the same. Both modulators produce an output consisting only

of sidebands.



15

It will be shown; however, that where as both upper sidebands leads the

input carrier voltage by 90o

, one of the lower sidebands leads the reference

voltage by 90o

j and the other lags it by 90o. The two lower sidebands are thus out

of phase, and when combined in the adder, they cancel each other.

The upper side bands are in phase at the adder and thus add and thus add,

giving SSB in which the lower sideband has been cancelled. The foregoing may

be proved as follows. If taken for granted that two balanced modulators are also

balanced with respect to each other, then amplitudes may be ignored, as they do

not affect the result. Note also that both balanced modulators are fed from the

same sources.

As before taking Sin (ωm t) as the carrier and Sin (ωc t+90o) s the

modulation, we see that the balanced modulator M1 will receive Sin (ωc t) and Sin

(ωc t+90o) where as modulator M2 takes Sin(ωc t+90

o) and Sin(ωc t). Following

the reasoning in the proof of the balanced modulator, we know that the output of

M1 will contain sum and difference frequencies. Thus

V1 = Cos [(ωc t+90o)-ωm t] - Cos [(ωc t+90

o)+ωm t]

=Cos [(ωc t+90o)-ωm t] - Cos [(ωc t+90

o) +ωm t] -----------� (1)

LSB USB

Similarly, the output of M2 will contain

V2=Cos [(ωc t - (ωm t -90o)] - Cos [ωc t - (ωm t -90

o)] ---------� (2)

The output of the adder is

Vo =V1+V2= 2 Cos [(ωc t+90o)-ωm t] -------------- �(3)

This is obtained by adding Equation (1) and (2) and observing that the first

term of the first equations 180o out of phase with the first term of the second

equation. We have that one of the sidebands in the adder is cancelled, where as

the other is the system as shown yields the upper side band. A similar analysis

shown that SSB with the lower sideband present will be obtained if both signals

are fed (phase-shifted0 to the one balanced modulator.



16

PROCEDURE:

1. Switch on the trainer and measure the output of the regulated power supply i.e., ±12V

and-8V.

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 90o out of phase with the direct output. The

output frequency is 100 KHz and the amplitude is ≥ 0.2VPP. (Potentiometers are 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 90o out of phase with the direct output. A

switch is provided to select the required frequency (2 KHz, 4KHz or 6 KHz). AGC

potentiometer is provided to adjust the gain of the oscillator (or to set the output to good

shape). The oscillator output has amplitude ≅ 10VPP. This amplitude can be varied using

the potentiometers provided.

4. Measure and record the RF signal frequency using frequency counter. (or CRO).

5. Set the amplitudes of the RF signals to 0.1 Vp-p and connect direct signal to one

balanced modulator and 90o phase shift signal to another balanced modulator.

6. Select the required frequency (2KHz, 4KHz or 6KHz) of the AF generator with the help

of switch and adjust the AGC potentiometer until the output amplitude is ≅ 10 VPP (when

amplitude controls are in maximum condition).

7. Measure and record the AF signal frequency using frequency counter (or CRO).

8. Set the AF signal amplitudes to 8 Vp-p using amplitude control and connect to the

balanced modulators.

9. Observe the outputs of both the balanced modulators simultaneously using Dual trace

oscilloscope and adjust the balance control until desired output wave forms (DSB-SC).



17

10. To get SSB lower side band signal, connect balanced modulator output (DSB_SC)

signals to subtractor.

11. Measure and record the SSB signal frequency.

12. Calculate theoretical frequency of SSB (LSB) and compare it with the practical value.

LSB frequency = RF frequency – AF frequency

13. To get SSB upper side band signal, connect the output of the balanced modulator to

the summer circuit.

14. Measure and record the SSB upper side band signal frequency.

15.Calculate theoretical value of the SSB(USB) frequency and compare it with practical

value.

USB frequency = RF frequency + AF frequency

Wave forms:

Carrier wave at 00:

Modulating wave at 00:



18

Carrier wave at 900:

Modulating wave at 900:

Balanced modulator o/p w.r.t message signal.

