[sdiy] Filter stability and self oscillation

[Nicholas Gregorich]

Just finished my second electronics course and feedback (negative) was one of the last subjects we studied.

I found it very interesting to learn about negative feedback and finally get some educational insight into what makes a resonant filter...resonate, but some things just aren't adding up and I'd like to request some clarification.

1. Would it be correct to call a filter that self oscillates "unstable"?  Meaning that the -180 degree phase frequency occurs before the unity gain frequency causing the system to be unstable and oscillate at omega -180.

2. In addition, all two pole systems are stable by nature because their maximum phase lag is -180 degrees but it occurs at an infinite frequency (at which case unity gain would have occurred at some point before).

If point 2 is true how do some 2 pole filters self oscillate?  The only example I can think of at the moment is the Korg MS20's self oscillation even though it is reported as a 12dB/oct filter (I've never used one myself).  My only explanation is that it actually not a two pole filter.

That's all I can think of at the moment.  Thanks for any help.

Nick.