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GF’s Godavari College of Engineering, Jalgaon.

Communication system-1 Lab Manual

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Dept. of Electronics & Telecommunication Engineering

Lab Manual

Communication System-I SE SEM I

Prepared by R.V.Patil(Asso.Prof.)



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LAB COURSE OUTLINE

Course Title Short Title & Course Code

Communication System-I CS-I

Course Description:

In this laboratory course emphasis is on the understanding of need of modulation and

demodulation and their uses.

Laboratory

Hours/Week No. Of Weeks Total Hours Semester Credits

2 14 28 1

Total Semester Credits: 1 Prerequisite Course(s): Analog signal and its fundamentals.

LAB COURSE CONTENT

(Note: Minimum FOUR Experiments from each group.)

Group A 1. Study of AM transmitter and calculate of modulation index of AM wave by envelope

method. a. Sketch and recognize the resulting waveforms for a sinusoidal carrier being

amplitude modulated by a single frequency audio signal.

b. Draw and analyze graphs to show the resulting waveform, and frequency

spectrum for a sinusoidal carrier amplitude modulated by an audio signal, to a

given depth of modulation, m;

c. Select and use the formula:

To calculate the depth of modulation for given amplitude modulated RF signal.

2. Analyze and generate A.M. Demodulation signal by diode detector.

a. Generate AM modulated wave form. b. Apply Modulated AM signal to demodulator.

c. Observe clipping effect .

d. Compare original modulating signal with demodulated output.



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3. Study of FM and calculate of modulation index of FM wave. a. Generate FM waveform.

b. Calculate Modulation Index.

c. Compare over with A.M. modulation.

4. F.M. Demodulation (Phase discriminator/Ratio detector method.) a. Generate FM modulated wave form.

b. Apply Modulated FM signal to demodulator.

c. Compare original output with demodulated output.

d. Plot S-curve

5. To Construct and Verify Pre-emphasis and De-emphasis and Plot the Waveforms.

a. Apply the sinusoidal signal as input signal to pre emphasis circuit.

b. By increasing the input signal frequency observe the output voltage and calculate gain.

c. Plot the graph between gain Vs frequency.

d. Repeat same procedure for de-emphasis circuit.

6. Study of Amplitude limiter circuit. a. Apply sinusoidal signal.

b. Find out limiting range of applied input signal.

c. Draw the graph for same and discussed about result.

Group B

1. Calculate gain for RF / IF stage with AGC and without AGC. a. Explain concept regarding with and without AGC. b. Calculate gain of RF/IF stages with AGC.

c. Calculate gain of RF/IF stages without AGC

2. DSB-SC signal generation using balanced modulator.

a. Verified Balance modulator.

3. Analyze voltage and waveform at various stages/points in A.M. radio

receiver (i.e. Super-heterodyne Radio Receiver).

a. Identify the different stages and write down the information about the individual stage.

b. Observation may be any available information such as number, value, type or any other

indication.

c. Observed and draw waveform of various stages.

d. Analyze signal each points.



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4. PAM modulator & demodulator. a. Generate pulse amplitude modulated waveform.

b. Observed waveform and made calculation.

c. Detection of modulated waveform.

d. Observed demodulated PAM waveform compute information.

5. PWM modulator & demodulator.

a. Generate pulse width modulated waveform.



d. Observed demodulated PWM waveform compute information.

6. PPM modulator & demodulator. a. Generate pulse position modulated waveform.



d. Observed demodulated PPM waveform compute information.

Reference Books:

1. G. Kennedy, B. Davis, “Electronic Communication Systems”, Tata McGraw Hill Edition, 4th

Edition, 1999.

2. H. Taub, D. L. Schilling and G. Saha, “Principles of Communication Systems”, Tata McGraw

Hill Edition, 3 rd Edition, 2012.

3. S. Kundu, “Analog and Digital Communication”, Pearson, ISBN 978-81-317-3187-1.

4. D. Roddy, J. Coolen, “Electronic Communications”, Pearson, 4th Edition, 2011.

Guide lines for ICA:

ICA shall be based on continuous evaluation of student performance throughout semester and practical

assignment submitted by the student in the form of journal.

Guide lines for ESE:

ESE will be based on practical assignment submitted by the student in the form of journal. In ESE the

student may be asked to perform any one practical out of 8. Evaluation will be based on paper work and

performance in the practical.



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Experiments list according to NMU Syllabus.

INDEX

Sr. No. Name of Experiment Page No.

1 Study of AM transmitter and calculate of modulation index of AM wave by envelope method.

2 Analyze and generate A.M. Demodulation signal by diode detector.

3 Study of FM and calculate of modulation index of FM wave.

4 F.M. Demodulation (Phase discriminator/Ratio detector method.)

5 Calculate gain for RF / IF stage with AGC and without AGC.

6 Analyze voltage and waveform at various stages/points in A.M. radio receiver (i.e. Super-heterodyne Radio Receiver).

7 PAM modulator & demodulator.

8 PWM modulator & demodulator.

9 PPM modulator & demodulator.



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Evaluation Sheet

Sr. No. Name of Experiment

Date of Performance

Date of Checking

Grade/ Marks Remark

1 Study of AM transmitter and calculate of modulation index of AM wave by envelope method.

2 Analyze and generate A.M. Demodulation signal by diode detector.

3 Study of FM and calculate of modulation index of FM wave.

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F.M.Demodulation. (Phase discriminator.)

