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