# Periodicity using Fourier transform

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aurc89 2016년 11월 18일
댓글: Walter Roberson 2016년 11월 18일
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 2016년 11월 18일
Are the times uniform intervals or irregular?
aurc89 2016년 11월 18일
In this case uniform but sometimes could be irregular in fact

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### 답변 (1개)

Walter Roberson 2016년 11월 18일
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 2016년 11월 18일
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 2016년 11월 18일
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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