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Windowing a signal in frequency space

  1. Jun 26, 2012 #1
    I'm trying to write a python script that takes a noisy multi frequency signal, transforms it to frequency space, windows it there with a gaussian, then transforms it back to time space. Here is what I wrote:

    Code (Text):
    Fs=1000     #sampling frequency
    fo=120      #center of gaussian  
    sigma=0.01  #inverse width of gaussian
    L=2**10     #number of samples
    t=arange(0,L)*T #time vector
    f=Fs*linspace(0,1,L)    #frequency vector
    x=0.7*sin(2*pi*50*t) + sin(2*pi*120*t)+randn(t.size)/sqrt(t.size)   #signal
    W=exp(-square(2*pi*sigma*(f-fo)))   #gaussian window
    y=ifft(W*x_fft)                      #windowed signal
    The problem I'm running into is the windowed signal y has non negligible imaginary parts. They're about the same order as the real parts. Does anyone know why I might be getting this?
  2. jcsd
  3. Jun 27, 2012 #2
    Any chance you are getting a complex vector when you were expecting a,b as in a*cos(n*t)+b*sin(n*t)?

    Or are you perhaps thinking you want the magnitude at each frequency and not the phase information?
  4. Jun 27, 2012 #3
    I just realized what was happening. When I multiplied the transformed signal by the gaussian, I cancelled out the negative frequency components. So of course when I transformed back the signal would be complex. I was expecting it to be a real sinusoid not a complex one.
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