[time-nuts] Voyager space probe question

[jimlux]

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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