(ADEV of 4E-16 at tau of 1000 seconds is a typical state of the art requirement)
…. and has been since the 1970’s when I first started talking with JPL people
about this :)….
They've gotten a lot smaller and probably draw less power since then.
There's also the testing problem: proving that it has that performance
is no easy matter.
It might well be that you could build a better one, but that other
errors in the whole ranging measurement might dominate (movement of the
antenna on Earth, etc.)
There's a lot of activity centered around making good flight atomic
clocks with trapped Hg ions. Those would essentially instantly improve
over the USO.
====
All that said, the real question is — can you change the fabrication of the
crystal in ways that improve it’s stability by
tuning a long way with DDS rather than a short way with reactance (select parts
plus varicap(s)).
The DDS synthesis thing is nice because you can, to a certain extent,
control where the spurs and noise are (at the cost of increased logic
complexity, but that's essentially free, these days), and you can open
up a trade space on crystals. You don't have to have certain "special"
frequencies.
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