5 Calculate gain for RF / IF stage with AGC and without AGC.

6 Analyze voltage and waveform at various stages/points in A.M. radio receiver (i.e. Super-heterodyne Radio Receiver)

7 PAM modulator & demodulator.

8 PWM modulator & demodulator.

9 PPM modulator & demodulator.

Total Marks

Average Marks



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Experiment No.-1 Date:

Aim: - Study of AM transmitter and calculate of modulation index of AM wave by

envelope method.

Objective:-

a. Sketch and recognize the resulting waveforms for a sinusoidal carrier being amplitude

modulated by a single frequency audio signal.

b. Draw and analyze graphs to show the resulting waveform, and frequency spectrum for a

sinusoidal carrier amplitude modulated by an audio signal, to a given depth of modulation, m;

c.Select and use the formula:

To calculate the depth of modulation for given amplitude modulated RF signal.

Apparatus: - Expt. Kit, function generator, dual channel CRO, probes etc.

Theory:-

Radio transmitter and other electronics system would be impossible without modulation. It

refers to low frequency signal controlling the amplitude, (Keeping frequency or phase Constant) of

high frequency signal such type of modulation is called as amplitude modulation. The high frequency

is called carrier & the low frequency signal is the modulating signal. Hundreds of carrier cycle

normally occurs during one cycle of modulating signal. For this reason, an AM wave forms on an CRO

look like the signal of figure (4.1)

When low frequency signal controls the amplitude of high frequency signal we get Amplitude

Modulation (AM). Let modulating voltage is given by

vm= Vm cosωmt

Where ωm is the angular frequency of modulating signal. And Vm is the amplitude of modulating signal. The

carrier voltage is given by

Vc = Vc cosωct

Where ωc is the angular frequency of modulating signal. The amplitude of carrier is no longer constant but

varies with time as given below

V(t) = Vc +ka.Vmcosωmt

Where ka.Vmcosωmt is the change in carrier voltage is given by



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V = V(t) cos ωct

V= Vc[ 1+( ka.Vm/ Vc) cosωm] .cos ωct

V= Vc[1+macosωmt] . cosωct

Where ‘ma’ is the modulation index or modulating factor is given by

ma = ka. Vm/Vc



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Percent Modulation:-

Amplitude Modulated waveforms has maximum peak to peak value of 2Vmax and minimum

peak to peak value of 2Vmin. The modulation coefficient is given by

2Vmax/2 – 2Vmin/2

ma = ---------------------------------

2Vmax/2 + 2Vmin/2

Vmax – Vmin

= ------------------------

Vmax + Vmin

Vmax – Vmin

% m = -------------------- x 100

Vmax + Vmin

The circuit provided on the front panel of kit generates AM waveforms as shown in figure. External

AF (audio frequency) I/P signal called the modulating signal is applied at AF I/P points provided on



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the kit .R1C1 from a low pass filter , RFC & C2 passes only modulating signal to the emitter of transistor

Q by adjusting the port of the amplitude modulated wave can be control called modulation index

control R4. R5 provides the base biasing voltage for the transistor. Capacitor C3 by pass if AC signal

appear at base of transistor. The carrier signal generated internally by collector of transistor &

capacitor C4 & C5. The capacitor C5 is used to impose the modulating signal on carrier by making a

positive feedback between collector and emitter. The oscillations are generated resulting in

amplitude modulated output. This amplitude modulated output to the AM demodulated section for

getting original modulating signal. The modulation index can be varied by changing the amplitude of

modulating AF input.

Experimental Kit Circuit Diagram:-



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Procedure:-

1] Study the circuit Amplitude modulation provided on the front panel of the kit.

2] Connect the CRO at the output to measure the carrier signal without applying

modulating signal.

Frequency, Fc=---------------Hz. Amplitude, Vc=--------------mV-

3] Apply the sine wave AF I/p at marked.

4] Adjust the amplitude of AF I/P and adjust potentiometer to get the amplitude o/p on CRO.

5] Vary the modulation index from signal generator.

6] For different amplitude of modulating voltage note down the modulation index by equation.

Vmax – Vmin

ma= --------------------------

Vmax + Vmin

7] Vary input frequency and amplitude. Note the corresponding change in modulation index.

8] Draw all observes waveforms on graph paper.

Observation Table:-

A] Constant Frequency of AF input, fm=---------------Hz.

Amplitude of

AF input

Vmax

( AM output)

Vmin

( AM Output)

Modulation Index

M= ( Vmax- Vmin )/

( Vmax+ Vmin )

Modulation Index

M=Vm/Vc



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B] Constant Amplitude of AF input, Vm=--------------V

Frequency of

AF input

Vmax of AM

Output

Vmin of AM

Output

Modulation Index

(m)

Result:-

Conclusion:-

Applications:-

Oral Based Questions.

Q:1 What is Amplitude Modulation ?

Q:2 What is the frequency range of AM ?

Q:3 What is modulation index and it’s unit ?

Q:4 What is the B.W. of AM ?

Q:5 What is the value of IF for AM ?



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Experiment No.:-2 Date:-

Aim: - Analyze and generate A.M. Demodulation signal by diode detector.

