The 5th harmonic should be only 14dB below the fundamental, although it will
drop fairly quickly as the sides of the input square wave deviate from vertical.

Does the 3.44MHz have a 50% duty cycle?

Are you filtering before amplifying (eg a high impedance 3 pole
bandpass/highpass L-C filter with a gain of about 5 at 17.2MHz).

Does the inverter supply a decent square wave under the load of the filter?

If all else fails, could you reverse the process - generate 17.2MHz and divide
it down to 3.44MHz?

(Many) years ago I made a functional TV modulator for an Apple ][ PC by pulling
out the 3rd harmonic of the 14.318MHz system clock. I know it was only 3rd
harmonic, but it was at ~43MHz, so I would expect similar or better logic should
be able to produce 17.2MHz for you.

On Fri, 12 Mar 2004 13:56:10 +0000, 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!

Tony (remove the "_" to reply by email)