st. martins engineering collegesmec.ac.in/sites/default/files/lab1/ac lab manual 2016-17.pdf ·...

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St. MARTINS ENGINEERING COLLEGE(Affiliated to JNTUH & Approved by AICTE, Certified by ISO 9001:2000)

Dhulapally, Secunderabad, T.S. INDIA-500014.

DEPARTMENT

OF

ELECTRONICS AND COMMUNICATION ENGINEERING

ANALOG COMMUNICATIONS LAB MANUAL

(Course Code: A50487)

Prepared By Approved By

Mr. K. Nishakar K.YADAIAHAsst. Professor Assoc. Professor. & HOD

Director

Prof. D. SHOBHA RANI

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INDEX SHEET

S. No EXPERIMENT NAME PAGE NO.

1AMPLITUDE MODULATION & DEMODULATION 3

2FREQUENCY MODULATION & DEMODULATION 8

3DSB-SC MODULATOR & DETECTOR 12

4PRE EMPHESIS & DE-EMPHASIS 18

5PHASED LOCKED LOOP AS FM DEMODULATOR 21

6SSB MODULATION & DEMODULATION 24

7AGC CHARECTERISTICS 28

8VERIFICATION OF SAMPLINGTHEOREM 31

9PULSE AMLITUDE MODULATION & DEMODULATION 34

10PULSE WIDTH MODULATION & DEMODULATION 38

11PULSE POSITION MODULATION & DEMODULATION 41

12TIME DIVISION MULTIPLEXING & DE-MULTIPLEXING 44

13SPECTRUM ANALYZER 47

14FREQUENCY SYNTHESIZER 50

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FREQUENCY DIVISION MULTIPLEXING & DE-

MULTIPLEXING53

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1. AMPLITUDE MODULATION AND DEMODULATION

1.1. Pre-Lab Questions:

1. What is modulation?

2. Why to modulate the analog signals?

1.2. Objective:

To study amplitude modulation and demodulation and to calculate modulation

index by changing the modulating signal’s amplitude.

1.3. Resources:

1. Amplitude Modulator and Demodulator Kit

2. Signal Generator

3. CRO

4. BNC Probes

1.4. Procedure:

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

2. Apply the 100 KHz carrier signal and amplitude of 6V(p-p) to the input of AM

modulator at 100 KΩ pot and 1 KHz of modulating signal to the AM modulator

at 100 KΩ pot as shown in the circuit diagram.

3. Apply the power supply of 12V as shown in the circuit diagram.

4. Observe the amplitude modulated wave synchronization with the modulating

signal on a dual trace CRO following figure shown the connections.

5. Adjust the 10 KΩ linear pot for carrier suppression and 100KΩ linear pot for

proper modulation i.e. 100%.

6. Now by varying the amplitude of the modulating signal, the depth of modulation

varies.

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1.5. Circuit Diagram:

1.5.1. AM Modulator

1.5.2. AM Demodulator

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Calculate the maxima and minima points of modulated wave on the CRO and calculate

the depth of modulation using formula.

1.6. Tabular Column:

1.7.Waveforms:

Inputs:

Fig A:Message Signal

Fig B:Carrier signal

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1.7.1. Different Types of AM modulated waves

1.7.2.Demodulated Waveform:

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1.8. Result:

Amplitude modulated signal is generated and original signal is demodulated from

AM signal Depth of modulation is calculated for various amplitude levels of modulating

signals.

1.9. Lab Assignment Questions:

1. What is the effect of Am and Ac on Amplitude modulated Signal?

2. What is the resonant frequency of the tank circuit?

3. What is the roll of the diode in demodulator circuit?

1.10. Post-Lab Questions:

1. How many types of analog Modulation are there?

2. What is the disadvantage of amplitude modulation?

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2. FREQUENCY MODULATION AND DEMODULATION


1. What is frequency range for speech information?

2. What are the reasons to modulate the analog signal or low frequency

Signal?

3. Define modulation index in FM?

2.2. Objective:

To study the process of frequency modulation and demodulation and to calculate

the depth of modulation by varying the modulating voltage.

2.3. Resources:

1. Frequency modulation and demodulator Kit

2. Signal Generator (2)

3. CRO

4. BNC Probes

2.4. Procedure:

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

2. Check the circuit properly and apply the power supply to the circuit.

3. Observe the carrier signal from the FM modulator at pin 2 of the IC 8038, which

is 82KHZ.

4. Apply the modulating signal frequency of 4KHZ, 6Volts (p-p) from the function

Generator to the FM input at pin 8 as shown in the figure below.

5. Trigger CRO with respect to CH1 adjust amplitude of the modulating signal until

we get Undistorted FM output. It is difficult to trigger FM on analog CRO.