Objective:-

a. Generate AM modulated wave form. b .Apply Modulated AM signal to demodulator.

c .Observe clipping effect .

d. Compare original modulating signal with demodulated output.

Apparatus:-Amplitude Demodulation kit, C.R.O. function generator, probes.

Theory:

AM broadcast signals use carrier frequencies between 540 and 1600kHz. In the studio, an audio

signal modulates the carrier to produce an AM signal. A transmitting antenna of suitable length then

radiated this AM signals into space. Miles away, a receiving antenna picks up the modulated RF

signal. After being amplified, this signal is demodulated (the audio is recovered). The most common

circuit in use is the envelope detector which consists of detector diode as shown in figure 1.

In figure2, the peaks of the amplitude modulated (AM) I/p signals are detected to recover the upper

envelope. For this reason the circuit is called an envelope detector if detector diode is connected as

shown in figure(D=D1). During each carrier cycle the diode turns ON briefly and charges the capacitor

to peak voltage of the particular carrier cycle between peaks, the capacitor discharges through

resistor by making RC time constant much greater than the period of the carrier, we get only a slight

discharge between cycles. In this way most of the carrier signal is removed. The output then looks

like the upper envelope with a small ripple as shown in figure1.we can remove the small RF ripple

that remains on the detected signal. After removing the carriers the circuit provides Demodulated



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O/P which is same as the modulating signal. By changing capacitor C1 or C2 the ripple contains in

O/P can change. Through C0 demodulated O/P without DC component (AGC voltages is obtained).


Procedure:-

(1) Study the circuit provided on the front panel of the kit.

(2) Connect CRO at O/P terminals. Apply AM I/P, observe & note rectified AM O/P without

connecting capacitors.

(3) Now connect capacitor C1 in the circuit. Note the frequency & amplitude of demodulated O/P

Vo. Compare this with modulating I/P. The frequency of demodulated & modulating signal

will be same.

(4) Repeat the above steps (3) for the capacitor C2 in the circuit.



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Observation Table:-

Sr noSS Modulating Signal Demodulating Signal Capacitor

Attached Amplitude Frequency Amplitude Frequency

1

2

Result:-

Conclusion:-

Oral based Questions:-

Q:1 What is the use of LPF in AM Detector ?

Q:2 Which type of rectifier is used in AM detector?

Q:3 What are the types of AM detector ?

Q:4 What are the drawbacks of simple diode detector ?

Q:5 Give the significance of AGC in practical diode detector ?



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Experiment No.-3 Date:-

Aim:- Study of FM and calculate of modulation index of FM wave.

Objective:

a. Generate FM waveform.

b .Calculate Modulation Index.

c .Compare over with A.M. modulation.

Theory:

In the communication information like audio signal or speech writing signal is to be

transmitted from one point to another. But if it is transmitted directly through antenna then

transmitting antenna must be ¼ wavelengths long. To reduce the antenna size, the low frequency

information (audio) signal is mixed with higher frequency. Such process is called as frequency

modulation (FM). This higher frequency is called as carrier signal.

At the receiver side, this signal is converted back to audio frequency called as demodulation

process.

Type of modulation:

If the characteristics of carrier wave like amplitude, frequency or phase is change in accordance with

the modulating signal (information signal) then such type of modulation is called as amplitude

modulation (AM), Frequency modulation(FM) or phase modulation(PM) respectively.

Frequency modulation:

In this modulating signal ‘em’ is used to vary the carrier signal frequency fc. Let the change in the

carrier frequency be ‘km’ where k is constant known as frequency deviation constant, then the

instantaneous carrier freq is given by

Fi = fc+km

Where fc is the un-modulated carrier frequency

In this the instantaneous frequency of the carrier is caused to vary by an amount proportional to the

amplitude of the modulating signal. The amplitude is kept constant.

More complex than AM this is because it involves minute changes in frequency. FM is more immune

to effects of noise. FM and PM are almost similar.



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Assume we have a carrier at a frequency of 100MHz, called the resting frequency. If a modulating

signal is then applied, this will cause the carrier to shift (deviate) from its resting frequency by a

certain amount. If the amplitude is increased then the amount of deviation also increases. The rate is

proportional to frequency of the intelligence signal. If the signal is removed then the carrier

frequency shifts back to its resting frequency.

This shift in frequency compared with the amplitude of the modulating voltage is called the

deviation ratio.

The deviation ratio is also called the deviation constant and it defines how much the carrier

frequency will change for a given input voltage level. The units are kHz/V

The following equation can be used to represent FM

Where fc is the unmodulated carrier frequency, k is proportionality constant and the last term is the

modulating voltage It will be seen that the maximum deviation will occur when the cosine term is

unity and hence we obtain

maximum deviation,

Figure.1 shows the FM waveform if modulating signal increase in positive direction towards positive

peak, then carrier frequency at O/P (FM) decreases it becomes max at positive peak. If modulating

signal decreases from positive peak towards negative half cycle it increases the carrier frequency at

O/P. When modulating signal returns to 0, the carrier frequency zero at O/P. Therefore the

modulating signal produces a frequency modulated waveform as shown in figure.1

It can be shown that the instantaneous value of the FM voltage is given by

The modulation index for FM is defined as

)cos1( tkVff mc

)1( mc kVff

cm fkV

t

ftAv m

m

c

sinsin

m

ff

m

frequency modulating

deviationfrequency max



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To solve for the frequency components of the FM requires the use of Bessel functions

To evaluate the individual values of J is quite tedious and so tables are used.

