Neil,

Well, your transfer function for it is something like: (from memory so
it may be wrong.)

                     1
=3D> -------------------------------------------
   s^2(R1C1R2C2) + s(R1C1 + R2C2 + 2*R1C2) + 1



                1/(R1C1R2C2)
   --------------------------------------
=3D>       s(R1C1 + R2C2+ 2*R1C2)       1
   s^2 + ---------------------- + --------
               R1C1R2C2           R1C1R2C2



You can then calculate the roots if you like.

Such circuits usually yield poor Q (theoretical best value=3D1/2) unless
you have an op-amp setup as a positive feedback amplifier.

If you are making an actual filter (and not doing this for just the
math) then look into Sallen-Key filters. There should be some
information about them on the web. Most people usually solve Sallen-Key
through simulation. I did it through pspice and it did a very accurate
job.=20

HTH

Shahid

-----Original Message-----
From: Picdude [mailto:picdude@NARWANI.ORG]=20
Posted At: Wednesday, March 31, 2004 3:01 AM
Subject: [EE:] RC filter calculations?


For some reason, I can't find (thru google) the formula for calculating
the=20
best values of R's and C's in this circuit for a given cutoff
frequency...

   >----`\/\/\,----o----`\/\/\,-----o------>
           R1      |       R2       |
                  ---              ---
               C1 ---           C2 ---
                   |                |
   >---------------o----------------o------>

Any of you have the formula?

Thanks,
-Neil.

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