[sdiy] Filter stability and self oscillation

[harry bissell]

insightful, perhaps ignorant comments inline...

Nicholas Gregorich wrote:
> 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.
>   
beats me. I would call a filter that self-oscillates with amplitude 
increasing to infinity "unstable" but
if there is some mechanism to control the amplitude... I'd call it an 
oscillator :^P

> 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.
>   
Who says the feedback is necessarily negative.  Some designs have 
positive feedback and are almost
guaranteed to oscillate.

The MS-20 (Sallen Key) is ont that is almost guaranteed to oscillate, 
and needs an amplitude limiter in the feedback.  The 'other' 
architecture... the 'state-variable' will almost never oscillate... 
adding feedback
reduces the resonant peak.  But if you add gain you can force it to 
oscillate.

H^) harry

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