# Why my frequency domain graph looks so weird?

3 views (last 30 days)
ong jia eek on 1 Oct 2019
Answered: Star Strider on 1 Oct 2019
This is the result of my frequency modulated signal in frequency domain, may I ask whether how can I avoid the big gap between each cycle
below is my coding
%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;% this will define the the resolution.
z=fft(y);
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)
ylabel('amplitude');xlabel('frequency domain');title('Frequency Modulated signal');
Walter Roberson on 1 Oct 2019
What are the inputs you are using in response to the input() requests?

Star Strider on 1 Oct 2019
You are plotting only the real part of a two-sided Fourier transform.
Try this:
fs=1000;% this will define the the resolution.
fn = fs/2; % Nyquist Frequency
z=fft(y)/numel(y);
nfft = fix(length(y)/2);
f = (0:1/nfft:1-1/nfft)*fn; % 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,abs(z(1:nfft))); % t (time-domain) is replace with f (frequency-domain)
ylabel('amplitude');xlabel('frequency domain');title('Frequency Modulated signal');

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