[time-nuts] Voyager space probe question

[Jeremy Nichols]

As the RTGs aged, the Voyagers were commanded to shed parts of their load.
Did the RTG voltage drop, was it the current-supplying capability (or both)
and how did that affect the oscillators, if at all?

Jeremy
N6WFO


On Sun, Nov 29, 2020 at 10:11 AM jimlux <jimlux at earthlink.net> wrote:

> For those interested in a "typical" (hah!) TCXO spec for a space radio,
> generically similar to the Electra UHF radios on and orbiting Mars, it's
> attached.
>
> The excessively precise frequency (49.244..... MHz) is because it
> matched a particular channel assignment for S-band, and the idea was to
> have the PN code (which is about 3 MHz, proportional to the carrier) be
> exactly 16 samples long.
>
> This is in pre "we can trust an NCO/DDS" days.  When it takes years to
> build your spacecraft, ordering a crystal with a 24 month lead time to
> get the frequency "just right" isn't considered a problem. Historically,
> the SDST used a VCXO with a crystal at the frequency at 8*f0, where f0
> is about 9.xx MHz, and multiplies up by 880 to the transmit frequency
> between 8.4-8.45 GHz.
>
> Today, we use 50 or 100 MHz oscillators (Electra uses 24 MHz, but it's
> an older design) and synthesize the carrier with a DDS feeding a PLL.
> For instance, the Iris cube-sat transponder uses a 50 MHz oscillator,
> and that drives a DDS running at 20 MHz, which is multiplied up in an
> integer N PLL to the carrier frequency.
>
> This is because the missions are shorter development time, and we don't
> want to have to know the frequency until after the radios are built (or
> at least, the oscillators are ordered). For Iris, there were 7 of them
> built for the Artemis-1 mission, and the frequencies are all over the
> space science X-band allocation.
>
> The SDST and older used a DRO as the microwave oscillator, and they just
> don't have the tuning range needed to cover 50-100 MHz tuning range (and
> lordy, we tried a bunch of techniques) - not do mention that DROs have
> noticeable microphonics because the physical cavity is part of the
> resonator.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>
-- 
Jeremy Nichols
Sent from my iPad 6.