John's homebrew pages

Comb generator

Yet another little sideline as a result of the 23cm transverter project.

I had a problem in that I was not happy that the 96MHz was coming out of the OCXO properly at a suitable level. Unfortunately 96MHz was (a) in a gap between two old wavemeters I had (see the old projects page), and (b) on top of a very strong BBC Radio 4 FM signal! So I decided that I needed something to measure it with - a suitable wavemeter - which would need calibrating - hence this comb generator. I thought I could knock something together quickly with an old crystal oscillator module (off a very old Creative Soundblaster cad - one of the big long ones!) which had a frequency (45 and a bit MHz) which would produce enough harmonics at useful frequencies, but would not be on top of the frequencies I'm trying to generate with the transverter local oscillator. Here's the result:

comb generator (annotated)

The circuit is below. The 2N3866 (VHF transistor) provides a low impedance output buffer to drive the diode multiplier (the 1N4148 works fine), and the MMIC bumps the signal level up for output.

comb generator circuit

The comb generator was fine for calibrating the low VHF wavemeter, but needs some harmonics to be selected and amplified for it to be really useful with the microwave wavemeter. You can see there's space left on the board for that.