Periodicity using Fourier transform

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aurc89 2016-11-18

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https://ww2.mathworks.cn/matlabcentral/answers/312850-periodicity-using-fourier-transform

评论： Walter Roberson 2016-11-18

Hi everyone, I have a simple question concerning how to find the periodicity of an oscillating function using the Fourier transform. Let's suppose to have an oscillating function y=f(x) where x and y are two vectors and x is the time vector expressed in second (s). Can you suggest me a code to retrieve the frequency (or frequencies) of this function, expressed in herz (i.e. 1/s) by using the Fourier transform? Thanks a lot.

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Walter Roberson 2016-11-18

Are the times uniform intervals or irregular?

aurc89 2016-11-18

In this case uniform but sometimes could be irregular in fact

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Answer 1

Walter Roberson 2016-11-18

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If the times are uniform intervals then do peak finding on fft(y-mean(y))

See the fft example for how to calculate the frequency from bin and fs and length

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aurc89 2016-11-18

So if I have a vector in second (s) how can I get the frequency of the oscillations (in 1/s) by using fft ?

Walter Roberson 2016-11-18

在 MATLAB Online 中打开

For x at equal time intervals,

L = length(y);
Fs = 1/(x(2)-x(1));
f = Fs*(0:(L/2))/L;

Now the first length(f) positions in fft(y-mean(y)) are associated with corresponding frequencies in f, so if you find a peak at position K then the frequency corresponding is f(K)

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