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@BlueCoder's nice answer to Syncom-3 was the first geostationary satellite. Anyone know of a description of the onboard electronics? mentions that the the master oscillator was doubled eight times to get to (nearly) X-band.

The answer links to NASA Technical Report R-252 “Syncom Engineering Report, Volume II” and in the neighborhood of pages 10 to 12 there is a description of the RF system and Figure II-7 shows a block diagram.

28.8917 Mz times 256 ($2^8$) does indeed equal 7396.275 MHz, but the block diagram only shows seven frequency doublers rather than the eight mentioned in the text.

How does the "missing" doubling come about? What is a "Multiplier Inverter" and is that the "secret sauce" that does it?

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The first frequency doubler is part of the "Master Oscillator". Note the "X2" in "28.8917X2". A reason to include it in the drawing like this might be that, together with the oscillator itself, it is the only 'active' component in the frequency generation block. The first stage is the only one build with a transistor amplifier. The base oscillator requires such a stage to extract a clean and strong signal which then can be further amplified to the stated 300 mW. All stages after that are 'passive' in a sense that the output signal is always weaker than the input signal - like the 32x multiplier that only forwards one fifth of the input power.

About the function of the "multiplier inverter" I can only speculate, especially because it doesn't have any inputs. It might be some feedback loop to stabilize the circuit.

I found a nice paper detailing how these "Varactor based frequency doublers" work: https://www.jhuapl.edu/Content/techdigest/pdf/APL-V04-N02/APL-04-02-Thompson.pdf

If you're interested how this circuit on Syncom-3 might have looked like, the paper contains an image of the circuit used in the GEOS satellites in 1965 on page 10.

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  • $\begingroup$ Well I am not sure yet. If you look at the full size diagram, there are TWO identical oscillators. If you look at the dashed lines, the upper region is called Quadrant 3 and the lower Quadrant 1. Each has a 28.8197 MHz oscillator, so I'd assumed that this "X2" indicated that there were two instances of the oscillator in the diagram. I wonder if I am overinterpreting? $\endgroup$
    – uhoh
    Commented Aug 20, 2018 at 10:54
  • $\begingroup$ @uhoh The whole circuit exists twice for redundancy. If the "X2" refers to that, you would have to add it to every single box, e.g. to each "Attenuator", to "Multiplier X32" and so on. $\endgroup$
    – asdfex
    Commented Aug 20, 2018 at 11:22
  • $\begingroup$ No one would not "have to" do that. An oscillator used by a spacecraft's communication system has a higher status than an attenuator, and might receive special attention if there were two rather than one of them on board, whereas it's likely going to have a plurality of amplifiers or attenuators with or without redundancy. That said, Occam tells us that you are probably right about the "2X" instance in question meaning exactly the same thing that every other "2X" instance means! $\endgroup$
    – uhoh
    Commented Aug 20, 2018 at 11:36
  • $\begingroup$ Thanks for the legacy varactor article! Enjoying it now. $\endgroup$
    – uhoh
    Commented Aug 20, 2018 at 11:38
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This is quoted from the text on page 13 of that wonderful R-252 Tech. report: "The frequency multiplication is accomplished in eight doublers, of which all but the first involve the use of varactor diodes connected push-push in efficient duo-mode networks. The first doubler uses a conventional transistor circuit"

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  • $\begingroup$ 100% agree with "wonderful"! I love reading these NASA reports, thorough and clear. Thanks for pointing that out - so it makes even more sense now for it to be incorporated within the same block of the diagram. $\endgroup$
    – uhoh
    Commented Aug 24, 2018 at 1:39

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