MATLAB: How to convert time domain to frequency domain

principal communication

This is my code, I failed to convert time domain to frequency domain, as I not sure on how to create the right frequency for frequency domain graph
%FM generation
clc;
clear all;
close all;
fc=input('Enter the carrier signal freq in hz,fc=');
fm=input('Enter the modulating signal freq in hz,fm =');
m=input('Modulation index,m= ');
t=(1/fc)*20;
t=0:0.0001:t;
c=cos(2*pi*fc*t);%carrier signal
M=sin(2*pi*fm*t);% modulating signal
subplot(3,1,1);plot(t,c);
ylabel('amplitude');xlabel('time index');title('Carrier signal');
subplot(3,1,2);plot(t,M);
ylabel('amplitude');xlabel('time index');title('Modulating signal');
y=cos(2*pi*fc*t-(m.*cos(2*pi*fm*t)));
subplot(3,1,3);plot(t,y);
ylabel('amplitude');xlabel('time index');title('Frequency Modulated signal');
fs=1000;
z=fft(y);
subplot(2,1,4);plot(t,z);
figure;
ylabel('amplitude');xlabel('frequency domain');title('Frequency Modulated signal');

Best Answer

  • Replace the following line :
    subplot(2,1,4);plot(t,z);
    figure;
    by the following code
    nfft = length(y);
    f = (0:1/nfft:1-1/nfft)*fs; % define frequency-domain
    figure; % figure should be written before subplot to open new figure
    subplot(2,1,1); % subplot(2,1,4) will give error beacause for a 2x1 vector valid indeces are 1&2, 4 is wrong
    plot(f,z); % t (time-domain) is replace with f (frequency-domain)
    Let me if you have any doubts