[time-nuts] Symmetricom 2700 Time Source

Peter Bell bell.peter at gmail.com
Fri Nov 18 15:31:16 EST 2011


I haven't used this specific unit, but I have used CDMA based timing
devices - the long term stability is obviously not an issue, since
it's ultimately locked to GPS - the short term stability varies
depending on how many different pilots you can track in your location.
If it's only one, the result is going to be slightly worse than using
a GPS directly - if you can pick up many pilots with good quality,
then the results can actually be better than using a single local GPS,
although obviously you are subject to far more external factors.

The other thing is more of a conceptual issue - that box appears to be
intended to be used as a BITS for a telecom network so you will likely
find that the default settings are not the ones that a timenut would
use - the main concern for telecoms people (especially in wireline
service) is to make sure that you never get enough clock error to
result in a frameslip - this tends to result in them using quite short
integration periods for the frequency adjustment and frequent
oscillator tweaks - so if you find it seems to have a lot of jitter,
make sure to check the settings.

Regards,

Pete Bell


On Fri, Nov 18, 2011 at 11:05 AM,  <Brucekareen at aol.com> wrote:
> Following a recent posting from a group member advising that Symmetricom
> 2700 CDMA Primary Reference Sources contain SRS PRS-10 rubidium oscillators,
> I  was able to buy one of these units at reasonable cost from the usual
> auction  source.  After removing the PRS-10 oscillator and while evaluating it
> with a laptop computer on the bench, I began to wonder if rather than trying
> to  discipline it with a Thunderbolt GPS receiver, it would make sense to
> just utilize it in the Symmetricom box as intended.  The Symmetricom 2700
> has a 10 MHz sine-wave output.
>
> From reading the Symmetricom manual, it appears the model 2700 was designed
>  to serve as a precision, GPS-based source for system timing.  It  avoids
> the requirement to directly receive GPS satellite transmissions by  utilizing
> multiple CDMA cell phone service base stations  as intermediaries.  The
> cell phone transmitters are locked to GPS  satellites and their 1900 MHz
> terrestrial transmissions are easier to  receive than GPS.  The manual says an
> indoor antenna is usually  adequate.  If your CDMA cellphone works in a given
> location, this box  should too.
>
> But with the cell phone base stations serving as intermediaries, by
> monitoring GPS signals and using them to synchronize the cellphone base station
> pilot-frequency transmissions, would the performance of  Symmetricom's
> disciplined oscillator be as good as one directly disciplined  from GPS satellite
> transmissions?  It would seem that since the system  should never even slip
> one Hertz over the years, the long-term accuracy should  be good.  The
> remaining consideration would be intermediate-term jitter  such as caused by tree
> branches blowing in the wind and affecting the 1900 MHz  transmission
> phase.  Short-term behavior might be fairly good as  the SRS-10 has a very stable
> crystal oscillator and low overall phase  noise.
>
> I would be interested in opinions and experience with the Symmetricom
> equipment or this method of disciplining an oscillator.
>
> Bruce, KG6OJI
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