optical limiter problem

[Rene Schmitz]

At 15:37 04.02.1998 +0100, you wrote:
>A few weeks ago I tried to build a stereo optoelectronic limiter, using
>a Vactrol 5C4/2, and I ran into a problem I did not expect.
>
>The idea was:
>
<snip>
>What I suspect is this:
>
>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.
>
>This is hard to optimize:
>Decreasing loop gain would lead me away from a rather hard peak
>limiting.
>Making the electrical time constant faster than the optical results in
>almost unchanged
>  distortion.
>Making the electrical time constant longer than the optical one results
>in far too long
>  response time.
>Making the electrical time constant approximately the same as the
>optical one means
>  the poles are identical, which will produce the highest overshot.
>
>So far, I have not experimented with a real electrical peak detector in
>front of the
>LED driver, but I have just connected elkos across the LED of the
>Vactrol. So
>I have combined an asymmetrical (good!) optical response with a
>symmetrical (suboptimal)
>electrical respomse. So I am sure there is room for improovement.
>Meanwhile, I have put this project aside for some time. But I'd love to
>get some hints
>from others: Have you been running into similar problems? What did you
>do?
>How are the commercial optical limiters optimized? (BTW: *are* there
>actually
>optical hard peak limiters, or do they have limited gain reduction in
>the first place ?)
>
>Any ideas ?
>
>JH.
>

Why don't you try compensating the vactrols nonlinear response, and use a 
feedforward solution instead, that way you would get arround the loop, and the
associated problems. Usually a loop will be slower than feedforward.
One way could be determining the R/I characteristic, and use a nonlinear
function that compensates for the nonlinearities. OR use two vactrols, and 
use one for the signal, and the other into a feedback-servo-loop to determine
the input current of the two vactrols, that way the linearity will be improved.
(Partially perhaps even the time response, but I'm not sure..)

Bye 
  Rene


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