[sdiy] more-pole filter

[Aaron Lanterman]

On Aug 10, 2008, at 1:04 AM, Ian Smith wrote:

> Just a late night random question. Everywhere I look, I see 4-pole  
> Low-Pass Filter. So, why 4? why not 3, or, heck, let's be all Tim  
> Allen for a moment and go with why not 8 or 12 poles?

It has to do with how the feedback resonance path works.

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.

See the videos for Sessions 15, 16, 17, and 18 here:

http://users.ece.gatech.edu/~lanterma/ems

Warning: I wasn't very coherent during that series of lectures (which  
it why it took me four sessions, I had to go back and clean up some  
stuff I said that wasn't quite right or wasn't clear.)

- Aaron