6. That is why you adjust modulating signal amplitude until small distortions notified

in Fm output.

7. Calculate maximum frequency and minimum frequency from the FM output and

calculate modulating index using table shown in below.

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2.5.1Frequency Modulator using IC 8038

2.5.2. Frequency Demodulator using LM 565

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2.6. Waveforms:

2.6.1.Modulated Waveform:

2.6.2. Demodulated Waveform:

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2.7. Result:

The frequency modulated signal is generated and original signal is demodulated and

modulation index is calculated for FM signal.


1. Effect of the modulation index on FM signal?

2. In commercial FM broadcasting, what is highest value of frequency deviation and

audio frequency to be transmitted.


1. What is band width requirement for FM ?

2. What are the Advantages of frequency modulation, FM ?

3. What are the disadvantages of frequency modulation, FM ?

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3. DSB-SC MODULATOR & DE-MODULATOR 3(A). BALANCED MODULATOR

3.A.1. Pre-Lab Questions:

1. What is a balance modulator ?

2. How will you generating DSB-SC AM?

3. Limitations of Amplitude Modulation (DSB-SC) ?

3.A.2. Objective:

1. To construct and properly adjust a balanced modulator and study its operation.

2. To observe that the output is a double sideband with a suppressed carrier

signal.

3. To adjust it for optimum carrier suppression.

4. To measure carrier only output and the peak sideband output and to calculate

the carrier suppression.

3.A.3. Resources:

1. Balanced Modulator Kit


3. CRO

4. BNC Probes

3.A.4. Procedure:

1. Switch on the trainer kit.

2. Connect 200Hz sine wave and 100 kHz square wave from the function generator.

Adjust R1, (1k linear pot).Connect oscilloscope to the output.

3. Vary R1 (1K) both clock wise and counter clock wise (1k linear pot).

4. Disconnect the sine input to R1 (1k).The output should be nearly zero.

5. Increase the oscilloscope vertical input sensitivity to measure the output voltage

Eout carrier only.

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6. Set the vertical input control to 1v/cm .Connect the sine input to R1(1k) and

adjust R1 for maximum output without producing clipping measure the peak side

band output voltage.

7. calculate the carrier suppression in dB

DB=-20 log (Epk sideband/Eout carrier only)

Turn off your experiment and disconnected your circuit.

3.A.5. Circuit Diagram:

Balanced modulator using IC 1496

Observations:

1. AF signal frequency= 200 Hz.

2. RF signal frequency = 100 KHZ.

3. Varying R1 ↑, DSBC amplitude (p-p) ↑ proportionally.

4. After disconnecting Sine input to R1.

5. Eout carrier only = 20 mV (p-p).

6. Epk sidebands = 2.4V (p-p).

7. Carrier suppression in db = 20 log EPK Sideband / Eout carrier only =41.5

3. A.6.Waveforms:

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Input and output waveforms of balanced modulator

3. A.7. Result:

The Double side band suppressed carrier signal is obtained (Balanced output i.e.,

100% modulation is obtained).

3. A.8. Lab Assignment Questions:

1. What is the efficiency of the DSB-SC modulating system?

2. What are the applications of balanced modulator?

3. What is the effect of amplitude of message on DSB-Sc signal?

3. A.9. Post Lab Questions:

1. What is up conversion?

2. What is the BW of DSB-SC signal?

3(B). SYNCHRONOUS DETECTOR

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3.B.1. Pre-Lab Questions:

1. What is Synchronous Detection ?

2. Considerations of Synchronous Detection ?

3. What are the demodulation methods for DSB-SC signals?

3.B.2. Objective:

To obtain the demodulating (or) message signal using DSB or SSB synchronous

or coherent detector.

3.B.3. Resources:

1. Synchronous Detector Kit


3. CRO

4. BNC Probes

3.B.4. Procedure:

1. A connection was made as per circuit diagram.

2. A balanced modulator circuit was connected for both Transmitter and

Receiver.

3. Carrier signal was same and was applied to two balanced modulator circuits.

4. Apply the power +12V to pin 8 of both the circuits.

5. The output of the Receiver balanced modulator circuit was connected to low

pass filter.

6. Low pass filter circuit was designed according to the requirement.

7. And the modulating signal was obtained at the LPF.

3.B.5. Circuit Diagram:

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3.B.6.Wave Forms:

3.B.7.1.AM-SC Wave Form:

3.B.7.2.Demodulated signal:

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3.B.8. Result:

Thus the modulating signal obtained from the DSB detector.

