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 __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>