[time-nuts] Voyager space probe question

[Andrew Rodland]

RTGs, much like solar cells, have a terminal voltage that drops pretty much 
linearly with the current you draw from them, and a "maximum power point" 
where V*I reaches a maximum (see for example page 37 and several later pages 
from https://apps.dtic.mil/dtic/tr/fulltext/u2/a057483.pdf ). As they age, the 
open-circuit voltage goes down, and so does the maximum-power current. So the 
goal with load shedding is to keep the current to a level where the RTG 
develops a sufficient voltage to make everyone happy. Exact details depend on 
what kind of regulators you have downstream (I have no clue, in Voyager's 
case).

Andrew

On Sunday, November 29, 2020 3:39:37 PM EST Jeremy Nichols wrote:
> 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.
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