I made the "frequency mixer with op amps" that appeared in EDN some
time ago. I used the Tellun MUUB-4 pcb as a daughter card for a MOTM-
890 mixer that I recently purchased from the August sale. I used two
of the 5 inputs for the frequency multiplier and normalled the output
to input 5. It works pretty neat and functions as a full wave
rectifier with only a single input.
Schematics and photos at
http://modularsynthesis.com/modules/DJB-FM/djb-fm.htmDave