3.B.9. Lab Assignment Questions:

1. Write the applications of synchronous detector?

2. What are the drawbacks of synchronous detector?

3. What is the Effect of Carrier signal on output signal?

3. B.10. Post Lab Questions:

1. Why Synchronous detection?

2. What is the difference between detector and demodulator?

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4. PRE-EMPHASIS AND DE-EMPHASIS

4.1. Pre-lab Questions:

1. What is pre-emphasis and de-emphasis ? Why is it required ?

2. What is the necessity of pre-emphasis and de-emphasis ?

3. What is the role of this two concepts in Communication ?

4.2. Objective:

To study the frequency response of the Pre-emphasis and De-emphasis circuits and

draw the graphs.

4.3. Resources:

1. Pre-Emphasis & De-Emphasis Kit


3. CRO

4. BNC Probes

4.4.Procedure:

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

2. Observe the I/P waveform on the CRO in channel.

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

voltage, say 4V or 6V or 10V etc.

4. Measure the O/P voltage in the CRO and note down in the observation table.

5. Calculate the attenuation and Log f values as shown in the observation table.

6. Draw the graph frequency (X-axis) and attenuation in db (Y-axis) to show the

emphasis curves on a semi log graph.

7. Various values of R and C are available so that the time constant in suitably

selected depending upon the application.

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4.5.Circuit Diagram:

4.5.1. Pre-Emphasis: I/p voltage= 4Volts

4.5.2.De-Emphasis:

I/p Voltage=volts

Frequency in Hz Output in volts LogfAttenuation in db

20 log Vo/Vi

Frequency(Hz)

Output(volts)

Log f Attenuation(db)

20 log eo/ei

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4.6.Frequency response of Pre-emphasis &De-emphasis

4.7. Result:

The frequency response of Pre-emphasis and De-emphasis circuits obtained.


1. What is the value of time constant used in commercial pre-emphasis circuit?

2. For which modulated signals pre-emphasis and de-emphasis circuits are used.

3. On what parameters fc depends?

4. Explain the pre-emphasis and de-emphasis characteristics?


1. Which range of frequencies are more prone to noise interference?

2. How to reduce the noise during transmission in FM?

3. Which technique is used at the receiver side to reconstruct the original signal?

4. What should be the time constant for the de-emphasis circuit?

5. Why pre-emphasis is done after modulation

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5. PHASE LOCKED LOOP


1. What is phase-locked loop (PLL) ?

2. What are the three stages through which PLL operates?

3. Define lock-in range of a PLL?

4. Define capture range of PLL?

5.

5.2. Objective:

To determine the Free running range, Lock range and capture range using PLL.

5.3. Resources:

1. IC LM 565

2. PLL Trainer kit

3. CRO

4. Function Generator.

5. BNC probes.

5.4. Procedure:

1. Connect + 5V to pin 10 of LM 565.

2. Connect -5V to pin 1.

3. Connect 10k resistor from pin 8 to + 5V

4. Connect 0.01 μf capacitor from pin 9 to – 5V

5. Short pin 4 to pin 5.

6. Without giving input measure (fO) free running frequency.

7. Connect pin 2 to oscillator or function generator through a 1μf capacitor, adjust

the amplitude around 2Vpp.

8. Connect 0.1 μf capacitor between pin 7 and + 5V (C2)

9. Connect output to the second channel is of CRO.

10. Connect output to the second channel of the CRO.

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11. By varying the frequency in different steps observe that of one frequency the

waveform will be phase locked.

12. Change R-C components to shift VCO center frequency and see how lock range

of the input varies.

13. Now compare the theoretical values and practical values using the given

formula.


5.6. Waveforms:

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5.6.1Theoretical Values:

(a).Lock Range of PLL (fL):

Lock range of PLL is in the range of frequencies in which PLL will remain lock, and this is given by

fL = ± 8fo / Vcc Where fo is the free running frequency

Vcc=Vcc-(-Vcc)

=2Vcc

(b).Capture Range of PLL (fC):

Capture range of PLL is the range of frequencies over which PLL acquires the lock. This is given by

Where fL is the lock range

5.7. Result:

Hence we determine free running frequency, capture range and lock range.

Lock range=_________________

Capture range=_______________

Phase detection=______________


1. Write the application of PLL?

2. What is the capture range of PLL.

3. What is the effect of R1 and C1 values and Vcc on output signal?


1. What are the advantages and applications of PLL?

2. What is the need to generate corrective control voltage?

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6. SINGLE SIDE BAND SYSTEM


1. How to generate SSB signal?

2. Define SSB-SC.

6.2. Objective:

To generate a SSB signal using Balanced Modulator.

6.3. Resources:

1. SSB Trainer Kit.

2. CRO.

3. Function Generator.


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6.5. Procedure:

1. Study the circuit operation of SSB system thoroughly.

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

RF generator, one is direct output and another is 90º phase shift with the direct

output. The output frequency is 100 KHz and amplitude is ≥0.2 Vpp

(potentiometers are provided to vary the output amplitude).

