LAB MANUAL – COMMUNICATION SUBJECT CODE : · PDF fileLAB MANUAL – COMMUNICATION...

LAB MANUAL – COMMUNICATION

ENGINEERING & CODING THEORY

CSE DEPARTMENT

SUBJECT CODE :

CS491

CREDIT POINTS : 2

LIST OF EXPERIMENTS

1. Generate a 30 KHz sine wave from function generator and display

it using CRO.

2. Generate a 50 KHz, 1 MHz square wave/ pulse from function

generator and display it using CRO.

3. Generate a 50 KHz triangular wave and display it using CRO.

4. Design a class A amplifier.

5. Design a monostablemultivibrator using IC 555.

6. Design anastablemultivibrator using IC 555.

7. Generation of Amplitude Modulation (Design using transistor or

Balanced Modulator Chip (to view the wave shapes)).

8. Generation of FM using VCO chip (to view the wave shapes)

9. Generation of PAM.

10. Generation of PWM (using IC 555 Timer).

11. Generation of PPM (using IC 555 Timer).

Experiment Name: Design a monostablemultivibrator using 555 timer:

Theory:

A monostablemultivibrator (MMV) often called a one-shot multivibrator, is a pulse generator

circuit in which the duration of the pulse is determined by the R-C network, connected externally

to the 555 timer.

Circuit Diagram :

The operation of the circuit is explained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cutoff, the capacitor C begins charging toward +VCC through resistance RA with a time constant equal to RAC. When the increasing capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output of the timer goes low, as illustrated in figure.Thus the output returns back to stable state from quasi-stable state. The output of the MonostableMultivibrator remains low until a trigger pulse is again applied. Then the cycle repeats. Trigger input and output voltage are shown in figure.

The time during which the timer output remains high is given as tp = 1.0986 RAC where RA is in ohms and C is in farads.

http://www.circuitstoday.com/555-timer-ic-introduction

Sample Data:

Signal Frequency Amplitude Offset Duty cycle

Trigger input(pulse) 10 Hz 3 Vpp 3V 50%

RA = 47 Kilo ohm. C= 1 Micro Farad

Procedure:

Construct the circuit using IC 555, resistors, capacitors and diode.

Set the triggering signal in function generator. Trigger can be given manually as well.

Check the waveform in oscilloscope.

Calculate pulse width.

Change the value of resistance and capacitance, repeat the process and check pulse width again.

Write down your observations.

MonostableMultivibrator Waveform:

Conclusion:

Experiment Name: Design an astablemultivibrator using 555 timer:

Theory:

An astablemultivibrator, often called a free-running multivibrator, is a rectangular-wave generating

circuit. Unlike the monostablemultivibrator, this circuit does not require any external trigger to

change the state of the output, hence the name free-running.

Circuit Diagram :

AstableMultivibrator Operation:

In figure, when Q is low or output VOUT is high, the discharging transistor is cut-off and the

capacitor C begins charging toward VCC through resistances RA and RB. Because of this, the charging

time constant is (RA + RB) C. Eventually, the threshold voltage exceeds +2/3 VCC, the comparator 1

has a high output and triggers the flip-flop so that its Q is high and the timer output is low. With Q

high, the discharge transistor saturates and pin 7 grounds so that the capacitor C discharges

through resistance RB with a discharging time constant RB C. With the discharging of capacitor,

trigger voltage at inverting input of comparator 2 decreases. When it drops below 1/3VCC, the

output of comparator 2 goes high and this reset the flip-flop so that Q is low and the timer output

is high. This proves the auto-transition in output from low to high and then to low as, illustrated in

fig ures. Thus the cycle repeats.

The time during which the capacitor C charges from 1/3 VCC to 2/3 VCC is equal to the time the

output is high and is given as tc or THIGH = 0.693 (RA + RB) C, which is proved below.

Sample Data:

Signal Frequency

Trigger input(pulse) 10 Hz

RA = 10 kilo ohm RB = 10 kilo ohm C= 1 Micro Farad

Procedure:

Construct the circuit using IC 555

Check the waveform in oscilloscope

Calculate pulse width.

Change the value of resistance and capacitance, repeat the process and check pulse width again.

Write down your observation.

AstableMultivibrator Waveform:

Conclusion:

Amplitude Offset Duty cycle

3 Vpp 3V 50%

using IC 555, resistors, capacitors and diode.


