FM

clear all;
clc;
clf;

fm = 500;                      % freq of message signal
Am = 3;                         % amplitude

fc = 15000;                     % freq of carrier signal
Ac = 10;

Fs = 32*fc;                     % sampling freq

N = 3*Fs/fm;                    % samples

n = 0:N-1;

signal = Am*sin( 2*pi*fm/Fs*n );

subplot(411); plot(n, signal);  % message signal

%% Modulation

kf = 150*pi;                        % modulator freq sens [ Hz / V ]
%beta = kf*Am/fm;
%mod_fm = Ac*cos(2*pi*fc/Fs*n+beta*sin(2*pi*fm/Fs*n));
mod_int = kf*1.e-4*cumsum(signal);
mod_fm = Ac*cos(2*pi*fc/Fs*n + mod_int);
subplot(412); plot(n, mod_fm);

%% Demodulation

demod_fm = diff(mod_fm);
demod_fm = [ 0, demod_fm ];
demod_fm = abs(demod_fm);

%%

%% Filter

f_3dB = 1500;
rzad = 3;
[B, A] = butter(rzad, f_3dB/Fs*2);
demod_filter = filter(B, A, demod_fm);

subplot(413); plot(n, demod_filter);

%% FIR
% %
% % N = 100;                                                             % Order of Filter
% % Wn = 2.e-4;                                                    % Pass Band Edge Frequency.
% % a = fir1(N,Wn);                                             % Return Numerator of Low Pass FIR filter
% % b = 1;                                                                % Denominator of Low Pass FIR Filter
% % rec = filter(a,b,demod_fm);
% % subplot(414); plot(n, rec);