AMPLITUDE MODULATION AND DETECT AN AMPLITUDE
CARRIER
Aim :- To study amplitude modulation and detect an amplitude modulated carrier.
Apparatus :1.
2.
3.
4.
5.
Experimental Kit ACL-01 & ACL-02
20 MHz Dual Trace oscilloscope
CRO probes
Connecting Chords
Power supply.
Theory :Amplitude modulation:
In amplitude modulation, amplitude of a high frequency carrier signal is varied by
modulating signal, whose frequency is variably lower than that of carrier. Let the
modulating and carrier voltages be Vm and Vc respectively.
Vm = Vm Sin ωt
Vc = Vc sin ωt
Modulation Index:
It is defined as ratio of modulating signal amplitude to carrier signal amplitude.
m=Vm/Vc
When m=1 modulation is 100٪
When m<1 modulation is under modulation
When m>1 modulation is over modulation.٪
Formula :- % Modulation index m = Vmax – Vmin / Vmax + Vmin
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Demodulation: Figure shows a diode detector AM signal is given as input to the
diode detector.
As soon as voltage at anode of the diode D1 increases diode becomes forward
biased and acts as a rectifier. In forward bias condition of diode the capacitor
charges to maximum input voltage, when the input starts decreasing, diode
becomes reverse biased and capacitor discharges through R. Since RC time
constant is very large, by the time the capacitor discharges by a small value, the
anode input voltage again increases. The diode again becomes forward biased to
charge capacitor to maximum input voltage and the process continuous. The
voltage across capacitor C1 contains both DC and audio signal. The DC part is
blocked by capacitor C2. The audio frequency signal appears across the R2.
Connection Diagram :-
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Connection Diagram :-
PROCEDURE:Modulation:
1) Switch on the Kit and connect CRO at output terminals of amplitude
modulation kit . observe the carrier frequency waveform.
2) Apply SINE wave of 1 KHz frequency at input terminals of the amplitude
modulation kit, from a function generator and observe AM wave.
3) By varying amplitude of modulating signal, Observe the change in output.
4) Calculate modulating index for 3 conditions i) under, ii) 100 % & iii) over
modulation.
5) Plot the waveforms on graph paper.
Demodulation:
1) Connect amplitude modulation signal at input terminal of demodulation kit
and observe the output.
2) Change frequency of carrier signal and observe its effect on output.
3) Plot the waveforms on graph paper.
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Observation table:
Case
V max (V)
Vmin(V)
m٪
Conclusion: -
Outcome
PLO 1
PLO 2
PLO 3
Performance
Attendance
Total
Score
Weight
20
10
30
20
20
100
E&TC DEPARTMENTTCET
Date of Performance:
_________Date of
Correction: __________
Score
Roll No: _
Marks:
/ 100
Signature of Faculty:
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