How to convert time domain to frequency domain

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ong jia eek
ong jia eek 2019-9-30
编辑： Rumana Afrin 2022-6-14
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');
  1 个评论
Adam
Adam 2019-9-30
You need to plot
abs(z)
or
abs(z).^2
if you prefer, rather than just z, which should be complex and will default to plotting the real part.

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采纳的回答

Shubham Gupta
Shubham Gupta 2019-9-30
编辑：Shubham Gupta 2019-9-30
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
  2 个评论
ong jia eek
ong jia eek 2019-9-30
yes it is working but may I ask why the result shows as below?
1.JPG
Shubham Gupta
Shubham Gupta 2019-10-1
I am glad it worked. In the above graph, every peak in y-axis shows that there is a signal of corresponding frequency shown in x-axis.

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更多回答（1 个）

Rumana Afrin
Rumana Afrin 2022-6-14
编辑：Rumana Afrin 2022-6-14
%Parameters
y1 = A_norm*sin(2*pi*100*t);
y2 = A_norm*sin(2*pi*200*t);
y3 = A_norm*sin(2*pi*300*t);
y4 = A_norm*sin(2*pi*400*t);
y5 = A_norm*sin(2*pi*500*t);
y6 = A_norm*sin(2*pi*600*t);
y = [y1; y2; y3; y4; y5; y6];
%Frequency content
X = fft(y);
fft_abs = abs(X);
figure;
for i = 1:6
A_norm(i) = A_norm(:,i);
X(i) = fft(y(i));
fft_abs(i) = abs(X(i));
subplot(3,3,i)
plot(fft_abs(i))
xlabel('Frequency(kHz)')
ylabel('Magnitude')
title('Frequency domain')
end
Hi, I do not understand where have I done wrong. Where A_norm is

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