ac 3-6.doc
TRANSCRIPT
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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.