numpy.fft.fftfreq

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

Return the 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

The array of length n, containing the sample frequencies.

Examples

>>> signal = np.array([-2, 8, 6, 4, 1, 0, 3, 5], dtype=float)
>>> fourier = np.fft.fft(signal)
>>> n = signal.size
>>> 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])