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From: Jim Hannon jmhannon@........
Date: Mon, 11 Sep 2000 17:57:22 -0500

I am quite sure it is a Butterworth. The characteristics of a Butterworth 
are, all the resonators are tuned to the same frequency and the gains of 
the individual stages match the Butterworth tables I have.
BTW It is easy enough to convert the filter to Bessel by changing the R and 
C values. If anyone is interested I will calculate the values and post them.

The design is a little suspest in that it was advertized as a 1 Hz filter 
but the components are for a 1.59 Hz. I think the designer forgot to 
convert from radians to Hz.

At 02:59 PM 9/11/00 -0700, you wrote:
>Hi Jim --
>I agree with Sean-Thomas regarding the use of Bessel filters.  Other kinds
>of filters will distort the waveforms.
>I don't think the filter shown in the schematic you referenced is even a
>Butterworth.  I entered the component values into a simulation program and
>found it has an 8:1 peak near its cutoff frequency.  That's not good for
>seismic work.

Jim Hannon


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