-> > 1)  Use a dual speed PLL to initially lock onto the strong
-> Would it really stay locked to the weak fundamental?

Yes, I believe so. A PLL CAN track a signal outside its capture range if
it has been locked onto it in the first place and its loop
characteristic are such that it resists change that is too fast (a
change from the fundamental to a harmonic can be considered a fast
change). When the harmonic comes into the capture range (close enough in
frequency and strong enough compared to the fundamental) the PLL won't
move to it because its loop characteristics inhibit fast change.

-> It can't be that easy, can it?  I'm already amplifying and heavily

No, you're right, I goofed. I'm still trying to find the reference in
"The Art of Electronics" that got me confused on that one (An excellent
reference BTW. I highly recommend it).

-> > 4)  Use the pic to implement an FIR or IIR filter.
->
-> What are these?

Digital filters. You need a very fast CPU with an analog front end.

-> How does one "purify a spectrum" with a PLL?  What does that mean?
-> But yes of course it is possible to track changes in a harmonic as
-> readily as the fundamental.

What I mean by that is that A PLL will lock onto one spectral component
(frequency) if it is coherent, stays within the tracking range, and
doesn't change too fast (slew rate). If you take a standard audio range
PLL and pluck a guitar string the PLL's oscillator will be one
frequency; The base note (terminology correct?), and the phase should be
shifted at around 90 degrees from the fundamental. The PLL will either
stay locked onto the fundamental or switch abruptly to a harmonic if
that harmonic is strong enough.

By measureing the PLL's oscillator frequency, you arn't getting a lot of
noise due to harmonics. Its only one frequency, changing over time.

Good luck with your project.

Regards, Dana Frank Raymond - Foxtrot Systems Ltd.
Internet: dana.raymond@canrem.com. Compuserve: 73362,3052