PSN-L Email List Message

Subject: Preamp-Filter Modifications
From: jmhannon@........
Date: Tue, 12 Sep 2000 09:42:36 -0500 (CDT)


Converting the A Loomis low pass filter 
http://www.info2000.net/~aloomis/seisfilt.htm  
to a Bessel response. 
For a 1 Hz Bessel response change the 20K resistor connected to the
negative input of the first filter op amp to 
11.0K. Change the 160K resistor connected to the negative input of the
second filter op amp to 100K. The first
two 0.1 UF capacitors associated with the first filter op amp should be
changed to 0.111UF. The second
two 0.1 UF capacitors associated with the second filter op amp should be
changed to 0.0991 UF. You will
need to measure and test select the capacitors and possibly parallel parts
to get within at least 5% of 
the values specified. All resistors in the circuit should be 1% metal film
parts.

The 1 Hz design is easily changed to any other frequency within reason by
dividing the value of the 4 1 Meg 
Ohm resistors by the desired frequency. For example if you want a 5 Hz low
pass filter you would use 200K 
resistors instead of 1 Meg.

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

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Larry Cochrane <cochrane@..............>