Joel Koltner wrote:
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?


Yes, anything below the desired 8x frequency and anything above that
isn't a harmonic of it will cause spurious mixing products when your
intent is to drive the LO port of a DBM with it. What happens within
your octupler doesn't matter much as long as you filter the final output.

BTW, in the olden days we preferred 3x, 5x per stage. Two 3x stages
would give you 9x. The most brazen ones (not me) did a direct 7x or even
9x. We just drove transistors in class C real hard.

--
Regards, Joerg

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