T

Time (s)

0.00 100.00u 200.00u 300.00u 400.00u 500.00u

Voltage (V)

-4.00

-2.00

0.00

2.00

4.00

SSB output:



19

Demodulation / output of synchronous detector

T

Time (s)

0.00 100.00u 200.00u 300.00u 400.00u 500.00u

Voltage (V)

-4.00

-2.00

0.00

2.00

4.00

Result: The SSB modulation demodulation is performed



20

6. MIXER CHARACTERISTICS

AIM: To perform the mixer characteristics

Apparatus and Components required:

1. Mixer characteristics trainer kit

2. Dual trace CRO

3. probes and patch chords

Circuit diagram:

T1 BC107

R1 4

.7k

R2 22kR3 100k R4 100k

R5 1

0k

R6 1

0k

C1 100n

C2 1

n

C3 1

n

V1 1

2

C1 100n

f y

f x

THEORY:

The heterodyne means to mix. This process involves a simple change a

translation of carrier frequency and this change in a carrier frequency is achieved

by heterodyning or mixing. Generally, mixing is done in mixer or frequency

mixer. Both the local oscillator voltage whose frequency of and the signal voltage

frequency is are applied to the frequency mixer.



21

PROCEDURE:

1. Connect the circuit as shown in the figure

2. Connect the sine wave form through Fx

3. Adjust the amplitude control to 2Vp-p and frequency of 10 KHz.

4. Connect sine waveform another AF oscillator through Fy and adjust the

amplitude control to 2Vp-p and frequency 10 KHz.

5. Connect CRO terminals across Vout and ground.

6. If Fx =Fy=100 Hz, the output is zero.

7. Vary the frequency Fx to 101KHz and observe the different frequency Fx-Fy,

the frequency will be 1 KHz.

If we increase and decrease the frequency to 101 KHz or 90 KHz, then we can

observe the different frequency as 1 KHz. Similarly the Fx and note down

different frequency for different values.

WAVEFORM:

Amp

t

Result: The characteristic of the Mixer has studied.



22

7. PRE-EMPHASIS & DE-EMPAHSIS

AIM: To perform the frequency response of the pre-emphasis & de-emphasis circuit and

draw the graphs.

APPARATUS REQUIRED:

1. Dual Trace CRO

2. function generator

3. pre emphasis and de emphasis trainer kit

4. patch chords

CIRCUIT DIAGRAM:

THEORY:

Filtering will remove the noise in RF circuits, but noise control in the low

frequency (audio) amplifier is achieved through a high pass filter at transmitter (

pre-emphasis) a LPF at receiver (de-emphasis). The measurable noise in low

frequency electronic amplifier is most pronounced over the frequency range of 1

KHz to 2 KHz.

At the transmitter, the audio circuits are tailored to provide a higher level

of audio signal at the upper audio frequency range for aired noise level, the



23

greater signal voltage yields a better S?N ratio. at the receiver, when the upper

audio frequency signals are attenuated to form a flat frequency response, the

associated noise level is also attenuated.

A time constant was selected rather than s set of component values for the

pre-emphasis circuit to allow product designer free choice of component values to

match either the RC or L/R. L/R circuit are less common because large inductance

values and small resistance values required to achieve the 75 µsec of time.

The dB increase for any frequency over the range can be form as dB gain

= 20 log 1+ ( f/f1)where f1 is frequency at 3dB point and f is the frequency under

investigation. The de-emphasis curve for 75 µsec can be form as

dB loss =20 log 1+(f/f1)

The dB improvement in the under at any frequency is formed as dB a

power ratio dB= 20 log (1/3)(f/f1).

PROCEDURE:

1. Construct the circuit as shown in the circuit diagram.

2. Observe the input waveform on CRO CH1.

3. Adjust the amplitude of the sine wave using the amplitude knob to a

particular voltage, say 4v,6v,10v etc.

4. Observe the o/p waveform on CRO in CH2.

5. Measure the o/p voltage in the CRO and note down in the observation

table.

6. Calculate the attenuation and log f values as shown in the observation

table.

7. Draw the graph frequency (X-axis) and attenuation in dB (Y-axis) to

show the emphasis curves on a semi log graph.



24

TABULAR FORMS:

a) pre-emphasis input voltage=______ volts

Frequency (Hz) O/p voltage (volts ) Log f (Hz) Attenuation in dB

20 log Vo/Vi

b) de-emphasis input voltage=______ volts

Frequency (Hz) O/p voltage (volts ) Log f (Hz) Attenuation in dB

20 log Vo/Vi

WAVEFORMS:

Gain in dB

: Pre -emphasis

10-

5-

0-

-5-

-10-

:

De-emphasis

Log f

RESULT:

The frequency response of pre-emphasis and de-emphasis circuits are obtained.



25

8. PHASE LOCKED LOOP

AIM: To measure lock range and capture range of a given of a given PLL

COMPONENTS & APPARATUS REQUIRED.

