Edge response, dynamics, and really ugly time constants

For all the digital (i.e., switching and class D) circuits in my induction
heater, I am now right back to a good old analog amplifier. I have a
control voltage input to adjust frequency, and three control voltage outputs
relating frequency (an absolute/open loop control), phase and voltage
characteristics of the circuit. Problem: I have a monster time constant in
the circuit, a reasonable Q resonant tank. (Heh, and you thought tube amps
were bad things to wrap NFB around!)

This is the edge response for a square wave input:
http://webpages.charter.net/dawill/Images/Induction725.jpg

Counting cycles, it appears to be about 22.5kHz for the high-amplitude
region, and 23.8kHz for the low-amplitude region. The ringing appears to be
the difference between driven frequency and characteristic (resonant)
frequency. (This is above resonance.) I don't remember exact resonance
frequency, but if my hypothesis is correct (I don't know anything formal
about dynamic resonances), it should be around 21.2kHz.

The signal input is square, 250Hz, 50% duty cycle, at some small amplitude
(I didn't measure it). I also didn't measure the phase detector's signal
output.

As long as I'm posting, I might as well add a question.. how do you all
prefer to deal with cantankerous loop responses like this?

I just need to stabilize the output within 0.1 seconds or so, a feedback
response time certainly long enough for the tank to stabilize inbetween
increments, but I get the feeling it'll hunt a lot over that time, too...

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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