This is an approximate method used to predict the required bandwidth necessary for FM

transmission

About 98% of the total power is included in the approximation

Figure.1 : FM waveform

]...}4sin4[sin

]3sin3[sin

]2sin2[sin

]sin[sin

sin{

4

3

2

1

ttmJ

ttmJ

ttmJ

ttmJ

tmJAv

mcmcf

mcmcf

mcmcf

mcmcf

cfo

maxmax2 sfBW



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Observation Table:-

Modulating I/P Signal F.M. O/P Signal

Fm Vm Max.freq

Deviation

m

ff

m

frequency modulating

deviationfrequency max



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Procedure:-

1) Study the circuit diagram provided on the front panel of the kit.

2) Connect dual trace CRO at input and output side

3) Without applying modulating i/p signal observe and note Sinusoidal o/p on CRO (carrier signal)

fc=…………………………Hz,

Vc=……………………….V

4) Connect sine wave i/p of around 100Hz, 10Vp-p from signal generator to the point marked as “AF

i/p“ keep it’s amplitude constant and vary the potentiometer R1 to change amplitude of o/p signal.

5) Adjust the amplitude and frequency of modulating

Vfm=…………………………V

6) Now increase Time/div control of CRO. You will get deviated waveform.

Note its difference which is called frequency deviation ,

7) Find modulation index, m= /fm

8) By Changing position of R1 amplitude of modulating signal will change the amount of frequency

deviation, Note its minimum &maximum frequency deviation.

9) Keep pot R1 at mid position change Freq of modulating signal, observed & note the change in

frequency deviation it should remains const.

Result:-

Conclusion:-

Application:-



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Oral Based Questions

Q.1: What is Frequency Modulation & Give frequency range for FM ?

Q.2: State the different types of FM modulators ?

Q.3: Specify the bandwidth requirement for FM ?

Q.4: Explain Modulation Index & Percent Modulation for FM ?

Q.5:What is Deviation ratio & What is the function of IC-8038 ?



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Experiment No:-4 Date:-

Aim:- F.M. Demodulation (Phase discriminator method.)

Objectives:-

a. Generate FM modulated wave form.

b. Apply Modulated FM signal to demodulator.

c. Compare original output with demodulated output.

d. Plot S-curve

Apparatus:- Experimental kit, CRO, function generator, patch chords.

Theory:-

Modulation is a fundamental requisite of communication systems. It is defined as the process

by which some characteristics of signal called carrier is varied in accordance with instantaneous value

of another signal called modulating signal. The signal containing information to be transmitted are

referred to as modulating signals. The carrier Frequency is greater than the modulating frequency.

The signal resulting from the process of modulation is called modulated signal.

The process of extracting a modulating signal from a frequency modulated carrier is known as

demodulation. Electronic circuits that perform the demodulation process are called FM detectors.

The FM detector performs the extraction in two steps.

1) It converts the frequency modulated signal into a corresponding amplitude modulated signal

by using frequency by using frequency depends on input frequency called frequency

discriminator.

2) The original modulating signal is recovered from this AM signal by using a linear diode envelop

detector.

A simple R-L circuit can be used as discriminator, but this circuit has a poor sensitivity as

compared to a tuned LC circuit. The various types of LC discriminators are there in FM. But

they suffer from the threshold effect in presence of excessive noise. The threshold can be

improved by pre-emphasis, and de-emphasis circuits.

FM discriminator can be divided into two types.

1) Slope detectors &

2) Phase difference discriminators



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In slop detectors the principle of operation depends on the slop of the frequency response

characteristics of a frequency selective network. It is catagorized as

1) Simple slop detector

2) Balanced slop detector.

The circuit consists of an inductively coupled double tuned circuit in which both primary and

secondary is connected to top of the primary through a capacitor C.

This capacitor performs following functions.

1) It blocks dc from primary to secondary.

2) It couples the signal frequency from primary to center trapping of the secondary.

The primary (V3) thus appears across the inductance L. Almost entire V3 appears across L1 except a

small drop across C may be kept negligible.

The center trapping of the secondary has an equal and opposite voltage across each half

winding. Therefore V1 & V2 are equal in magnitude but opposite in phase. The radio frequency

voltages Va1 & Va2 applied to diodes D1 & D2 are given as

Va1 = V3 + V1

Va1 = V3 –V2

Voltages Va1 & Va2 depends on the phasors relation between V1,V2,V3.

The diodes are so arranged that the output voltage Vo is equal to the arithmetic difference

given by Vo = Vo1 – Vo2

Therefore, the voltage Vo will instantaneous frequency in accordance with difference of (Vo1

– Vo2). The detected O/P Vo can also be taken at any one of diode cathode w.r.t to ground.



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Procedure:-

1] Study the circuit provided on the front panel of the kit.

2] Apply the modulating i/p signal to frequency to modulation kit, of around 80Hz- 250Hz, and adjust

its amplitude to get FM Output.

3] Apply the FM o/p signal from frequency modulation kit to the FM input of foster- seeley/phase

shift discriminator kit.

