numpy.fft.fftfreq

numpy.fft.fftfreq(n, d=1.0)

Discrete Fourier Transform sample frequencies.

The returned float array contains the frequency bins in cycles/unit (with zero at the start) given a window length n and a sample spacing d.

f = [0,1,...,n/2-1,-n/2,...,-1]/(d*n)         if n is even
f = [0,1,...,(n-1)/2,-(n-1)/2,...,-1]/(d*n)   if n is odd

Parameters:

n : int

Window length.

d : scalar

Sample spacing.

Returns:

out : ndarray, shape(n,)

Sample frequencies.

Examples

>>> signal = np.array([-2.,  8., -6.,  4.,  1., 0.,  3.,  5.])
>>> fourier = np.fft.fft(signal)
>>> n = len(signal)
>>> timestep = 0.1
>>> freq = np.fft.fftfreq(n, d=timestep)
>>> freq
array([ 0.  ,  1.25,  2.5 ,  3.75, -5.  , -3.75, -2.5 , -1.25])