[sdiy] more-pole filter

[Antti Huovilainen]

On Sun, 10 Aug 2008, Aaron Lanterman wrote:

> Each pole gives a 45 degree phase shift at the cutoff frequency. So if you 
> have four poles, that gives you 180 degrees of phase shift. If you then take 
> the signal and feed it back (with a minus sign), you accentuate the 
> frequencies around the cutoff. This gives you a nice, predictable way to 
> place that peak.

Well, yes and no. Because the poles are separate, the overall filter 
attenuation at said "cutoff" frequency is 12dB. Not quite the 3dB you'd 
perhaps expect. So there's no reason you couldn't use anything from 3 
poles up. Only the resonance frequency would change (and filter 
attenuation of course). You do have to scale the feedback to take into 
account the total attenuation at the resonant freq (where total phase 
shift is 180 degrees).

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
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