4] Adjust the amplitude & frequency of modulating signal to get the FM demodulated output on the

CRO. The o/p contains noise.

5] Observe the modulating signal applied to the FM kit on the channel of the CRO & the Demodulated

o/p on the other channel.

6] Vary the i/p frequency and amplitude & observe the change in the demodulated o/p.

7] Also observe and note the FM signal converted into AM signal at Various test points.

8] Draw the modulating i/p , carrier , FM O/P & Demodulated o/p on graph paper.



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Observations:-

1) AF Modulating i/p :-

Frequency = ………………………… Amplitude = …………………………

2) Demodulated o/p :-

Frequency = ………………………… Amplitude = …………………………

Conclusion:-


Q:1 Why foster seeley discriminator called as phase shift discriminator ?

Q:2 Why discriminator must be preceded by limiter stage ?

Q:3 Is there any difference between discriminator & demodulator ?

Q:4 What are the advantages & disadvantages of discriminator ?

Q:5 Comment on frequency response of discriminator ?



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Experiment:-5 Date:

Aim:- Calculate gain for IF stage without AGC.

Objective:-

a. Explain concept regarding with and without AGC. b. Calculate gain of IF stages without AGC

c. Plot the graph of gain Vs frequency

Apparatus:- Experimental kit, CRO, patch cords.

Theory:- A radio receiver must perform a number of functions; the receiver must separate a

desired radio signal from all other radio signals that may be picked up by the antenna and reject the

undesired once. Next, the receiver must amplify the desired signal to a usable level. Finally, the

receiver must receives the information from the radio carrier and pass it on to the user.

IF amplifier is one of the critical part in the AM receiver. At this stage must of the gain and

selectivity is obtained. It is a compromise between selectivity and stability, which is best, obtained at

higher frequencies. One of the most common IF values is 455KHz. It is low enough to provide good

selectivity and make high gain with minimum instability.

The IF amplifier are tuned class A amplifier. Usually move than one stage of IF amplifier is

used to get good sensitivity. The o/p of the IF amplifier appears across a tuned transformer ckt. For n

stages of IF amplifier the total no. of IFT needs (n+1).

When the i/p frequency is in the VHF range and beyond, a very high value of IF chosen. Most

FM receiver operating in the 88 to 108 MHz band uses an IF of 10.7 MHz. In many communication

receivers operating at higher frequencies, double conversion is used to solve the image and

selectivity problems. In some high frequency receivers operating in the 3-30 MHz range, single

conversion is used but a very high IF of 9Mhz is chosen as it effectively solve the image problems.

In an IF amplifier the selectivity should not be too sharp. If the IF bandwidth is too narrow, it

would cause side band cutting. This means that the higher modulating frequencies will be greatly

reduced in amplitude thereby distorting the received signal.

In order to widen the band width of the amplifier the over coupled tuned circuits are used

between the two stage of the IF amplifier. The o/p voltage Vs frequency for such coupled circuits is



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strictly dependent upon the amount of coupling or mutual inductance between primary and

secondary windings. That is spacing between the winding s is determine how much of magnetic

produced by primary will cut the turns of secondary. This affects not only the amplitude of o/p vtg,

but also the bandwidth.


Procedure:-

1] Study the circuit provided on the front panel of the kit.

2] Switch ON the power supply connect the CRO at o/p side.



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3] Apply AM i/p of 455KHz from AM generator to the i/p of ckt . Adjust the amplitude so that o/p

should not be in saturation.

4] Keep i/p voltage constant (Vi), vary the AM i/p frequency from 300KHz-1650KHz & note the

corresponding voltages at the o/p of first stage & second stage. Find out the gain Vo/Vin. This will

provide max gain for IF 455KHz (approximate).

5] Connect o/p Vo to detector diode, AGC bias. o/p to AGC bias i/p. Observe & note demodulated o/p

Vo which is modulating signal.

6] Plot the graph AM i/p frequency Vs gain Vo/Vi .

Observation Table:-

AM I/P Vi = ………………….Constant.

AM I/P

IF Frequency

Fi (KHz)

O/P

Voltage

Vo

Gain

Vo/Vi



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Result:-

From the experiment it is concluded that the IF amplifier amplifies those frequencies for

which it is tuned and rejects the unwanted frequencies. Also it is observed that the o/p voltage is

maximum for the tuned IF frequencies.


Q:1 How IF is generated ?

Q:2 Give the different values of IF for different receivers ?

Q:3 What is the role of AGC in IF amplifier ?

Q:4 Draw & Comment on IF band pass characteristic of IF amplifier ?

Q:5 Draw & Comment on Ideal response of IF amplifier ?



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Experiment No. : 06(A) Date :

Aim:- Analyze voltage and waveform at various stages/points in A.M. radio

receiver (i.e. Super-heterodyne Radio Receiver).

Objective:



indication.



Apparatus: Digital multimeter for voltage at different point, single trace CRO for observing waveforms, CRO

probes.

Theory: Transmission of radio signal

Radio waves propagate with different modes in atmosphere. We very some of its property in with

information so by catching those radio waves information can be extracted e.g in AM amplitude of

radio carrier is varied in accordance with information and in FM, frequency of carrier is varied

accordingly.

Different stations are operating on different carrier frequencies. Therefore, they can be

separately received.

