ASM1 filter self oscillation

[Don Tillman]

   From: gstopp at fibermux.com
   Date: Thu, 19 Dec 96 11:12:55 PST

I think the big issue behind the difference in the behavior between
state-variable and ladder filters as they approach oscillation has to
do with the clipping behavior.  In the ladder filter, each "rung"
provides gradual limiting, while on the state variable you generally
depend upon opamp/ota integrator clipping, which is going to be very
sudden.

   State-variable filters are notoriously unstable in the high-Q regions. 
   I'm not exactly sure what the engineering reasons for this are, but it 
   may have something to do with the fact that the state-variable goes 
   into a higher-Q condition when feedback is *reduced*, but the 4-pole 
   (cascaded integrator) filter goes into a higher-Q condition when 
   feedback is *increased*.

Interesting idea, but it's misleading; while you do reduce feedback on
a local stage on the state-variable filter to increase Q, that has the
effect of increasing the gain of the global integrator-integrator-inverter
feedback loop, which is where the real filtering action is taking place.

  -- Don