I read in sci.electronics.design that John Fields jfields@austininstrum
ents.com wrote (in ) about
'Frequency multiplication', on Mon, 16 Feb 2004:

Starting with a perfect square wave at f1, bang the hell out of a diode
with it, and then bandpass it and the 3rd harmonic (f2) separately, then
mix them to get f1, f2, f1+f2, and f1-f2. Using a doubly balanced mixer
will get rid of f1 and f2, then notching out f1+f2 will leave f1-f2,
which will be 2f1, that non-existent second harmonic.

No need to abuse any diodes. The third harmonic is already there, just
10 dB down. You only need a bit of gain after the peaky filter.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk