CODEWITHSUNDEEP
pythonnumpyfft
ma.aero
ma.aero

Reputation: 107

properly not getting frequencies in numpy.fft

I have a signal in frequency domain.Then I took numpy.fft.ifft of signal.I got time domain signal.Again I took fft of same time signal properly I'm not getting negative and positive frequencies(Plot 3 in Figure). enter image description here

     time       = np.arange(0, 10, .01)
     N          = len(time)
     signal_td  = np.cos(2.0*np.pi*2.0*time)
     signal_fd  = np.fft.fft(signal_td) 
     signal_fd2 = signal_fd[0:N/2]
     inv_td2    = np.fft.ifft(signal_fd2)
     fd2        = np.fft.fft(inv_td2)

Upvotes: 1

Views: 1742

Answers (1)

Frank M
Frank M

Reputation: 1570

General comment: I avoid using time as a variable name because IPython loads it as a "magic" command.

Something I find at times confusing about matplotlib is that when you plot a complex array, it actually plots the real part. In the code snippet:

tt       = np.arange(0, 10, .01)
N          = len(tt)
signal_td  = np.cos(2.0*np.pi*2.0*tt)
signal_fd  = np.fft.fft(signal_td) 
signal_fd2 = signal_fd[0:N/2]
inv_td2    = np.fft.ifft(signal_fd2)
fd2        = np.fft.fft(inv_td2)

The following arrays have dtype of float64: tt and signal_td. The others are complex128. The reason you only see one peak in fd2 is because it is a transform of exp(4j*np.pi*tt) rather than cos(4*np.pi*tt).

Upvotes: 1

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