Scott Stephens wrote in message news:tnamc.36397$0H1.3120575@attbi_s54...
If I have a waveguide, coax, or even lumped varactor tuned LC tank,
could I simply select which harmonic I want it to oscillate at, by
tunning a negative resistance?

A coaxial resonator will resonate on (or near) odd harmonics of the
lowest resonance, but you'll probably have trouble getting an LC tank
to have significant response at harmonics. Then the tuning will be
problematic, I think: you need to cover 3:1 for the lowest resonance,
and only 5:3 for the second, 7:5 for the third, etc., but capacitive
tuning would need a capacitance for the lowest resonance which is
awkward for the higher ones. Maybe you could only use the resonator,
say, at and above the 7th harmonic of the lowest resonance, and forget
about the 1st, 3rd and 5th.

Perhaps with the use of a "register key", such as is on flutes and
clarinets? I suppose that functions as a high-pass filter on the line,
when the hole is unplugged.

Just as an overtone crystal oscillator selects the overtone, you
should be able to do similarly with a coaxial resonator oscillator.
There's likely a practical limit. (The coupling will change with
which resonance is selected, no?)

I'm interested in a multi-decade VCO, tuned with one control voltage and
just a few switches.

You can switch both the C and the L of an LC tank and cover a wide
range...but it's often easier to just build separate oscillators and
switch among them.

No, I don't want to heterodyne or synthesize, because I'm interested in
the regenerative/super-regenerative capabilities.

Any applicable app notes or articles? Dream on?

I predict there will be a whole lot to learn on your way to your
goal...but remember the old regen radios that used plug-in coils for
different bands? Some of them could cover quite a range. You may be
able to accomplish something similar with diode switching.

Cheers,
Tom