Karl,
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.
>
>Regards,
>Karl
>

Jim Hannon
http://soli.inav.net/~jmhannon/
42,11.90N,91,39.26W
WB0TXL

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