Just did an interesting little 'speriment. Standard diode "full wave
rectifier" frequency doubler. Transformer is 16 trifilar turns on an
FT50-43 core (should be a bit more than 50uH per section). One of the
triplet is the primary, and the other two are connected as a
center-tapped secondary. The diodes are 1N4007 -- yep, the 1kV
mains-freq rectifiers. Excitation comes from an HP3326, set to square
wave output, source impedance 50 ohms. 50 ohm load impedance on the
doubler output (input to spectrum analyzer; DC coupled load). HP3326
square wave risetime is about 10 nanoseconds, I believe. Excite at
0.5MHz, +/-2V (4Vp-p) Output waveform observed on a fast scope is
frequency-doubled, close to 50% duty cycle, with fast falling edges
and slow (200nsec) rising edges. Amplitude about 2Vp-p. Strong
spectral output on all even harmonics; all odds suppressed about 20dB
from the low evens, and I'm sure would be much lower with better
matching of the diodes.

Explanation left as an exercise for the reader, but should be obvious
from previous discussion here. I'd guess 1N4148-type diodes would
behave similarly for an input around 100MHz.

Cheers,
Tom

(Tom Bruhns) wrote in message om...
(Avery Fineman) wrote in message ...
... Suffice to say
that a square wave cannot be used with a passive diode doubler; all
the energy is contained in the short transition times and that is rarely
enough to be worth it.

?? Lots of energy in the fundamental; filter to extract the
fundamental and feed it to your full-wave rectifier doubler.
Efficiency can be high if the filter does not cause dissipation in the
source at the harmonics.