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EXP: 3 SPECTRUM ANALYSIS OF DSB-SCSIGNAL USING

SPECTRUM ANALYZER

AIM : To observe the frequency spectrum of DSB-SC signal by using a Spectrum Analyser

and Calculate Resuidal carrier power, Side bands power, Harmonicpower of a DSB-SC signal

APPARATUS:

1. CRO

2. Function generator- 2No's

3. Spectrum Analyzer

4. Balanced Modulator kit

THEORY :

The spectrum analyzer is an important and useful tool for examining signals in the

frequency domain. The spectrum analyzer can be thought of as a band pass filter whose center

frequency is varied linearly over a range of frequencies. The center frequency of the filter is

plotted on the horizontal axis and the output of the filter is plotted on the vertical axis. The filter

must be swept over the frequency range slowly enough so we obtain the steady state filter output.

The internal circuitry of the analyzer automatically changes the sweep rate as the filters

bandwidth is changed to insure this condition is met. There is a default setting for the filter

bandwidth, but it can be changed using the front panel controls. A wide bandwidth allows a more

rapid sweep, but a narrow bandwidth is necessary to resolve closely spaced signals. Suppose for

example that we wanted to observe a signal that consisted of two sine waves, one at 500 kHz and

one at 505 kHz. If we had a filter bandwidth of 30 kHz we would not be able to resolve them as

two separate signals, they would appear as one. However, if we reduced the bandwidth to 1 KHz

we would be able to resolve them into two distinct signals, and be able to measure their

frequency separation and their individual amplitudes.

In operating the spectrum analyzer there are three principal parameters that the operator should

set. These are, center frequency, span and amplitude. The center frequency setting determines the

frequency that corresponds to the middle of the screen. The span determines the range of

frequencies from one side of the screen to the other. For example, if we set the center frequency

to 10MHz and the span to 2 MHz, the screen would cover the range 9 MHz to 11 MHz and any

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signals in that frequency range applied to the input would be seen on the screen. The amplitude

can be set to either the log or linear mode. The log mode is useful for looking at two or more

signals that have large differences in amplitude. In this mode the scale is calibrated in

dB/division, with a given reference value corresponding to the top line. In the linear mode, which

is good for observing small differences in amplitude, the top line can be set to correspond to a

given voltage

BLOCK DIAGRAM

EXPECTED WAVEFORMS(TIME DOMAIN):

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PROCEDURE

1. Apply an AF signal (message signal) with an frequency of 100KHZ, from

function generator to the kit

2. Apply an RF signal (carrier signal) with an frequency of 1MHZ, from function

generator to the kit

4. Switch on the trianer kit.

3. Observe the output signal(modulated signal ,DSB-SC) with the help CRO.

4. Now observe the frequency spectrum of DSB-SC signal with the help

frequency spectrum.

5. Keep the centre frequency at 1MHZ, span at 1MHZ.

6. Place the maker on the spectrum and note down the frequencies & power of

respective components.

7. Calculate the ratio of signal power to the resuidal carrier.

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8. Calculate the ratio of signal power to the spurious in Harmonic power

SAMPLE READINGS:

Readings for AM signal

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

(MHz)

(fc - fm)

(MHz)

1 1.5 10 1.51 1.49

MODEL GRAPHS:

OBSERVATIONS:

Parameters Amplitude(CRO) Frequency Power(Dbm)

AF Signal

RF Signal

Modulated

Signal(LSB)

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Modulated

Signal(USB)

PRECAUTIONS:

1. Check the probe connections.2. Observations should be done carefully

INFERENCES:

Spectrum Analyzer can be used to measure bandwidth and output power of

modulated signals.

EXP -5 SINGLE SIDE BAND SYSTE

AIM:

1. To study about the single side band system with modulation and demodulations.2. Observe the conditions of summer (Fx + Fy) and subtractor (FxFy).

APPARATUS:

1. Single side band system trainer kit.2. Cathode Ray Oscilloscope.

3. Digital multi meter.

4. Connecting wires & probes.

THEORY:

Single side band signal generation using Phase shift method and demodulation of SSB signal

using Synchronous detector.

This exp consists of1. R.F generator.2. A.F generator.3. Two balanced modulators.4. Synchronous

detector5. Summer6. Subtractor

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1.RF generator: Colpitts oscillator using FET is used here to generate RF signal of approximately

100KHz frequency to use as carrier signal in this experiment. Phase shift network is included in the

same block to produce another carrier signal of same frequency with 900 out of phase. An

individual controls are provided to vary the output voltage. Facility is provided to adjust phase of the

output signal.

2. AF generator:

This is a sine cosine generator using OP-OMP. IC TL 084 is used as an active component;

TL084 is a FET input general purpose quad op-amp integrated circuit. A three position switch is

provided to select output frequency . An individual controls are provided to vary the output

voltage.AGC control is provided to adjust the signal shape.

