Hi Randy,

I think the sharp cutoff on the high frequency side of the FFT, is 
because of your sample rate.   Winquake limits the FFT on the high side 
to about 1/2 of the sample rate for reasons of "aliasing", which relates 
to the Nyquist frequency and the ability of digital sampling to 
accurately reconstruct a sine wave.

Here is a link for further study.
> http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem
Roger

psn-l-digest-request@.............. wrote:
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> Subject: Surface Waves
> From:    "Randy" 
> Date:    Tue, 29 May 2007 20:34:40 -0500
>
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> Hi All,
>
> I have been having trouble recording any surface waves lately.  I used =
> the instrument several years ago and had no problem but now I do not =
> seem to get low frequency.   I am using the PSN schematic for the =
> preamp, filter and final stages.  There is both an unfiltered quake and =
> an FFT posted at http://mit.midco.net/rpratt/  I noted the high =
> frequency side of 2 peaks seems to have hard cutoff on this quake but =
> have not noticed this before.  Where should I look to trouble shoot?  My =
> suspicion is the electrolytic cap coupling to the final.
>
> Randy
>
>   
>   
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