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

AF generator, one is direct output and another is 90º phase shift with the direct

output. A switch is provided to select the required frequency (2K, 4K, and 6K).

AGC potentiometer is provided to adjust the gain of the oscillator (or to select the

output to good shape). And the amplitude is≈10Vpp (potentiometers are provided

to vary the output amplitude).

4. Measure the RF signal frequency using frequency counter.

5. Set the amplitude of the RF signal to 0.1Vpp and connect 0º phase shift signal to

one balanced modulator and 90º phase shift to another balanced modulator as

shown in figure.

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

of the switch and adjust the AGC potentiometer until output amplitude is≈10Vpp

(when amplitude controls are in max condition).

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

8. Set the AF signal amplitudes to 8 Vpp using amplitude control and connect to the

balanced modulators as shown in fig.

9. Observe the outputs of both the balanced modulators simultaneously using dual

trace oscilloscope and adjust the balance control until you get the output

waveforms (DSB-SC).

10. To get SSB lower side band signal, connect balanced modulator output (DSB-SC

signals) to substractor.

11. Measure and record the SSB signal frequency using frequency counter.

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

value LSB=RF freq – AF freq

13. To get SSB upper side band signal, connect balanced modulator output (DSB-SC

signals) to summer.

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

counter.

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

value USB=RF freq + AF freq

16. Connect SSB signal from summer (or) sub-tractor to the SSB signal input of the

synchronous detector and RF signal (0) to the RF input of the synchronous

detector.

17. Observe the detector output using CRO and compare it with the modulating

signal(AF).

18. Observe the SSB signal for the different frequencies of the modulating(AF) signal.

6.6. Waveforms:

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6.7. Result:

Thus the SSB signal was generated using balanced modulator.


1. What are difficulties in practical implementation of SSB-C system?

2. Why SSB-SC is not used in broadcasting?


1. What are the advantages of signal sideband transmission?

2. What are the disadvantages of single side band transmission?

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7. AUTOMATIC GAIN CONTROL


1. Define is AGC

2. Classify AGC

3. What are the applications of AGC ?

7.2. Objective:

To study the AGC characteristics of a radio receiver.

7.3. Resources:

1. AGC Characteristics Trainer kit.

2. 20 MHz dual trace oscilloscope

3. Patch chords

7.4. Procedure:

1. Select carrier frequency of 1000 KHz, AF frequency of 1KHz and apply AM signal

to the input of receiver set amplitude to around 1mv.

2. Connect CRO at the output of the audio amplifier.

3. Tune the mixer – local oscillator for maximum AF signal output at detector output

and measure the audio signal.

4. Including the RF level in appropriate steps and note down corresponding output AF

signal amplitude.

5. Plot the AF output Vs RF input on graph which will be as shown in figure.

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7.5. Block Diagram:

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7.6. Result:

The gain is controlled automatically to the strength of message signal is verified.

7.7. Lab Assignment Questions ?

1. What are the functions of IF Amplifier ?

2. What are the functions of RF Amplifier?


1. What is the need for AGC ?

2. What are the drawbacks of AGC and solution?

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8. SAMPLING THEOREM VERIFICATION


1. What are the various Sampling techniques?

2. Explain various sampling circuits?

8.2. Objective:

To verify sampling theorem.

8.3. Resources:

1. Sampling theorem verification trainer (AET-47).

2. C.R.O (20MHz)

3. Patch cords.

8.4. Procedure:

1. Connect trainer to mains and switch on the power.

2. Observe the output of AF generator and pulse generator using CRO and note

that AF signal is approximately 3Vp-p of 100HZ frequency and pulse generator

output is pulse train of 10VP-P with frequency between 200 HZ and 4 KHz.

3. Connect pulse output and AF output to the respective inputs of sampling circuit.

4. Connect one of the inputs of oscilloscope to the sampling circuit output and

another to AF signal.

5. Initially set the amplitude of the AF generator to minimum level and sampling

frequency to 200Hz (by adjusting the potentiometer). Observe the output of

sampling circuit by varying the amplitude of modulating signal. You can notice

the amplitude of sampling pulse is varying in accordance with the amplitude of

the modulating signal.

6. Connect sampling circuit output to reconstructing circuit.

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7. Observe the output of reconstructing circuit (AF signal) at 200Hz sampling

frequency until you get the original signal. Statement: The Nyquist Theorem

states that in order to recover the original signal from sampled signal when

signal sampled at fs ≤ 2fm.


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8.6. Expected wave forms:

8.7. Result:

Hence verified the sampling theorem and reconstructed the original signal.


