How to convert time domain to frequency domain

Question

ong jia eek 2019-9-30

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/482769-how-to-convert-time-domain-to-frequency-domain

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
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Adam 2019-9-30

在 MATLAB Online 中打开

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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Answer 1

Shubham Gupta 2019-9-30

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/482769-how-to-convert-time-domain-to-frequency-domain#answer_394072

编辑：Shubham Gupta 2019-9-30

在 MATLAB Online 中打开

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
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ong jia eek 2019-9-30

yes it is working but may I ask why the result shows as below?

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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Answer 2

Rumana Afrin 2022-6-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/482769-how-to-convert-time-domain-to-frequency-domain#answer_985130

编辑：Rumana Afrin 2022-6-14

在 MATLAB Online 中打开

%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