In article , Paul Burridge
writes:

Is there some black magic required to get higher order harmonics out
of an oscillator?

[did you miss a class at Hogwarts? :-) ]

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.

As others have suggested, the duty cycle may be off such that
the 5th harmonic is not as strong as it should be (it would be only
about 10% of the fundamental frequency with the 'best' duty cycle).

Part of the problem can be in trying to L-C tune logic gate outputs,
presuming a great deal that such is what you are doing. If you must
use logic gate inverters, use an open collector kind and put a 5th
harmonic parallel-tuned circuit there and couple it to a relatively high
impedance buffer amplifier input. Or, use a transistor stage and tune
the collector (or drain if FET) to the 5th harmonic.

With TTL gates the output characteristics are non-linear in that a
conducting-to-logic-0 state is a rather low impedance source while
output conducting to a logic-1 state is a medium impedance source.
The resulting loading is not good for trying to filter out a 5th or higher
harmonic. An open-collector output allows the average output Z to
be higher with less upset of a tuned circuit.

The above also applies to a series-tuned L-C circuit for the 5th but
that may be an advantage with the curious impedance of TTL gate
inputs. Using CMOS logic gates over all might prove to be an
advantage since their input impedances are quite high and most
output characteristics don't differ as much between logic 0 and 1.

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?

Varactors don't create harmonics all by themselves. Those need to be
"tuned" either through resonant circuits or harmonics selected via
L-C filters. What you want to do can be done with stock parts but in
different arrangements.

Len Anderson
retired (from regular hours) electronic engineer person.