[sdiy] Notch Filter for Logon's Human-Capacitor Filter

[Don Tillman]

   > Date: Thu, 14 Dec 2006 05:09:05 -0500 (EST)
   > From: Aaron Lanterman <lanterma at ece.gatech.edu>
   > 
   > I'm a student in Aaron Lanterman's synthesizer class. I'm working
   > on a circuit that allows the user to control the cutoff
   > frequencies of a high pass and low pass filter through
   > capacitance with an antenna.

Hey Logan, Aaron,

Are you using hand capacitance to directly filter audio frequencies?
That's doomed... the capacitance is so small you need an impractically
high impedance to tune it to audio, and that's going to be extra
susceptable to all sorts of background noise.

Maybe in a Faraday cage, but that's gonna drastically limit musical
performance possibilities.

   > However, one of the problems I have encountered is 60 Hz wall
   > voltage being coupled into my circuit through the antenna. To me,
   > the easiest solution seems to be putting a 60 Hz notch filter at
   > the output of the circuit.

And all harmonics of 60 Hz.  The 60 Hz and other noise will be too
great to filter out.  

   > In an effort to avoid inductors, I found this circuit which can
   > be downloaded as a PDF from this
   > link. www.national.com/ms/LB/LB-5.pdf or can be found quickly
   > through a search engine by searching for "High Q Notch Filter." I
   > am trying the circuit configuration in figure 1 in the pdf
   > file. However, my simulations with PSPICE are showing me a
   > frequency response that is hardly a notch filter at all. 

(That application note is 38 years old, so you'd think they'd have
time for a correction.  :-) )

The filter works, it's a classic.  But the notch is *really* steep and
*really* narrow, so all your simulation parameters have to be
specially tweaked to have any hope of catching it.

If you use this filter, fine tuning it is going to be critical and
very difficult.  And when it drifts a little, it will no longer be
filter out the 60Hz.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com