Jim and co.,

Seismic filters have always been of the "Bessel" response because
of the uniform time delay, and timing is the most important feature
in seismology. The "Butterworth" response is flatter, with a sharper
cut off, but this is not necessary for dealing with environmental
noise as with seismometers. The amplitude overshoot of a four pole
Butterworth filter to an impulsive input is 11% near the cut off
frequency, which is also undesirable in seismology. Bessel filters
do not have an overshoot response.

For some time I have posted the schematic of the seismic preamp
and 4-pole filter that has been used in over 100 telemetry stations
from Alaska to Greece. The filter design is from the NASA filter 
handbook (info below).  The schematic shows options for frequency 
scaling, as for a 2.7 second "Wood Anderson Seismometer" response 
that works well in an urban setting (the vault on SLU campus) with
a 15-second long period seis as input. THe preamp/filter can easily
assembled on a Vector 3677 perforated/solder pads circuit board cut 
in half lengthwise. The cmos 4250 amplifiers can be replaced with more
available higher power devices if battery operation is not needed.

http://www.eas.slu.edu/People/STMorrissey/index.html
stmmisc.html" PSN INFO ... SLU Seismic Network


(repeat from previous post:)

Among the references I use, two give very workable designs and tables
for multi-pole filters. The NASA publication, "An RC Active Filter
Design Handbook", NASA SP-5104, 1977, gives standard designs for up 
to 8 poles, and uses a constant resistance algorithm for unity gain 
1 khz filters that unfortunately results in very uncommon capacitor 
values. The designs are impedance and frequency scaled by multiplying/
dividing the R and C values. I have found that the odd capacitor values
can usually be made up with two common values. The unity gain design
is stable and simplifies inclusion in precision amplifier designs.

The other reference is the Sams publication "Active-Filter Cookbook" by 
Don Lancster; Howard W. Sams & co, 1975, Indianapolis; ISBN 0-672-21168-8;
Library of Congress 74-33839. He uses an algorithm that gives equal
values for all the capacitors, with the frequency and damping changed
by the filter and feedback resistors. The designs are also scaleable
from 1 khz. Since the damping of each stage is controlled by the feedback
resistance, this results in gain variability depending on the response
selected and number of poles. There are excellent tables for filters
of 7 different characteristics and up through sixth order (or poles) 
with all the resistance values, gains, and component tolerances
calculated and graphs for frequency scaling the capacitor values.

I prefer the NASA filters because they are all unity gain and often use
a constant resistance for all the stages, which helps quantity buying
of 1% values. The capacitor values are usually made up with a larger
value, like 0.47, which can be purchased in quantity, and smaller parallel
values, which cost much less and can be selected for value with a meter.


Regards,
Sean-Thomas
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