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Another Jim here putting his $0.02 worth in.

If you run across step recovery diodes (SRDs), there is another time parameter you need to be aware of, minority carrier lifetime. Turns out that while you want a FAST transition or snap time so that high frequency harmonics are generated efficiently, you want a SLOW minority carrier lifetime so that you can put a low enough frequency in to get your frequency comb out.

Say you're putting in 100 MHz; you'd want a minority carrier lifetime >> 1/100 MHz = 10 ns. So probably 20 ns minimum would be a good value to use. From what I understand, if the lifetime is too short, the carriers will recombine before the next edge of the input waveform comes around, and you won't get much power out at multiples of the input frequency.

OTOH, suppose you're putting in 1 GHz to get 2, 3, 4 GHz, etc. out. A smaller lifetime would be in order, but if you wanted up to 20 GHz out, you'd need to use an SRD with a transition time of around 1/20 GHz = 50 ps. Longer transition times than that would result in lower power out at the higher harmonics.

Good luck!

Jim Ford
Laguna Hills, California, USA

"And we know that God causes all things to work together for the good of those who love God and are called according to his purpose for them." Romans 8:28 (NLT)