3. Balanced Modulator:

This has been developed using MC 1496 IC, is a monolithic integrated circuit Balanced

modulator/demodulator, is versatile and can be used up to 200MHz. These modulators are used in

this experiment to produce DSB_SC signals. Control is provided to balance the output.

4. Summer and Sub tractors:

These circuits are simple summing and subtracting amplifiers using OP-AMP. IC TL084 is

used as an active component, TL 084 is a FET input general purpose quad OP-AMP integrated

circuit. The phase shift method makes use of two balanced modulators and two phase shift

networks as shown in figure. One of the modulators receives the carrier signal shifted by 900 and

the modulating signal with 00 (sine ) phase shift, where as the other receives modulating signal

shifted by 900 ( cosine ) and the carrier ( RF) signal with 00phase shift voltage.

Both modulators produce an output consisting only of sidebands. It will be shown

that both upper sidebands leads the reference voltage by 900, and the other lags it by 900. The

two lower side bands are thus out of phase and when combined in the adder, they cancel each

other. The upper side bands are in phase at the adder and therefore they add together and give

SSB upper side band signal. When they combined in the subtractor, the upper side bands are

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canceling because in phase and lower side bands add together and gives SSB lower side band

signal

CIRCUIT DIAGRAM:

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PROCEDURE:

1. Switch on the experimental board.

2. Observe the output of the RF generator using the CRO. There are two outputs from theRF generator, one is direct output and another one is 90 phase shift with the direct

output.

3. Connect RF output 0 to the CH1 and RF output 90 to the CH2 of the CRO andobserve the phase difference between the two signals.

4. The output signal frequency of the RF generator is100 KHz with 2.5V.

(Potentiometers are provided to vary the output amplitude).

5. Observe the output of the AF generator using the CRO. There are two outputs fromthe AF generator, one is direct output and another one is 90 phase shift with the direct

output.

6. Connect AF output 0 to the CH1 and AF output 90 to the CH2 of the CRO and

observe the phase difference between the two signals.

7. A variable frequency potentiometer is given to select the required frequency like 1KHz, 2 KHz and 4 KHz with 10V. AGC (automatic gain control) pot is provided to

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

(Potentiometers are provided to vary the output amplitude).

8. Measure and record the RF and AF signal frequencies using the DMM.9. Connect the RF output 0 to RF input 0 in the modulator circuit(X).10.Connect the RF output 90 to RF input 90 in the modulator circuit(Y).11.Connect the AF output 0 to AF input 0 in the modulator circuit(Y).12.Connect the AF output 90 to AF input 90 in the modulator circuit(X).13.Observe the both output signals of the balanced modulator simultaneously using the

CH1 and CH2 of the CRO, and adjust the balance between the signals until you get the

output waveforms (DSB-SC) as shown in the figure.

14.To get SSB lower sideband signal, connect balanced modulator outputs (DSB-SCsignals) to subtractor circuit.

15.Measure and record the SSB lower sideband signal frequencies using the DMM.

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16.Calculate the theoretical frequency of SSB (LSB) and compare it with the practicalvalues.

LSB = RF frequencyAF frequency

Ex: if RF frequency is 100 KHz and AF frequency is 2 KHz

Then the LSB = 100 KHz2 KHz = 98 KHz.

17.To get SSB upper sideband signal, connect balanced modulator outputs (DSB-SCsignals) to summer circuit.

18.Measure and record the SSB upper sideband signal frequencies using the DMM.19.Calculate the theoretical frequency of SSB (USB) and compare it with the practical

values.

USB = RF frequency + AF frequency

Ex: if RF frequency is 100 KHz and AF frequency is 2 KHz

Then the USB = 100KHz + 2 KHz = 102 KHz.

20.Connect SSB output signal from the summer (or) subtractor to the SSB input terminalin the demodulator section.

21.Connect RF 0 from the RF generator to the RF input of the demodulator section.22.Observe the demodulated output signal by using the CRO.23.Observe the modulating signal in CH1 and demodulated signal in CH2 of the CRO.24.Observe the SSB (USB and LSB) signals for the different frequencies of the

modulating signal.

EXPECTED WAVEFORMS:

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OBSERVATIONS:Amplitude Time period

RF signal

AF signal

Modulated signal

Demodulated signal

CALCULATIONS:

Theoretical frequency of SSB (LSB)

LSB = RF frequencyAF frequencyTheoretical frequency of SSB (USB)

USB = RF frequency + AF frequency

S.No RF

(KHz)

AF

(KHz)

Fx + Fy

(Theoritical)

(KHz)

Fx - Fy

(Theoritical)

(KHz)

USB(practical)

(KHz)

LSB(practical)

(KHz)

1

2

RESULT:

The modulated and demodulated waveforms of the single side band system has been

observed and verified.