1. PLL Trainer Kit

2. Function Generator

3. Dual Trace CRO

4. CRO Probes and Patch chords

CIRCUIT DIAGRAM:



26

BLOCK DIAGRAM:

Input e (t)

PLLO/p

Feed-back path

THEORY:

The block diagram represents PLL. It consists of a phase detector, a LPF,

and a voltage controlled oscillator (VCO). The phase detector compares the input

frequency fin with feed-back frequency fo, the output voltage of the phase

detector is proportional to the phase difference between fin and fo. The output

voltage of the phase detector is DC voltage and therefore is often refers as error

voltage. The output of the phase detector is produces a DC level. The DC level is

the input of the VCO. The output of VCO is directly proportional to the input DC

level. The VCO frequency is compared with the input frequencies and adjusted

frequency constant at the input frequency.

The PLL goes through 3 stages

1. Free running stage

2. capture stage

3. phase lock stage

Before input is applied the PLL is in free running stage, once the input

frequency is applied the VCO frequency starts to change and the PLL is said to be

in the capture range.

Phase

Discriminator

Low-pass

filter (LPF)

VCO (Voltage

controlled

Oscillator)



27

The VCO frequency continues to change until it equals the input

frequency and the PLL is then in the phase locked stage. When phase is locked

the loop tracks any change in the input frequency through its respective action.

PROCEDURE:

1. Switch ON the experiment trainer kit

2. R1 and C1 values are designed for center frequency fo

3. Sine wave applied to the input terminal using function generator.

4. The input frequency is varied and the frequency at which output frequency

becomes equal to input frequency is noted and it is denoted by F1.

5. The input frequency further increased and the output voltage falls down to

input and it is noted and denoted by F2.

6. The input frequency further increased and the output frequency becomes equal

the frequency is noted as F3.

7. The input frequency further decreases the output frequency raises thus that

frequency is noted as F4.

WAVEFORMS:

Capture range

Lock range

|

f4 f1 fo f3

f2

f

RESULT: PLL lock range and capture range is measured.

1. lock range =_______

2. capture range =________



28

9. ANALOG LINK USING FIBRE OPTICAL CABLES

AIM:

The aim of the experiment is to observe the frequency response of optical

fibre cable, and also observe the losses which are occurred in the optical fibres

and measure losses in dB of optical fibre chords at the wavelength of 650nm. The

coefficients of attenuation per meter at these wavelengths are to be computed

from the results.

Apparatus required

1. Fibre Optic analog link Transmitter kit

2. Fibre Optic analog link Receiver kit

3. Dual trace CRO

4. Optical fibre cable (5m & 1m)

5. Patch chords and CRO probes

BLOCK DIAGRAM:

AF input OFC AF

THEORY:

Attenuation in an optical fibre is a result of a number of effects. We will

confine our study to measurement of in two cables ( 1m and 5m ) employing an

SMA- SMA in line adaptor. We will also compute loss per metre of fibre in dB.

We will also study the special response of the fibre at wavelength of 650 nm.

650nmLED

Photo device/

Photo transistor

TX unit

RX unit

Output



29

PROCEDURE:

1. Switch ON the experiment Trainer kit ( Both transmitter and receiver )

2. Apply the sinusoidal input signal from the function generator with finite

amplitude (3Vp-p) and frequency to the transmitter and connect the input to

the CH1 of the CRO and note down the readings.

3. Connect the 5metre optical fibre cable (OFC) between the transmitter and

receiver.

4. Connect the output terminal to the CH2 of CRO. Vary the frequency knob in

the function

generator and observe output and note down the values.

5. Tabulate the output values and calculate the attenuation using formulae

Attenuation in dB=20 log (Vi/Vo).

6. Plot the graph on a Semi-Log graph sheet for different attenuation values

with different frequencies

7. Repeat the steps from 4 -6 for 1 metre cable.

TABULAR FORMS:

1) AF input Vi=_______ Volts for 5 metres

2) AF input Vi=_______ Volts for 1 metres

S. No Frequency

(Hz)

Output Voltage

( Volts)

Attenuation in dB

=20 log (Vi/Vo)

S. No Frequency

(Hz)

Output Voltage

( Volts)

Attenuation in dB

=20 log (Vi/Vo)



30

WAVEFORM:

Attenuation

in dB

‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘

‘

0 1 10 100 1000

10k

Log f

RESULT:

The attenuation losses of the Optical Fibre Cable (both 1m & 5m) for

different frequencies are observed.

analog communications lab manual

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