Martin writes:
ML>        Thanks all for the suggestions.  I am curious about Gary Skinner's
ML>suggestion.  How much higher than 60Hz is "much higher"?  I do a single PWM
ML>control loop that drives 8 separate actuators.  They all function with the
ML>same master cycle.  All turn on at the same time and I just toggle the bit
ML>for each off when the time for that one runs out.  The 20MHz PIC is doing
ML>about 230Hz at best the last I checked.  Is that fast enough to avoid some
ML>sort of beat frequency effect?

I don't really know exactly what much higher means.
The PWM is chopping up the sinewave into pulses.  If it chopped up each
cycle into at least 3 or 4
pieces, then there should not be any "beat frequency" type of problems.
I suppose you could do a lot of math and verify some results, but I
would probably build a
simple device and measure the RMS output as a function of PWM on/off
ratio.
I expect it will be pretty linear with 230Hz period.

Please let us all know how the experiment works out.

Thanks,  Gary Skinner  ESI