The summer and subtractor frequencies are also calculated.

1) The RF frequency is =__________2) The AF frequency is =________3) The USB frequency (Theoretical )is =_________4) The USB frequency (practical)is =__________5) The LSB frequency (Theoretical)is =_____6)The LSB frequency (practical)is =________

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PRECAUTIONS:

1. Give the connections correctly.2. Handle the apparatus / equipment with care.3. Take the readings /values correctly.4. Turn off the equipment when not in use.5.Switch off the supply while leaving the lab.

---Matlab program for ssb-sc modulation and demodulation

Clc;

clear all;

close all;

fm=input('enter the message signal');

fc=input('enter the carrier signal');

pi=3.14;

t=0:0.001:1;

% message signal

m1=sin(2*pi*fm*t);

subplot(3,3,1);

plot(t,m1);

xlabel('time');

ylabel('amplitude');

title('message signal 1');

%message signal

m2=cos(2*pi*fm*t);

subplot(3,3,2);

plot(t,m2);

xlabel('time');

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ylabel('amplitude');

title('message signal 2');

%carrier signal 1

c1=sin(2*pi*fc*t);

subplot(3,3,3);

plot(t,c1);

xlabel('time');

ylabel('amplitude');

title('carrier signal 1');

%carrier signal 2

c2=cos(2*pi*fc*t);

subplot(3,3,4);

plot(t,c2);

xlabel('time');

ylabel('amplitude');

title('carrier signal 2');

%dsb sc1 modulated output

x=m1.*c1;

subplot(3,3,5);

plot(t,x);

xlabel('time');

ylabel('amplitude');

title('DSB-sc 1');

%dsb sc2 modulated output

y=m2.*c2;

subplot(3,3,6);

plot(t,y);

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xlabel('time');

ylabel('amplitude');

title('DSB-sc 2');

%ssb upper sideband generation

s=x+y;

subplot(3,3,7);

plot(t,s);

xlabel('time');

ylabel('amplitude');

title('ssb-sc modulated');

%ssb demodulation

d=s.*c1;

subplot(3,3,8);

plot(t,d);

xlabel('time');

ylabel('amplitude');

title('ssb-sc');

EXP:5 FREQUENCY MODULATION AND

DEMODULATION

AIM:

1. To study the frequency modulation (FM) and demodulation.2. Calculate the frequency deviation, modulation index and bandwidth.

APPARATUS:

1. Frequency Modulation and demodulation trainer kit.

2. Function Generator.

3. Cathode Ray Oscilloscope.

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4.Connecting wires & probes.

THEORY:

The process, in which the frequency of the carrier is varied in accordance with the instantaneous

amplitude of the modulating signal, is called Frequency Modulation. The FM signal is expressed as

s (t)= Accos(2fc+sin(2fm t))

Where AC is amplitude of the carrier signal, fC is the carrier frequency, is the modulation index of

the FM wave

CIRCUIT DIAGRAM:

Modulator

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Demodulator

PROCEDURE:

1. Switch on the experimental board.

2. Connect CH1 of CRO to the FM o/p terminal and observe the carrier output signal with

out any A.F input.

3. Connect an A.F signal (sine wave) of frequency 7 KHz with 6V from the function

generator to the A.F input of modulator circuit.

4. Now observe the modulated signal by using CH2 of the CRO and adjust the amplitude

as well as frequency of AF signal to get clear modulated waveform.

5. Vary the modulating signal frequency and amplitude and observe the effects on the

modulated signal.

6. Calculate the frequency deviation, modulation index and bandwidth by using the

following formula.

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= f /fm where frequency deviation f = fmaxfmin

fm = modulating signal frequency

Carsons formula for bandwidth = 2(+1) fm

7. Connect the modulated signal to the input of the demodulator circuit.8. Observe the de-modulated signal by using the CRO and to get clear waveform vary the

potentiometer provided in the de-modulator section.

9. Observe the modulating signal in CH1 and de-modulated signal in CH2 of the CRO.10.Change the modulating signal frequency to find out the different frequency deviation,

Modulation index and bandwidth values.

EXPECTED WAVEFORMS:

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EXPECTED WAVEFORMS (USING MATLAB)

OBSERVATIONS:

Amplitude Time period fmax fmin

Carrier signal -NA- -NA-

Message signal -NA- -NA-

Modulated signal

Demodulated signal -NA- -NA-

CALCULATIONS:

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= f /fm where frequency deviation f = fmaxfmin

Carsons formula for bandwidth = 2(+1) fm

RESULT:

The frequency modulated and de-modulated waveforms are observed and verified.

The frequency deviation and modulation index value is also calculated.

