[time-nuts] Symmetricom TimeSource 2500

Attila Kinali attila at kinali.ch
Tue Jan 23 18:03:21 EST 2018

On Tue, 23 Jan 2018 12:28:45 -0500
Bruce Hunter via time-nuts <time-nuts at febo.com> wrote:

> A key point is that no provision is made to control the frequency of the 
> PRS-10; it is simply monitored and the offset from GPS continually 
> recorded.  In other words, the PRS-10 does not influence the accuracy or 
> noise of the 10 MHz output unless the GPS reference is lost. 

And it's exactly that hold-over performance that you need for a
communication system. UMTS demands IIRC to keep sync between adjacent
base-stations better than 10µs at any time. LTE can be as low as 1.5µs.
The lawmaker used to demand 24h hold-over and most countries seem to
have switched to 72h in recent years. 1.5µs over 72h requires
frequency stability at the level of 5e-12. Even if you have a
well characterized high-quality OCXO getting bellow 1e-10 at these
time scales needs a bit of fiddling and going below 1e-11 is not easy.
(unless you apply techniques that can cancel out flicker noise,
but as far as I am aware of, nobody does that and I have no
experimental numbers on that, just simulations)

As for GPS operation, the advantage of an Rb vs a good OCXO is not
that big. There is a range between a few 100s and 10-20ks where a
GPSDO with an Rb reference can perform better than one with just an
OCXO. But it's not going to be more than a factor of 10. Beyond..uhmm..
let me guestimate 50ks the GPS will dominate the frequency stability.

I hope that clarifies the need for an Rb in such a unit.

			Attila Kinali
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.

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