Mod_FM

clear all;
clc;
clf;

fm = 1000;                       % freq of message signal
Am = 2;                         % amplitude

fc = 10000;                     % freq of carrier signal
Ac = 1;

Fs = 32*fc;                     % sampling freq

N = 3*Fs/fm;                    % samples

n = 0:N-1;

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

subplot(311); 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 = cos(2*pi*fc/Fs*n + mod_int);
subplot(312); 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(313); plot(n, demod_filter);