Signal reception: Antenna which emits radio energy into atmosphere can also absorb energy from

atmosphere. So in the receiver we use antenna. Now this antenna is tuned to a particular frequency

using variable capacitor across it called gang. So we can get one prominent ratio carrier at a time.

Now this signal is very weak, it is amplified by tuned amplifiers and detected.

Super heterodyning Principle: Super heterodyning means mixing. There is no. of different carrier

Frequencies e.g 550 KHz to 1600KHz.By means of antenna we can stick to one carrier & extract



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information out of it. But then forgoing amplifiers are required to work over a large frequency span,

which is not possible. Over this span, characteristic of amplifying devices change and we will not get

required fidelity. To avoid the difficulty all these amplifiers are made to work on single frequency

called intermediate frequency [IF]. So better fidelity results. To do this antenna signal is mixed with

local oscillator signal and resulting signal frequency is held fixed, equal to IF . To achieve this local

oscillator frequency is also varied by gang so that antenna frequency and its frequency maintain fixed

frequency difference, equal to IF. This IF has typical values like 455 KHz, 550 KHz etc.

So each station is converted into IF using mixer and amplified signal is passed through detector

and we get information output. In our kit we have given AM generator with variable carrier freq.

stations. Modulating signal is variable. Since at a particular time modulating signal is of fixed

frequency set by operator, we can observe different stages in receiver for our required signal.

Here we connect our AM signal with patch cord to antenna, or even without actual connection

signal can be observed but power is less, so to observe signal clearly we connect it with patch cord.

Actually AM signal at the input of receiver required is less, but to observe it properly we have given

strong AM signals and in receiver we have provided potential dividers i.e. deliberately we have

reduced the gain of our receiver. Due to this we cannot hear weak commercial stations in our

receiver.

Procedure:

1] Connect mains cord of A.M to the 230/50Hz as supply.

2] Switch on the unit.

2] Tune the Radio receiver to any Station.(i.e .Jalgaon at 988KHz.)

3] Observe, Note the o/p(Frequency, Amplitude) at the R.F. Amplifier Stage ,Local oscillator stage,

I.F. stage, Mixer stage, Detector stage, Audio Amplifier stage.

4] Draw the waveforms observed at different points on kit



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Sensitivity test

1) Switch on the power supply.

2) Adjust Frequency of AM gen 625 KHz using carrier freq. adjust pots. Two pots are provided, one

for coarse adjustment (left side) & other for fine adjustment (right side).

3) Adjust freq of signal gen. to 400 Hz , & amplitude 1Vpp & connect it to signal i/p of AM

gen.Keep AM signal o/p to maximum amplitude using amplitude adjust pot.& adjust AM signal

such that we get 30% modulation index. For that purpose use DC bias pot of AM gen. block.

4) Adjust amplitude of AM signal 200mV p-p & connect it to I/P point of receiver.

5) At receiver to types of power amplifiers are used.

a) IC LM380 based power amplifier

b) Pushpull amplifier.

Keep both toggle S/Ws to left most position.i.e LM380 power amplifier is used.

6) Observe oscillator freq. at a point provided at right hand lower corner of PCB. Vary gang capacitor

to observe maximum & minimum freq. Note the direction.

7) Now adjust gang position such that we get audio o/p at o/p points of receiver. Keep audio pot at

maximum position.

8) Now sensitivity of receiver is defined as i/p signal required getting 50 mW o/p powers, when loud

speaker is replaced by equivalent resistance. We have used 8 ohm resistance. Corresponding to

that we required 1.78 Vp-p at o/p.

9) Here very fine adjustment of gang knob & AM o/p knob is required.

10) Adjust AM o/p knob such that we get o/p 1.78 Vp-p.



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11) Verify that station is properly tuned using gang knob.Measure AM signal amplitude.

11) Now Adjust AM gen. Carrier freq. to 1MHz. & repeat the procedure as above.

12) Adjust AM gen. carrier freq to 1400 KHz & repeat the procedure as above.

13) Plot the Graph of AM i/p signal required for 50mW o/p power Vs AM station frequency.

This is the sensitivity curve for given receiver.

Selectivity Test:

1) As above adjust AM o/p (carrier freq = 1MHz, modulation index = 30%)

2) Connect AM o/p to i/p of receiver.

3) Adjust gang knob & AM strength such that we get audio o/p 1.78 Vp-p corresponding to 50 mW

o/p power.

4) Measure carrier freq. using freq. counter.

5) Now slightly change the carrier freq. Using fine adjustment knob & measure o/p of receiver.

6) Take two to three readings for carrier freq. lower than 1MHz & two to three readings for carrier

freq. higher than 1MHz.

7) Plot the graph of o/p Vs carrier freq. This is the selectivity curve of receiver.

Fidelity Test:

1) Adjust AM o/p as in case of selectivity test.

2) Adjust gang knob & AM o/p strength such that we get o/p of receiver.

3) Now don’t change final AM o/p strength.

4) Now vary audio freq from minimum to maximum & each time measure audio O/P . Tabulate the

readings. Plot the graph of o/p Vs freq. of signal generator. This the fidelity curve.

5) Now Keep both toggle S/Ws to rightmost position i.e. pushpull power amplifier is used.