1. State the Shannon’s sampling theorem.

2. What is Nyquist rate?

3. What is meant by Aliasing?

4. What are the effects of Aliasing?

5. How to avoid Aliasing effect?


1. Why you need ahold circuit?

2. What is flat-top Sampling?

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9. PULSE AMPLITUDE MODULATION & DEMODULATION


1. Define PAM.

2. What are the advantages of PAM ?

3. Classify modulation techniques.

9.2. Objective:

1. To study the pulse amplitude modulation and demodulation technique.

2. To study the effect on amplitude and frequency variation of modulation

Signal on the output.

9.3. Resources:

1. Pulse Amplitude Modulation and Demodulation Kit.

2. Oscilloscope – 20MHz Dual Channel.

3. Patch Cards.

9.4. Theory:

Pulse Amplitude Modulation is used to transmit analog signals are sampled at

regular intervals. At receiver (Demodulation) the original waveforms may be reconstructed

from the information regarding the samples.

It is the simplest form of the pulse modulation. In PAM the signal is sampled at

regular interval and each sample is made proportional to the amplitude of the signal at the

instant of sampling. The pulses are then sent by either wire cable are used to modulated

carrier.

In PAM sampling has been done by two types --- I ) Natural sampling : in which

finite – width pulses are in the modulator but the tops of the pulses are forced to follow the

modulating waveforms.

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II) Flat – topped sampling: in which finite – width pulses are in the modulator but

the tops of the pulses are follow the modulating waveforms as pulse wise rather natural

sampling . it is often used because of ease of generating the modulated.

PAM signals are rarely used for transmission purpose directly and are distorted by

Noise, Cross talk and other forms of distortions.

9.5. Procedure:

9.5.1. Modulation

1. Connect the circuit as shown in figure.

a) Output of the sine wave to modulating signal input TP2 keeping the switch

in 1KHz position and amplitude position to maximum position.

b) 16KHz pulse output to pulse input TP1(keep the frequency plot in

minimum position in pulse generator block.

2. Switch ON or keeping the power supply.

3. Monitor the output at TP5, TP6 and TP7 and also observe the output by varying

amplitude plot.

4. Now vary the frequency solute switch position in modulating signal generator

block to 2KHz position i.e., Maximum.

5. Observe the output TP5,TP6 and TP7 and also observe the output by varying

amplitude plot.

6. Repeat all the above steps for pulse frequency of 32MHz.

7. Switch OFF the power supply.

9.5.2. Demodulation

1.Connect the circuit diagram as shown in figure.

a) Output of Sine wave to modulating signal input TP2 by keeping the Switch in

1KHz position amplitude plot in maximum position.

b) Connect the pulse output to pulse input TP1.

c) Sample output, Sample &Hold output Flat – Top output respectively to input

of LPF( low pass filter ) TP9 and LPF output to AC amplifier inputTP1.

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2. Observe the output of LPF and Ac amplifier at TP10, TP12 respectively

Corresponding to input from TP5,TP6 and TP7.

3. Now set switch position in modulating signal generator to 2KHz and observe the output

at TP10 and TP12 respectively corresponding input from TP5,TP6 and TP7.

4. Vary the Frequency of pulse to 32 KHz.

5. Switch OFF the power supply.

9.6. Expected Graph:

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9.7. Result:

Pulse Amplitude Modulation and Demodulation has been done and corresponding waveforms have been plotted.


1. Compare natural and flat-top sampling with the help of waveforms.

2. What is meant byaperture effect?

3. What is meant byideallyor instantaneous sampled PAM?


1. What are the demodulation methods for the flat-top sampled signal?

2. What are thedisadvantages of PAM ?

3. Applications of PAM.

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10. PULSE WIDTH MODULATION (PWM) & DE-MODULATION


1. What are thedifferent types of Pulse timemodulation systems?

2. What are themethods to generate PWM?

10.2. Objective:

1. To study the process of pulse width modulation and demodulation.

2. To observe the Effect of Amplitude and frequency of modulating signal on

PWM output.

10.3. Resources:

1. Pulse Width Modulation and Demodulation trainer kit.

2. Cathode Ray Oscilloscope 20 MHz dual channel.

3. Patch Chords.

10.4. Theory:

Pulse Width Modulation is one of the methods of Pulse Time Modulation in which

the amplitude is fixed & starting at each pulse but the width of each pulse is proportional

to the amplitude of the signal at that instant.

PWM will converts varying amplitude message signal into square wave with

constant amplitude & frequency but which changes duty cycles to correspond to the

Strength of the message signal.

It is generated by applying trigger pulses( at sampling rate )to control the starting

time of pulses from a monostable multivibrator and feeding in the signal to be sampled to

control the destination of these pulses. When PWM signals comes at destination the

recovery circuit used to decode the original signal is a sample is a simple Integrator (LPF).

The disadvantage of PWM is of Varying width & power content due to the

transmitter power not enough to handle the maximum – width pulses.