Change the value of resistance and capacitance, repeat the process and check pulse width


Duty cycle

50%

Change the value of resistance and capacitance, repeat the process and check pulse width

Experiment Name: Generation of PAM

Theory:

Pulse amplitude modulation is a scheme, which alters the amplitude of regularly spaced

rectangular pulses in accordance with the instantaneous values of a continuous message signal. A

train of very short pulses of constant amplitude and fast repetition rate is chosen the amplitude of

these pulse is made to vary in accordance with that of a slower modulating signal the result is that

of multiplying the train by the modulating signal the enve lope of the pulse height corresponds to

the modulating wave .the Pam wave contain upper and lower side band frequencies .besides the

modulating and pulse signals. The demodulated PAM waves, the signal is passed through a low

pass filter having a cut – off frequencies equal to the highest frequency in the modulating signal. At

the output of the filter is available the modulating signal along with the DC component PAM has

the same signal to noise ratio as AM and so it is not employed in practical circuits.

Circuit Diagram:

Sample Data:

Signal Frequency Amplitude Offset Duty cycle

Carrier Signal(pulse) 10 Hz 3 Vpp 3V 50%

Modulating Signal(sine)

1 KHz 50 Vpp -3V 50%

Procedure:

Construct the circuit using transistor and resistors.

Set the carrier signal and modulating signal in function generator.



PAM Waveform:

Conclusion:

Experiment Name: Generation of PW

Theory:

In PWM, the width of the modulated pulses varies in proportion with the amplitude of modulating

signal. The amplitude and frequency of the PWM wave remains constant, only width of the pulse

changes. That’s why information is contained in the width variation.

Circuit Diagram:

Sample Data:

Signal Frequency

Carrier Signal(pulse) 44 Hz


10 Hz

Procedure:


Set the clock signal and modulating signal in function generator.



Generation of PWM using 555 timer:

e width of the modulated pulses varies in proportion with the amplitude of modulating


changes. That’s why information is contained in the width variation.

Amplitude Offset

10 Vpp 0V

7 Vpp 6V


signal and modulating signal in function generator.



e width of the modulated pulses varies in proportion with the amplitude of modulating


PWM Waveform:

Conclusion:

Experiment Name: Generation of PP

Theory:

In PPM, the amplitude and width of the pulses is kept constant but the position of each pulse is

varied in accordance with the amplitude of the sampled values of the modulating signal. The

position of the pulses is changed with respect to the position of refe

going to derive the PPM pulses from PWM pulses.

Circuit Diagram:

Sample Data:

Signal Frequency

Carrier Signal(pulse) 44 Hz


10 Hz

Procedure:


Set the clock signal and modulating signal in function generator.


Write down your observation

Generation of PPM using 555 timer :

PPM, the amplitude and width of the pulses is kept constant but the position of each pulse is


of the pulses is changed with respect to the position of reference pulses. Here, we are

going to derive the PPM pulses from PWM pulses.

Amplitude Offset

10 Vpp 0V

7 Vpp 6V


signal and modulating signal in function generator.


Write down your observation

PPM, the amplitude and width of the pulses is kept constant but the position of each pulse is


rence pulses. Here, we are

PPM waveform:

Conclusion:

Amplitude Modulation Aim: 1. To generate amplitude modulated wave and determine the percentage modulation.

Apparatus Required:

Name of the Component/Equipment

Specifications/Range Quantity

Transistor(BC 107) fT = 300 MHz

Pd = 1W Ic(max) = 100 mA

1

Diode(0A79) Max Current 35mA 1

Resistors 1KΩ, 2KΩ, 6.8KΩ, 10KΩ 1 each

CRO 20MHz 1

Function Generator 1MHz 2

Regulated Power Supply 0-30V, 1A 1

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

stAc1Kamtcos2f ct

Where Ka is a 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

envelop 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.

(E E )

The Modulation Index is defined as,m= max min

(Emax Emin )

Where Emax and Emin are the maximum and minimum amplitudes of the modulated wave.

Circuit Diagrams: For modulation:

Fig.1. AM modulator

Procedure:

Fig.2. AM demodulator

1. The circuit is connected as per the circuit diagram shown inFig.1.

2. Switch on + 12 volts VCCsupply.

3. Apply sinusoidal signal of 1 KHz frequency and amplitude 2 Vp-p as modulating signal, and carrier

signal of frequency 11 KHz and amplitude 15Vp-p.

4. Now slowly increase the amplitude of the modulating signal up to 7V and note down values of Emax

andEmin.

5. Calculate modulation index usingequation.

Observations

Table1: fm= 1KHz, fc=11KHz, Ac=15 V p-p.

S.No. Vm(Volts) Emax(volts) Emin (Volts) m %m (m x100)

.

Waveforms and graphs:

Precautions:

1. Check the connections before giving the powersupply

2. Observations should be donecarefully.

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