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6) Repeat step 1 to 4. Plot fidelity curve for pushpull amplifier.

Circuit Diagram:

65K

47

K

4.7K

2ND

IFT

I/P

220K

BF 195

3.3K

1UF 0.047UF

100K

3rd IFT

[WHILE]

OA

79

0.01

0.01

4K7

1K

BF 195

200K

BC

1480.022UF

220

8550

8550

5V

100UF

8 ohm

SPEAKA

R100UF

330

220

330

POWER AMPLIFIER

2ND IFT

Result:

Oral based Questions:

1. What is roll of RF amplifier in Radio Receiver?

2. What is roll of Local Oscillator in Radio Receiver?

3. What is roll of IF stage in Radio Receiver?

4. What is roll of Mixer stage in Radio Receiver?

5. What is roll of Audio amplifier in Radio Receiver?



35

Experiment No. : 06(B) Date :

Aim:- Analyze voltage and waveform at various stages/points in F.M. radio

receiverceiver).

Objective:



indication.



Apparatus: Digital multimeter for voltage at different point, D.S.O.(200MHz) for observing waveforms, CRO

probes, F.M receiver trainer kit,etc.

Theory:

There are two types of analog system is used,

1) F.M transmitter when function of carrier is charged in accordance with modulating signal.

2) A.M transmitter where amplitude amplr of carrier is charge in accordance with the modulating signal.

Where we will discuss about freq. modulating analog transmission system. The F.M receiver is

superhetrodyne receiver & the dig. In panel shows, first how similar it is to the A.M receiver however the basic

differences are as follows,

1] Generally much higher operating freq. in F.M.

2] Hence for timing & deemphasize is F.M.

3] Need for timing & deemphasize in F.M.

4] Totally different match of demodulator.

5] Different match of obtaining AGC.



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Common circuit with an A.M receiver:

A no. of section of the F.M receiver corresponds exactly to the use of others types of receiver.

R.F amplifier:

R.F amplifier is always used is on F.M amplifier receiver. The main purpose is to reduce noise fig.

Control otherwise be a problem, because of large needs of an F.M.

Frequency channels:

The oscillator ckt takes of usual forms with the colpits & clap freq. Dominant being suited to VHF.

Operation taking is normally such a non-problem in F.M broad cast receiver. This is because the tuning freq.

Range is unity 12.5% much than in A.M broad casting.

Intermediate freq. & R.F amplifier:

Again the type & operation D.C not differ from their A.M counter parts. It is worth

nothing however that if & switch required are higher than in A.M broad cast receiver operating in the 88 to

108 MHz.

Amplitude limiting:

In order to make the use of the advantages offered by a demodulator must be proceeded by amplifier

amplitude changed in the signal fed to the F.M and demodulator.

Basic F.M demodulator:

The function of a freq. FM demodulator is to change the freq. Deviation of the incoming carrier into an

A.M amplifier variation.

About trainer:

In this IM transmitter trainer a transistorized IM transmitter on 88 to 188 MHz band is provided to

obtain GND. Range & clarity in receiver IC base function generator is provided as built in further to enable

students to carry out the measurement with a transistorized.

Procedure:



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1] Connect mains cord of F.M transmitter to the 230/50Hz as supply switch off the unit.

2] Tune the Radio receiver to Jalgaon Station.i.e.106.4MHz.

3] Observe, Note & Draw the o/p (Frequency, Amplitude) at the R.F. Amplifier Stage,

Local oscillator stage, I.F. stage, Mixer stage, Discriminator stage, Audio Amplifier stage.

Conclusion:

Signature with Date

Subject In charge



38

Experiment No. : 07 Date :

Aim:- Study of PAM and Demodulation Technique.

Objective: a. Study of sampling, effect of variable sampling rate

b. Observation of Flat top and Natural sampling.

c. Reconstruction of original signal using filter

d. Observed and draw waveform of various stages.

Apparatus: Experimental kit, DSO or CRO, connecting wires, CRO probe.



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Theory: According to sampling theorem, sampling rate should be rapid enough so that at least two samples

are taken during the course of the period corresponding to the highest frequency spectral component. i.e. if our signal frequency is 1 kHz then sampling frequency must be 2 kHz minimum for proper reproduction of original signal from sampled signal. It is called as Nyquist criteria.

In this Kit we have provided two different types of sampling

1] Natural Sampling.

2] Flat Top Sampling.

1] Natural Sampling: - In this case the sampled signal consists of sequence of pulses of varying amplitude whose tops are not flat but follow the waveform of I/p signal.

2] Flat Top Sampling: - A Flat topped pulse has constant amplitude established by the sample value of the signal at some point within the interval. We have arbitrarily sampled the signal at the beginning of the sampling pulse. To generate flat top sample the signal is held using a holding capacitor after sampling pulse.

After holding time the capacitor is dumped using a dump pulse. So that between sampling pulse & dump pulse we get a fiat topped sample pulse.

In this kit we have provided 1 kHz sine wave & crystal clock to observe stable waveforms on normal oscilloscope. To verify sampling theorem you can vary freq of i/p signal from minimum to maximum & observe reconstructed o/p. OR IC 555 clock is given. By varying basic clock freq. sampling frequency can be varied & its effect on reconstructed signal can be observed.

