PSN-L Email List Message

Subject: Re: Preamp-Filter Modifications
From: Jim Hannon jmhannon@........
Date: Tue, 12 Sep 2000 21:08:05 -0500

I was worried that the 4 1 Meg would be miss interpeted. I am referring to 
the four      1 Meg Ohm resistors not to a 4.1 Meg ohm resistor.
My eyes are not working too well, upon closer examination of the schematic 
the resistor values look like 1M6 rather than the 1MG I thought I was 
reading. This is a notation I have not seen too often for 1.6 Meg. That 
value makes more sense because it gives the filter a 1 Hz 3 dB response 
than the 1.59 Hz I had calculated using 1 Meg Ohms. Anyway when I get to 
work tomorrow I will go over my calculations and also try to figure out why 
Karl and I are getting different results from our analysis.

As for the capacitor values 0.0991 UF is what the equations give. I did not 
try and round off or pick nearest values.  All of this illistrates the 
difficulty of building active filters and getting the desired response. You 
have to account for all the component tolerances and check to see how badly 
it messes up the desired response. Some circuit topologies are more 
tolerant to component variations than others. So even the type of circuit 
needs to be considered.

>     ??? Sorry, but I don't quite understand. 0.0991 UF is within 1% of 0.1
> > 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.
>     Where is the 4.1 M Ohm coming from? The diagram of the four pole filter
>gives pairs of 1.6 M Ohm input resistors. Can you confirm the R value for 1
>Hz please?
>     Regards,
>     Chris Chapman
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Jim Hannon


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