In order to modulate a message signal with conventional AM, first multiply the message signal m(t) with modulation factor ka. Then, add to this signal one. After the addition of one, multiply this signal with a carrier signal c(t) = Accos(2πfct) to obtain the modulated signal.
Figure 1: Modulation of Conventional AM
In order to demodulate this signal, first, take the square of the modulated signal. Then, filter this squared signal with a low pass filter (LPF). After taking the square root of the filtered signal, reach an estimate of the message signal. This technique is called Square-Law detector.
Figure 2: Demodulation of Conventional AM
3.2 Conventional Amplitude Modulation
(a) Obtain the time vector with the duration of t = 0.1 seconds and with the sampling frequency of Fs = 10000 Hz.
(b) Generate a carrier signal c(t) = Accos(2πfct) and a message signal m(t) = Amcos(2πfmt) where Am = Ac = 1 and fm = 100 Hz and fc = 1000 Hz with respect to time vector obtained in (a).
(c) Obtain the modulated signal, s(t).
(d) Plot the modulated signal s(t) for three modulation factors namely 0.5,1 and 2 in the same figure by using subplot.
(e) Comment on the effects of the modulation factors.
(f) Plot the magnitude of the frequency responses of m(t),c(t) and s(t) for ka = 0.5 in the same figure by using subplot.
3.3 Conventional AM Demodulation
(a) Demodulate the modulated signal s(t) when ka = 0.9 and ka = 2 by following the steps given in the preliminaries. Hint: You can use butter(.) with the suitable filter order and cut off frequency and filter(.) functions.
(b) Comment on your choice of the filter order and the cut off frequency.
(c) Plot the demodulated signals in the same figure by using subplot.
(d) Comment on the the results that you obtain in (c).