Also at receiver side effect of value of Holding capacitor (sample & hold ckt.) o reconstructed signal can be observed.

Procedure:-

1. Switch on the power supply.

2. Check the oscillator frequency & Input frequency given to Binary counter given at Pin no.2.

3. Check 1KHZ sin frequency at Pin no. 12

4.Check different frequencies at Pin no 3 To 8 (320K,160K,80K,40K,20K,10K Respectively)

5. Check frequency at Pin no 32

6. Through the selector switch to the internal position .So that internal frequency will become sampling freq.

7. Give 1KHZ sine signal as sine input at Pin no 34

8. Check the sampled output / sampled & hold output at different sampling frequency.

9. Give the input to second order LPF& check the response of output.

10. Similarly used fourth order LPF& check the response of output.



40

Result:- Thus we verified PAM and demodulation.


1. What are the types of Pulse modulation?

2. Explain advantages and applications of Pulse Modulation

3. Explain aliasing.



41


Aim:- Study of PWM and Demodulation Technique.

Objective: a. Study of timer IC 555 .

b. Study of monostable and astable multivibrator.


Apparatus: Experimental kit, Bread bord, IC555,Registers, Signal Generator,

Capacitor, DSO or CRO, connecting wires, CRO probe.

Theory: If we apply variable voltage at control input (pin 5) of IC555 in astable mode then the width of output

pulse will change accordingly. When we apply low frequency audio signal to the control input, internally

the control voltage will change the reference voltage of comparator so charging time of capacitor changes

every time and so width of o/p pulse also changes. So here as shown in figure the audio signal is applied at the

control input through positive clamper circuit. The values of RA, RL and C are chosen to generate pulse train of

around 64 KHz. The width of o/p pulse will change in accordance with the amplitude of audio signal as shown

in figure . Ra = 3k ,c = 0.02 micro farad, Rl = 1k



42

This pulse width modulator circuit need external pulse train input to trigger the the 555 monostable circuit. This trigger input sould be a square wave signal with a fixed frequency. The output of this pulse width modulator circuit will have same frequency with the trigger input, but with its pulse width proportional to the modulating input signal at pin 5.

Pin No Function Name

1 Ground (0V) Ground

2 Voltage below 1/3 Vcc to trigger the pulse Trigger

3 Pulsating output Output

4 Active low; interrupts the timing interval at Output Reset

5 Provides access to the internal voltage divider; default 2/3 Vcc

Control Voltage

6 The pulse ends when the voltage is greater than Control Threshold

7 Open collector output; to discharge the capacitor Discharge

8 Supply voltage; 5V (4.5V - 16 V) Vcc

Procedure:-

1 Make the connection as per the circuit Diagram.

2 Connect 6 volt DC power supply to Vcc.

3 give the 1kHz,2v square wave signal to pin2

4 connect modulating i/p (sine wave with 500Hz, 1v)in between pin 1 and 5

5 Observe o/p at pin3.

Result:-




43


Aim:- Study of PPM and Demodulation Technique.

Objective: a. Study of timer IC 555 .

b. Study of monostable and astable multivibrator.


Apparatus: Experimental kit, Bread bord, IC555,Registers, Signal Generator, Capacitor,

DSO or CRO, connecting wires, CRO probe.

Theory: PPM is also sometimes known as pulse-phase modulation

• The amplitude and width of the pulse is kept constant in PPM • The position of each pulse, in relation to the position of a recurrent reference pulse, is varied by each

instantaneous sampled value of the modulating wave • It often used in optical communication, such as fiber optics, in which there is little multipath way

interference • It is used exclusively for transferring digital signals and cannot be used with analog systems • It is used for transferring simple data and is not effective at transferring files.

Pulse position modulation is similar to pulse width modulation, but the frequency is not constant. Like pulse width modulator circuit, pulse position modulator can be easily constructed using 555 IC. If you look at the output signal plot in the above timing chart, and compare it with our previous pulse width modulation, you can find the key difference: the pulse width modulator modulate the active factor (the duty cycle), t(on)/(t(on)+t(off)), without changing the total period of t(on)+t(off), while pulse position modulator modulate the t(on) and keep t(off) fixed. You can see that in the pulse-position-modulation, the frequency of the pulse is modulated as well as the duty cycle.

PPM is simply PWM + monostable multi-vibrator. Means if we fed the PWM output to trigger input of IC555 in monostable mode then the output of monostable will be PPM wave. So to generate PPM we require one astable multi-vibrator to generate PWM signal and one monostable multi-vibrator to get desire PPM. The final circuit and the waveforms are as shown in the diagram.

http://www.simplecircuitdiagram.com/2009/05/20/pulse-width-modulator-using-555-ic/





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Procedure:

1 Make the connection as per the circuit Diagram.

2 Connect 6 volt DC power supply to Vcc.

3 give the 1kHz,2v square wave signal to pin2

4 connect modulating i/p (sine wave with 500Hz, 1v)in between pin 1 and 5

5 Observe o/p at pin3 ie PWM pulses.

6 connect the o/p to pin2 of second timer IC.

7 Observe the o/p at pin 3 of second timer IC.

Result:

Documents

Lab Manual - godavaricoejal.ac.in · amplitude modulated by a single frequency audio signal. b. Draw and analyze graphs to show the resulting waveform, and frequency ... Generate