Delete comment from: Ken Shirriff's blog
Thank you, another informative piece!
I suspect that rather than step recovery diodes, varactor diodes were used in the multiplier stages. Especially for the transmitter, they are very much more efficient than step recovery diodes would be. For instance: see a contemporaneous paper at https://core.ac.uk/reader/85250570
Also the 240/221 is easily 'explained' by considering the technology and techniques in common use at the time, and taking into account power, size and weight constraints. A minimum of a dual conversion receiver architecture would be suggested. Similarly, to maintain phase coherence, local oscillators with a 'nice' multiple of the reference are desired. Neither PLLs nor dividers of a suitably high frequency were in common use at the time.
So a received frequency of (LO1 = N1 x Fref) +/- (LO2 = N2 x Fref) +/- Fref is required, where N1 and N2 are readily factorised integers.
From the included block diagram of the transponder it's clear how multipliers are shared (and so minimised in number) between N1 and N2 for the receiver and the required x240 for the transmitter.
Also of interest from the Apollo (and earlier) programs is the VHF communications system. How it grew from a simple, effective voice system to incorporate an elegant 'bolt on' range finder. Well worth studying for more elegant solutions with the constraints imposed.
Apr 27, 2022, 8:59:26 AM
Posted to The digital ranging system that measured the distance to the Apollo spacecraft