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10.5. Procedure:

10.5.1. Modulation

1. Connect the circuit as shown in diagram

a) Sine wave output of modulating signal generator to modulating signal

Input. Keeping the switch the in 1Khz position and amplitude pot in maximum

position.

b) 16Khz pulse output (by varying the frequency pot in pulse generator block) from

pulse generator to input TP1.

2. Switch ON the Power Supply.

3. Observe the PWM output at TP2 and the different output signal at TP3

4. Vary the modulating signal generator frequency by switching the frequency selector switch to

2KHz.

5. Now again observe the PWM output at TP3.

6. Repeat the above the steps for the pulse frequency of 32 KHz.

7. Switch OFF the Power Supply.

10.5.2. Demodulation

1. Connect the circuit as shown in diagrama) Output of the sine wave to Modulating Signal input TP2 in PWM block.

b) 16 MHz pulse output from pulse generator block to pulse output Tp1.


3. Observe the output of LPF and A.C. Amplifier respecting at TP6 and TP8. The

output will be the replica of the input.

4. Now monitor the amplitude the frequency of sine wave by varying the amplitude

plot and Frequency selection switches to 2Khz and observe the PPM output.

5. Repeat the steps 10 and 11 for pulse frequency of 32 KHz.


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10.6.Expected Graph:

10.7. Result:

Pulse Width Modulation And Demodulation has been done and corresponding waveforms have been plotted.


1. What is the BW of PWM?

2. Applications of PWM?

10.9.Post-Lab Questions:

1. What are the advantages of PWM?

2. What are the disadvantages of PWM?

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11. PULSE POSITION MODULATION (PPM) & DEMODULATION


1. Define PPM

2. How to generate PPM?

11.2. Objective:

1. To Study the generation of PPM signal modulation and its demodulation.

2. To observe the effect of Amplitude and the frequency of modulating on it’s output

and observe the wave form.

11.3. Resources:

1. Pulse Position Modulating and demodulating trainer kit.

2. Cathode Ray Oscilloscope-20MHz, Dual pin.

3. Patch chords.

11.4. Theory:

Pulse Position Modulation is one of the methods of Pulse Time Modulation .PPM is generated by changing the position of a fixed time slot.

The amplitude & width of the pulses is kept constant while the position of each pulse in relation to the position of the recurrent pulse is valid by each instances sampled value of the Modulating wave.

Pulse position modulation simply obtained from PWM where the locations of the

leading edges are fixed and trailing edges not. Their position depends on width, it is said

that the trailing edges of PWM pulses are in fact position modulated. This has positive

going narrow pulses corresponding to leading pulses and has negative going pulses

corresponding to trailing pulses. If the position corresponding to the trailing edge of an un

modulated pulse is counted as zero displacement then the other trailing edges will arrive

earlier or later .Then they will have a time displacement other than zero. The time

displacement is proportional to the instantaneous value of the signal voltage. The

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differentiated pulses corresponding to the leading edges are removed with a diode clipper

or rectifier and the remaining pulses are position modulated.

PPM has the advantage of requiring constant transmitter power output & disadvantage is that it depends on the transmitter Synchronization.

11.5. Procedure:

11.5.1. Modulation

1. Connect the circuit as shown in diagram a) Output of the sine wave to Modulating Signal input (TP1) in PPM block.

b) Keeping the switch in 1 KHz Position and amplitude pot in max position.


3. Observe the PWM output at TP2 and the different output signal at TP8.

4. Now monitor the amplitude the frequency of sine wave by varying the amplitude

plot and frequency selection switch to 2Khz and observe the PPM output.


11.5.2. Demodulation

1. Connect the circuit as shown in the figure.

2. Switch ON the Power Supply

3. Monitor the output at TP3 by varying amplitude plot.

4. Observe the Demodulated signal at the output of Low Pass Filter at TP5.

5. Thus the recovery signal is at output of Low Pass Filter is actual replica of input.

6. Switch OFF the Power Supply

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11.6. Expected Graph:

11.7. Result:

Pulse Position Modulation and Demodulation has been done and corresponding

waveforms have been plotted.


1. Define PDM

2. What is the BW of PPM?

3. Applications of PDM and PPM?

11.9. Post-lab Questions:

1. What are theadvantages of PPM?

2. What are thedisadvantages of PPM?

44

12. TIME DIVISION MULTIPLEXING & DE-MULTIPLEXING


1. What is a multiplexer?

2. A basic multiplexer principle can be demonstrated through.

3. Distinguish between the two basic multiplexing techniques?

12.2. Objective:

1. To study of four channel analog multiplexing and de multiplexing techniques.

2. Study the effect of sampling frequency variation on the output.

3. Study the input signal amplitude on the output.

12.3. Resources:

1. Time Division Multiplexing Kit

2. Cathode Ray Oscilloscope – 30MHz dual

3. Patch cards

12.4. Theory:

Time Division Multiplexing is used for transmitting several analog message signals

over a communication channel by dividing the time frame into slots, one slot for each

message signal.

