I'm making a quick and dirty frequency "octupler" (x8) that uses a string of
three Mini-Circuits frequency doublers (AMK-2-13+, KC2-11+, KC2-19+) --
300-312.5MHz in, 2400-2500MHz output (the ISM band). I believe these are just
diode ring-based mixers hooked up and optimized to act as doublers; they have
an insertion loss of around 11dB. I have buffer amplifiers at the output of
each to make up for this loss, but I'm wondering... do I need to bother with
bandpass filters as well? Without them each doubler suppresses the
fundamental and 3rd harmonic by some 40-60dB (depends on input frequency); the
4th harmonic is suppressed by ~15dB. Is anything undesireable going to happen
if I just let everything through? That would mean that at the final (x8)
output, I'd still have plenty of lower frequencies around -- albeit pretty
well suppressed -- as well as some x12, x16, etc... but the x8 component
should still be the strongest by far.

For a diode ring-based mixer, it's not entirely clear to me whether or not
having some low-level "junk" to the RF input is all that bad. As a man once
said to me, "diode mixer RF ports like being driven by squares waves," so the
higher harmonics don't seem as though they'd matter much... but will the
lower-frequency signals tending to dither the exact turn-on point of the
diodes be deleterious?

---Joel