If I understand correctly, you're interested in harmonics up in the
70MHz region. To me it seems silly to use diodes capable of
generating a comb of harmonics out to 20GHz for that. An advantage--a
big advantage--of the tiny logic is that you get considerable power
gain in the stage, and the sot-23 package is likely no larger than the
diode you might have used anyway. The edges, as others have pointed
out, are PLENTY fast enough for what I believe you want to do.

Whether you use diodes or gates, your size problem will be the
harmonic-selection filter. As you've learned (and I trust not
forgotten already), you need either multiple poles in your filter or a
rather high Q. You _could_ get the high Q with crystals, but of
course then you're locked down to particular frequencies. For high
multiplication factors to low-VHF frequencies, it's probably hard to
beat a PLL for (potentially) small size.

In offering suggestions, it would also be helpful to us to know your
actual needs for signal purity, both close-in (phase noise) and
broadband (other harmonics, etc.)

If you do use diodes for higher-order harmonic generation, and not
just a simple full-wave-rectifier type frequency doubler, I suppose
you want something of the nature of a step recovery diode. That
implies minority carrier stored charge in the diode, and that would
preclude using a Schottky diode (which would work great in the
full-wave-rectifier type doubler). If you get into actually wanting
to generate harmonic combs out to microwave frequencies, it's probably
worthwhile looking for diodes actually characterized for step recovery
service. But I really think that's way beyond what you are trying to
accomplish right now.

Cheers,
Tom


Paul Burridge wrote in message . ..
On Sun, 11 Apr 2004 14:14:56 +0200, "Fred Bartoli"
r_AndThisToo wrote:

Don't let Paul dream too much about picoseconds.

Well, quite. I wouldn't know what to do with all those extra
harmonics, anyway. :-)
Diodes sound great, but I'm heavily put off by that comment earlier in
the thread that they're prone to self oscillation. :-( What's a
"parametric device" anyway?

In order to achieve results that good, one have to pay very careful
attention to "wiring". There's still a lot of room for Paul to improve his
PCB design skills ;-)

Fair comment. But I *am* working on it!