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 <jim...@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@lists.febo.com > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.