I was trying to turn a previous SV filter into a notch, (Freq ~3.3 kHz, Q~10-20, see my mech. PID thread.) I was thinking of just subtracting the BP from the input. But almost all the web stuff I could find said add the HP and LP. (That seemed crazy to me, at high Q both those are peaking.. but then again the SV filter seems a bit crazy to me... two integraters in a row, come on. :^)
So I first summed the LP and HP, (it was easier, than the BP, no invert and divide the signal by the Q.) Amazingly it worked. Unfortunately the depth of the notch varied inversely with the Q... At high Q the notch was not deep. (It's probably losses in the capacitors.) At a Q of 20 and 500 mV input min on the notch was 25 mV.
Depressing.
Anyway I then flipped over the BP, attenuated it by ~Q (with a pot to tweak the attenuation) and summed it with the input. That made a much better notch. ~5mV at Q=20, (500 mV in) I didn't have time to check other Q's.
The thing that stinks about this notch, is that the high Q means I have to throw away dynamic range on the input. Is there a high Q, lower gain band pass filter.... (I'm seeing Tim W., "Why not... A/D -> uC -> D/A " or do you also suffer dynamic range issues when implementing a notch in digital space?)
George H.