AM modulation and demodulation

Can somebody tell me how to synthesize a bandpass AM signal, s(t) = Ac[1 + Amcos(2pfmt)]cos(2pfct) ; where fm = 100 Hz, fc = 25 kHz, Am < 0.5 and t 1 sec. The sampling rate can be assumed.

(a) How to derive the Fourier Transform and draw the “true” amplitude spectrum.

(b) Obtain and plot the spectral components of this signal using Matlab's fft function.

(c) Using demodulation technique by multiplying the modulated waveform with the vector containing the carrier. Perform and interpret the fft analysis on the signal after multiplication. Could you apply filtering process to recover the demodulated signal and compare to original signal?

(d) Repeat the same process for two fm and two fc and make sure you can differentiate them in frequency spectrum observation. Please apply demodulation technique to recover both signals with your design of low pass or bandpass filter.

Answer 1

[Sai Kiran]

Hi,

As per my understanding you want to compute the fft on the amplitude modulated signal.

You can get the amplitude modulated signal by the following function 'ammod'.

You can pass the above signal to the 'fft' function to get the fourier transform.

To modulate and demodulate the signal you can use the 'NCO' block.

Please refer to the following documentations for more information.

https://in.mathworks.com/help/comm/ref/ammod.html

https://in.mathworks.com/help/matlab/ref/fft.html

https://in.mathworks.com/help/dsp/ref/nco.html

I hope it helps!

Thanks.