[time-nuts] GPSDO and oscillator steering - EFC vs DDS schemes?

[Jim Lux]

>> (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.