[time-nuts] High-end GPSDO's

[Attila Kinali]

On Sun, 19 Aug 2018 15:27:03 -0700
Ralph Devoe <rgdevoe at gmail.com> wrote:

> Thanks for the notes on the FS740.  I missed it back last November. It
> looks so user-friendly that I might have to get one. Most SRS stuff is
> really well done.

Yes, indeed. All SRS schematics I have read looked really like someone
who knew what he is doing did the design. The newer designs also include
comments in the schematics about the design decisions, which is really
nice IMHO.

> The question I have is whether it is practical to get to the 10(-13) level
> with a good Rb and a good GPSDO. The Rb's aging rate is low enough, but, as
> Corby has pointed out,  they are sensitive to temperature / pressure so
> that they can move faster than the GPSDO can correct for. I'm not clear how
> far this can be pushed.

If you have a HP5065 and use Corby's modification, then getting to
1e-13 should be reasonably easy. If you have a PRS10 or LPRO, you
will have to do temperature and probably also pressure compensation
to get to the stability needed. But I cannot say whether you can
actually get down to those numbers. I am currently building a setup
to measure the temperature and pressure dependence on an LPRO, 
but it will take some time until everything is ready, and then
it will take a couple of months until I have reliable data.

BTW: I recommend using some digital scheme for offset frequency generation
and not touching the C-field adjust of the Rb. Either a simple DDS with
enough bits should do, or a divider scheme like Rick's synthesizer[1].

			Attila Kinali

[1] "A narrow band high-resolution synthesizer using a direct digital
synthesizer pollowed by repeated dividing and mixing," by Rick Karlquist, 1995
http://www.karlquist.com/FCS95.pdf
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