MATLAB Answers

FFT of sine wave not plotting

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Eva Carrillo
Eva Carrillo on 25 Nov 2019
clc;
spectrum=[];
omega=2*pi*10;
t=0:0.001:10;
f=0:0.1:500;
for n=1:0.001:10
y(n)=sin(omega*y(n));
spectrum=abs(fft((y)));
for i=1:10001
if spectrum(i)<5001
spectrum2(i)=spectrum(i);
end
if spectrum(i)>5001
spectrum3(i)=spetrum(i);
end
end
end
plot(f,spectrum);
hold on;
xlabel('frequency');
hold on;
ylabel('power');
hold on;
xlim([0 1]);
So I have a loop that stores 10001 values into an array. I need to be able to plot the values that are greater than, or less that 5001. I developed this code but its not working. Help is greatly appreciated!

  1 Comment

KALYAN ACHARJYA
KALYAN ACHARJYA on 25 Nov 2019
There are issues in the code, likewise
for n=1:0.001:10;
y(n)=
For the above n=1, first iteration will evaluate y(1)..OK
2nd interation n=0.001, which will evaluate y(0.001)..Not OK
Matlab only allows pisitive integer indexing, Hence , y(1), y(2), y(3)...so on...OK. But y(0),y(-2),y(0.04) are not allowed in Matlab Platform.
Plese do the modification, its easy to implement the code of FFT of sine wave

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Answers (1)

Constantino Carlos Reyes-Aldasoro
In addition to the previous comment 2 issues:
1) FFT will give a complex answer so you will need to plot the absolute values like
plot(abs(spectrum))
This is better than using the abs previously as you may need to use the phase, or imaginary parts for other calculations
2) you are looping as it is normally done in C or other programming languages. In Matlab you can do this as matrix operations, no need to loop
omega=2*pi*10;
t=0:0.001:10;
y=sin(omega*t);
spectrum=fft(y);
spectrum2=spectrum(1:5000);
spectrum3=spectrum(5001:end);
plot(abs(fftshift(spectrum)))
also notice that you had a typo
spectrum3(i)=spetrum(i);

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