1) The frequency deviation f=__________2) The modulation index = _________3) The bandwidth (B.W) =

--- matlab code for frequency modulation and demodulation

Clc;

clear all;

close all;

fm=input('enter the message signal');

fc=input('enter the carrier signal');

b=input('enter modulation index');

pi=3.14;

t=0:0.001:1;

% message signal

m=sin(2*pi*fm*t);

subplot(2,2,1);

plot(t,m);

xlabel('time');

ylabel('amplitude');

title('message signal');

% carrier signal

c=cos(2*pi*fc*t);

subplot(2,2,2);

plot(t,c);

xlabel('time');

ylabel('amplitude');

title('carrier signal');%modulation signal

s=cos(2*pi*t*fc+b.*m);

subplot(2,2,3);

plot(t,s);

xlabel('time');

ylabel('amplitude');

title('freq amp');

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EXP: 6 PRE-EMPHASIS & DE-EMPHASISAIM: To study the frequency response of a pre-emphasis and de-emphasis circuit.

APPARATUS:

1. Pre-emphasis and de-emphasis trainer kit.2. Function Generator.

3. Cathode Ray Oscilloscope.

4. Connecting wires & probes.

THEORY:

Frequency modulation is much immune to noise than amplitude modulation and

significantly more immune than phase modulation. A single noise frequency will affect the output

of the receiver only if it falls within its pass band.

The noise has a greater effect on the higher modulating frequencies than on lower ones.

Thus, if the higher frequencies were artificially boosted at the transmitter and

correspondingly cut at the receiver, improvement in noise immunity could be expected. This

boosting of the higher frequencies, in accordance with a pre-arranged curve, is termed pre-

emphasis, and the compensation at the receiver is called de-emphasis.

If the two modulating signals have the same initial amplitude, and one of them is pre-

emphasized to (say) twice this amplitude, whereas the other is unaffected (being at a much lowerfrequency) then the receiver will naturally have to de-emphasize the first signal by a factor of 2,

to ensure that both signals have the same amplitude in the output of the receiver. Before

demodulation, I.e. while susceptible to noise interference the emphasized signal had twice the

deviation it would have had without pre-emphasis, and was thus more immune to noise.

Alternatively, it is seen that when this signal is de-emphasized any noise sideband voltages are

de-emphasized with it, and therefore have a correspondingly lower amplitude than they would

have had without emphasis again their effect on the output is reduced.

The amount of pre-emphasis in U.S FM broadcasting, and in the sound transmissions

accompanying television, has been standardized at 75 microseconds, whereas a number of other

services, notablyCCIR and Australian TV sound transmission, use 50 micro second. The usage of

microseconds for defining emphasis is standard. 75 microseconds de-emphasis corresponds to a

frequency response curve that is 3 db down at the frequency whose time constant is RC is 75

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microseconds. This frequency is given by f=1/2RC and it is therefore 2120 Hz; with 50-

microseconds de-emphasis it would have been 3180 Hz. Figure I shows pre emphasis and de-

emphasis curves for a 7 microseconds emphasis, as used in the united states.

If emphasis is applied to amplitude modulation, some improvement will also result, but it

is not as great as in FM because the highest modulating frequencies in AM are no more affected by

noise than any others. Apart from that, it would be difficult to introduce pre-emphasis and de-

emphasis in existing AM services since extensive modifications would be needed, particularly in

view of the huge numbers is receivers in use.

CIRCUIT DIAGRAM:

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PROCEDURE:

1. Switch on the experimental board.Pre-emphasis:

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2. Connect an A.F signal (sine wave) of frequency 1 KHz with 20mV from the functiongenerator to the AF input of Pre-emphasis circuit.

3. Connect a 75mH of inductor to the supply of the circuit.4. Observe the pre-emphasis output signal by using the CRO.

5. By changing the frequency of the A.F signal (1-20 KHz) in different steps, note down

the corresponding output voltages and tabulate it in a tabular form.

6. Repeat the above steps for 50mH of inductance also.

De-emphasis:

7.

Connect an A.F signal (sine wave) of frequency 1 KHz with 2V from the function

generator to the AF input of de-emphasis circuit.

8. Connect a 75K of resistor to the capacitive network.9. Observe the de-emphasis output signal by using the CRO.10.By changing the frequency of the A.F signal (1-20KHz) in different steps,

note down the corresponding output voltages and tabulate it in a tabular form.

7. Repeat the above steps for 50K of resistor also.8. Calculate the attenuation values for a pre-emphasis and de-emphasis circuit.

EXPECTED GRAPH:

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OBSERVATIONS:

Pre-emphasis:

Input Vi =20mV

S.No Frequency (KHz) Vo/p(V) Gain in dB 20 log(Vo/Vi)

De-emphasis:

Input Vi =2V

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S.No Frequency (KHz) Vo/p(V) Gain in dB 20 log(Vo/Vi)

RESULT:

The pre-emphasis and de-emphasis graphs are observed and verified.