In (rec.radio.amateur.homebrew), Paul Burridge wrote:
Hi all,

Is there some black magic required to get higher order harmonics out
of an oscillator?
I'm only trying to get 17.2Mhz out of a 3.44Mhz source and am thus far
failing spectacularly. I've tried everything I can think of so far to
no avail. All I can get apart from the fundamental is a strong third
harmonic on 10.32Mhz, regardless of what I tune for. I've tried
passing the osc output through two successive inverter gates to
sharpen it up, but still nothing beyond the third appears after tuned
amplification for the fifth. I no longer have a spectrum analyser so
can't check for the presence of a decent comb of harmonics at the
input to the multiplier stage but can only assume the fifth is well
down in the mush for some reason. I could change the inverters for
schmitt triggers and gain a couple of nS but can't see that making
enough difference. What about sticking a varactor in there somewhere?
Would its non-linearity assist or are they only any good for even
order harmonics?
Any suggestions, please. I'm stumped!

There must be something killing the fifth harmonic, which should be
present at (1/5) of the amplitude of the fundamental in a square
wave. That's a pretty strong component.

If you can amplify the output of the source and then square it up
sharply, the fifth harmonic ought to be pretty easy to extract. The
larger the amplitude of that square wave, the larger the amplitude of
the fifth harmonic, of course, so amplification is your friend here --
but you may want to shield very well indeed to keep other components
out of places where they don't belong and may cause trouble.

Have a look at
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/geowv.html
for a lot of stuff that you probably already know.

Best of luck, and please keep us posted.

--
Mike Andrews

Tired old sysadmin