In Multiplexing the four input signal all band limited by the input filters are

sequentially sampled the output of which is a PAM waveform containing samples of the

input signals periodically interlaced in time. The samples from adjacent input message

channels are separated by Ts/M where the number of input channels. A set of M pulses

containing of one sample from each of the input M – input channels is called a frame.

At De-multiplexing the samples from individual channels are separated by

carefully synchronizing and are critical part TDM. The samples from each channel are

filtered to reproduce the original message signal. There are two levels of synchronization.

FRAME Synchronization is necessary to establish when each group of samples begins and

WORD Synchronization is necessary to property separate the sample within each frame.

45

12.5. Procedure:

12.5.1. Multiplexing

1. Connect the Circuit diagram as shown in figure.

2. Switch ON the power supply.

3. Set the amplitude of each modulating signal as 5V peak – peak.

4. Monitor the output at test points 5,6,7,8. These are natural sampling PAM

outputs.

5. Observe the outputs by varying the duty cycle plot (TP5).The PAM output will

varying with 10% to 50% duty cycle.

6. Try to varying the amplitude of modulating signal corresponding each channel

by using amplitude plotsP1 and P4. Observe the effect on all outputs.

7. Observe the TDM output at pin No:13 of 4052. All the multiplexes channels are

observed during the full period of the clock (1/32 KHz).

12.5.2. De-Multiplexing

1. Connect the circuit as shown in figure.

2. Observe the de-multiplexed outputs attest points 13 to 16 respectively.

3. Observe by varying the duty cycle plot TP5 and see the effect on the outputs.

4. Observe the LPF outputs for each channel at test points 17,18,19,20 and at

sockets channels CH1,CH2, CH3,CH4. these signals are actual replica of the

inputs. These signals have been amplitude.

46

12.6.Expected Graph:

12.7. Result:

Four channel analog multiplexing and de-multiplexing has been done and

corresponding waveforms have been plotted.


1. What is the function of an enable input on a multiplexer chip?

2. Will multiplexing create additional harmonics in the system?

3. Can I accidentally switch a dimmer to multiplex mode?

4. Why guard bands are used in FDM?

5. Why sync pulse is required in TDM?


1. What is difference between Frequency Division multiplexing and Wave Division

multiplexing?

2. In what situation multiplexing is used?

47

13. SPECTRUM ANALYSIS OF AM AND FM SIGNAL USING

SPECTRUM ANALYZER


1. What is spectrum analyzer?

2. List the types of spectrum analyzer.

13.2. Objective:

To observe the spectrum of AM and FM signals and obtain the power levels in

dBm of fundamental frequency components by using spectrum Analyzer.

13.3. Resources:

1. Spectrum analyzer

2. AM/FM generator-0.1MHz-110MHz

3. CRO-30MHz

13.4.Theory:

A spectrum analyzer provides a calibrated graphical display on its CRT with

frequency on the horizontal axis and amplitude on the vertical axis. Displayed as vertical

lines against these coordinates are sinusoidal components of which the input signal in

composed. The height represents the absolute magnitude, and horizontal location

represents the frequency. This instrument provide a display of the frequency spectrum

over a given frequency band.

13.5. Block diagram:

Fig.1 Block Diagram of Spectrum Analyzer

48

13.6. Procedure:

1. AM signal is given to the spectrum analyzer.

2. Adjust the zero marker to carrier frequency and measure spectrum of AM.

3. For different values of fc and fm, observe the spectrum of AM.

4. Now remove AM signal and give FM signal to the spectrum analyzer.

5. Adjust the zero marker to carrier frequency and observe spectrum of FM.

6. Plot the spectrums of FM and AM.

13.7. Observation Table:

Table1: Readings for AM signal

S.No. fc (MHz) fm (KHz) (fm+ fc ) (MHz) (fc - fm) (MHz)

1

2

Table2: Readings for FM signal

S.No. fc (MHz) fm (KHz) (fm+ fc ) (MHz) (fc - fm) (MHz)

1

2

49

13.8. Model Graphs:

Fig.2 AM spectrum

Fig. 3 FM spectrum

13.9. Result:

Spectrum Analyzer is studied.


1. Distinguish between CRO and Spectrum analyzer?

2. What are the functions of span/div control and reference level control?


1. List some application of spectrum analyzer.

2. Differentiate between wave analyzer and Spectrum analyzer.

50

14. FREQUENCY SYNTHESIZER


1. What is meant by Frequency Synthesizer?

2. How does a frequency synthesizer work? 14.2. Objective:

To construct a frequency synthesizer circuit.

