optical limiter problem

[Don Tillman]

   From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
   Date: Wed, 4 Feb 1998 15:37:44 +0100

   Because of the very nonlinear R / I characteristic of a vactrol, I
   have to use a feedback rather than a forward compressor
   topology. So I have a feedback loop, and inside this feedback loop
   one lowpass filter (the inertia of the vactrol). But if I add an
   electrical smoothing as well (i.e. RC-filter), I have a second pole
   inside the feedback loop, and this will cause overshot in the
   regulation response.

Heh-heh...  It sounds like the Vactrol pole changes position for
rising and falling times, so that during the falling time (or rising
time depending on the values) the Vactrol pole is close enough to the
RC pole to cause an oscillation at that loop gain.

I would guess that making the RC filter nonlinear, say adding a diode
in with the resistor, you could move the RC filter's pole right along
with the Vactrol's.

(Depending on the details of the circuit you could get the
nonlinearities happening at different phases of the signal and end up
with some mighty weird behavir.)

  -- Don