[time-nuts] Symmetricom 2700 Time Source

[Brucekareen at aol.com]

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