[sdiy] Filter stability and self oscillation

[Antti Huovilainen]

On Mon, 29 May 2006, Nicholas Gregorich wrote:

> 1. Would it be correct to call a filter that self oscillates "unstable"?

Yes. If not for the clipping (either builtin or due to power rails), the 
oscillation amplitude would grow to infinity.


> 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

I'd say that all two pole filters using cascaded RC sections and negative 
feedback are stable.

> 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

MS-20 uses positive feedback and bandpass filter for self oscillation & 
resonance.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova