"Dana" bravely wrote to "All" (29 Sep 03 17:54:35)
--- on the heady topic of "400HZ Notch Filter how to."

Da From: "Dana"
Da Looking to notch out a 400HZ whine from some audio circuits connecting
Da remote manned sites to a central location.
Da The audio is microwaved from remote location to the central location.
Da Would like to know the formula for determing the resistor and
Da capacitor values.
Da The overall bandpass is 300HZ to 3KHZ.
Da Or would it be better to just use an High Pass filter.
Da Problem with that is getting the inductors locally.
Da Up here In Fairbanks our choices are very limited.

Da Thanks.

For a resistor capacitor combination the Twin-T Notch filter comes to
mind. The notch frequency is equal to 1/(2piRC).

: C C
: In o-+---||----+-----||---+-o Out
: | | |
: | R/2 |
: | _|_ |
: | |
: `----R----+-----R----'
: |
: =3d=3d=3d 2C
: _|_

Try and pick practical values of capacitance consistant with impedance
of the driving circuit and loading of the input circuit. The R/2 can
consist of a variable resistor to slightly tune the notch. Care in
matching the components will result in a deeper notch but I think 5%
parts should give at least -35dB rejection.

BTW it would be far preferable to remove the offending interference
signal at the original source than to try fixing it at the receiver.

Asimov
******

.... Is reactance then illusory? No, it just appears that way...