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I try to get a frequency response using FFT(output)/FFT(input). My input signal is white
noise. This works pretty good in low frequency. However in high frequency (next to the sampling
frequency) the gain curve goes fast to infinite but it shouldn't. Moreover the phase curve is
not exactly correct. It has the right form but it goes about 200 degrees too high. How should
improve the results?

Thanks!

-Joonas

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Joonas-

> I try to get a frequency response using FFT(output)/FFT(input). My input
> signal is white noise. This works pretty good in low frequency. However
> in high frequency (next to the sampling frequency) the gain curve goes
> fast to infinite but it shouldn't. Moreover the phase curve is not exactly
> correct. It has the right form but it goes about 200 degrees too high. How
> should improve the results?

Near the sampling rate you have practical limitations that could be making the
results look this way:

  -freq response of anti-alias filter used in data acquisition card

  -freq response in any subsequent processing, for example sampling
   rate conversion

For instance, the closer to "ideal"anti-alias cutoff near the sampling rate the
higher the cost of the data acq card.

-Jeff

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