14.3. Resources:

1. Frequency Synthesizer Kit

2. CRO 0-20MHz

3. Function generator 0-1MHz

4. Regulated Power Supply 0-30V, 1A

14.4. Theory:

The frequency divider is inserted between the VCO and the phase comparator of

PLL. Since the output of the divider is locked to the input frequency fin, the VCO is

actually running at a multiple of the input frequency. The desired amount of

multiplication can be obtained by selecting a proper divide– by – N network, where N is

an integer. To obtain the output frequency fOUT=5fIN, a divide – by – N = 5 network is

needed. One must determine the input frequency range and then adjust the free running

fOUT of the VCO by means of R1 (20k_pot) and C1 (10µF) so that the output frequency of

the divider is midway within the predetermined input frequency range. The output of the

VCO now should be 5fIN. The output of the VCO now should be adjusted from 1.5 KHz

to 15 KHz by varying potentiometer R1 .this means that the input frequency fin range has

to be within 300Hz to 3KHz. In addition, the input wave form may be applied to inputs

pin2 or pin3. Input – output waveforms forms for fOUT= 5fIN. A small capacitor typically

1000pf is connected between pin7 and pin8 to eliminate possible oscillations. Also,

capacitor C2 should be large enough to stabilize the VCO frequency.

51


Fig.1 Frequency synthesizer circuit

14.6. Procedure:

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

2. Measure the free running frequency of PLL (IC565) at pin no.4 with the

input signal set to zero volt.

3. Compare the output with the calculated theoretical value 0.25/RTCT.

4. Set the input signal (say2 Vp-p, 1KHz square wave form) using function

generator.

5. Vary the frequency by adjusting the 20K_ Potentiometer till the PLL (IC565)is locked.

6. Measure (frequency counter) the frequency of the output signal. It must be 5

times the input signal frequency.

7. Observe and note down the waveform and frequency of various signals using

CRO.

52

14.7. Observation Table:

S.No. f i/p (KHz) f o/p (KHz)

1.

2.

TABLE 1: Readings of a Frequency Synthesizer

14.8. Model Wave forms:

Fig.2 Output Wave Forms

14.9. Result:

Frequency Synthesizer is studied.


1. Applications of Frequency Synthesizer.

2. What are the advantages of digital transmission?


1. How to achieve fout = 2 fin ?

2. What is the effect of C1 on the output frequency?

53

15. FREQUENCY DIVISION MULTIPLEXING


1. What is FDM?

2. In what situation multiplexing is used?

3. Distinguish between the two basic multiplexing techniques.

15.2. Objective:

To construct the frequency division multiplexing and demultiplexing circuit and

to verify its operation.

15.3. Resources:

1. Function Generator 1MHz

2. RPS 0-30v

3. CRO 0-30MHz

4. CRO Probes and Patch cards

15.4.Theory:

When several communications channels are between the two same point’s

significant economics may be realized by sending all the messages on one transmission

facility a process called multiplexing. Applications of multiplexing range from the vital,

if prosaic, telephone networks to the glamour of FM stereo and space probe telemetry

system. There are two basic multiplexing techniques

1. Frequency Division Multiplexing (FDM)

2. Time Division Multiplexing (TDM)

The principle of the frequency division multiplexing is that several input

messages individually modulate the subcarriers fc1, fc2 etc., after passing through LPFs

to limit the message bandwidth. We show the subcarrier modulation as SSB, and it often

is; but any of the CW modulation techniques could be employed or a Mixture of them.

54

The modulated signals are then summoned to produce the baseband signal with the

spectrumXb9f), the designation “baseband” is used here to indicate that the final carrier

modulation has not yet taken place.

The major practical problem of FDM is cross talks, the unwanted coupling of one

message into another. Intelligible cross talk arises primarily because of non linearity’s in

the system, which cause 1 message signal to appear as modulation on subcarrier.

Consequently, standard practice calls for negative feedback to minimize amplifier non

linearity in FDM systems


15.6. Procedure:

1. Connections are given as per the circuit diagram.

2. The FSK signals are obtained with two different frequency pair with two

different FSK generators.

3. The 2 signals are fed to op-amp which performs adder operation.

4. The filter is designed in such a way that low frequency signal is passed through

the HPF.

5. Fixed signal is obtained will be equal to the one signal obtained from FSK

modulator.

55

15.7. Tabular Column:

15.8. Result:

Observed the Frequency multiplexing and demultiplexing process.


1. Why guard bands are used in FDM?

2. What is the difference between Frequency Division Multiplexing and Wave

Division Multiplexing?


1. What is Orthogonal Frequency Division Multiplexing?

2. Will multiplexing create additional harmonics in the system?

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