Matlab code:

%% message signal close all clear all clc ts = 1.e-4; t = -0.04 : ts : 0.04; Ta = 0.005; %m_sig = triangularPulse((t+0.01)/Ta) - triangularPulse((t-0.01)/Ta); m_sig = 10 * square(2*pi*(1/Ta)*t); figure(1) plot(t,m_sig); title('message signal') %% frequency domain representation

Lfft = length(t); Lfft = 2^ ceil(log2(Lfft)); M_fre=fftshift(fft(m_sig,Lfft)); freqm = (-Lfft/2 : Lfft/2-1)/ (Lfft*ts); figure(2) plot(freqm, abs(M_fre)) title('message signal in frequency domain') %% frequency modulation

B_m = 100; h=fir1(80, [B_m*ts]); kf = 160*pi; m_intg = kf *ts *cumsum(m_sig); s_fm = cos(2*pi*300*t+m_intg); figure(3) plot(t, s_fm) size(s_fm) title('FM signal output')

%% demodulation

s_fmdem = diff([s_fm(1) s_fm])/ts/kf; s_fmrec = s_fmdem.*(s_fmdem>0); s_dec = filter(h,1,s_fmrec);

figure(5) plot(t, s_fmdem) title('FM derivative')

figure(6) plot(t, s_fmrec) title('rectified FM derivative') %% Lfft = length(t); Lfft = 2^ceil(log2(Lfft)+1); s_fm = fftshift(fft(s_fm, Lfft)); freqs = (-Lfft/2 : Lfft/2-1)/(Lfft*ts); figure(4) plot(freqs, abs(s_fm)) title('FM signal output in frequency domain')

Impact of change of amplitude of message signal (freq = 100hz) :

Amplitude = 5 unit

Amplitude = 10 unit

Impact of change of frequency (freq = 200, amplitude = 10 unit)

FM demodulation : (freq =200, amp =10)