I can build a diode doubler to generate harmonics and then put a 1/4
wavelength of shorted-end coax across its output to select the 902 MHz
harmonic, but I'd like to be able to tune all freqs between the 512 and
1024 MHz without building a lot of shorted stubs. How does HP do it with
their doubler? Bandpass filtering? Could one bandpass filter cover that
much range?? Maybe something as simple as a low pass filter with an Fco
of 1050 MHz or so, but with this, how would I keep the fundamental freq
(256-512 MHz) out of the output??

Thanks for any leads!

--
Scott N0EDV

You don't need the filter. The "push pull" doubler will suppress the
fundamental, (By as much as 40 dB if it's well balanced). Just tune the 8640
to 451 MHz and calibrate the output. If you wish to gild the lilly, you
could install a HPF with lower corner frequency of 512 MHz, but the original
signal isn't much cleaner WRT harmonic content from the dividers.

W4ZCB