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Homework Help: Fourier Transform Power Spectrum

  1. Nov 13, 2007 #1
    Input: sine wave at 10Hz, amplitude 1.

    After the transform the plot has a spike at 10Hz with amplitude 0.5. If I vary the amplitude of the sine wave I get:

    sine amp. - FT spike amp.
    1 - 0.5
    2 - 2
    4 - 8

    So it seems A' = A^2/2

    Is this because power is proportional to A^2 and it is averaged over trough/crest so division by 2?
  2. jcsd
  3. Nov 13, 2007 #2
    Are you adding real and imaginary parts?

    The power should be the same in both domains.
  4. Nov 13, 2007 #3
    Sorry I think I asked my question poorly.

    I'm doing this in a lab using LabVIEW and it's doing the FT. When I input a sine wave (vs time) with varied amplitude 'A', I get an output spike of amplitude (A^2)/2 centered at some fixed frequency. Is this because [itex]P \propto A^2[/itex]? Is the half for 'average'?

    I'm just trying to make sense of what this VI is doing. All I know is "computes the averaged auto power spectrum of time signal".

    Does my data still make no sense?

    I'm not directly dealing with imaginary parts...
    Last edited: Nov 13, 2007
  5. Nov 13, 2007 #4
    OK- you probably forgot to add the power in -ve and +ve frequencies.
  6. Nov 13, 2007 #5
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