Modelling of an AM signal .Setting and measuring the depth of modulation.
Study of AM waveform and spectra.
Demodulation of AM signals by envelope detection.
Let m(t) represent an arbitrary waveform that is the message to be transmitted, and let the
constant M represent its largest magnitude:
The message might be just a simple audio tone of frequency:
A carrier wave is modeled as a sine wave:
In which the frequency in Hz is given by:
The constants and represent the carrier amplitude and initial phase, and are introduced
for generality. For simplicity, their respective values can be set to 1 and 0.
It is assumed that and that
Amplitude modulation is formed by the product:
A represents the carrier amplitude, which is a constant that demonstrates the modulation
y (t) can be written in the form
Therefore, the modulated signal has three components: a carrier wave and two sinusoidal
waves (known as sidebands), whose frequencies are slightly above and below
The simplest form of AM demodulator consists of a diode which is configured to act
as envelope detector.
Matlab Code for the Experiment:
FIG: AM SIGNAL m>1
Conclusion: On changing the value of m the waveform is changing. For values of m>1 the
envelope is not a copy of AM signal.
The Channel 1 we observed the AM signal,channel 3 is message signal and channel 2
demodulated signal of envelope detector.
The output signal is distorted when m>1.This was due to distortion of envelope of AM signal
which was fed to envelope detector.
CONCLUSION: We cannot use envelope detector for DSBSC signal demodulation as the
output signal and frequency double of message signal.
We need a coherent detector for demodulating DSBSC signal. One can use an envelope
detector if one has an idea of what is the sign of the signal initially