[time-nuts] Phase noise from Allan Deviation ?

[Charles Steinmetz]

Tom wrote:

>The 'unknown' is a Rubidium oscillator locked to CDMA pilot (TS2700)

There is very little information publicly available on the 
Symmetricom "BesTime" engine ("BTE"), but after playing with a few 
TS2700s for quite some time, including monitoring a number of 
internal signals, several things became apparent.  First, the 2700 
does not seem to discipline the PRS10.  The rubidium runs open loop 
and the BTE keeps track of the offset and the drift rate from "BTE 
time" (which is synthesized from all available sources -- however 
many CDMA signals it is receiving, plus any wireline telco timing 
signals and the PRS10 -- using a proprietary algorithm to estimate 
the reliability of each source and outputting BTE time and frequency 
using DDS).  Hobby users won't be feeding the unit any telco timing 
signals, so the BTE has only the CDMA signals to work from.  During 
holdover (and assuming no telco timing signals), the Rb is the sole 
input to the BTE, which uses the stored offset and drift to calculate BTE time.

I found that the TS-2700 is more than an order of magnitude less 
stable than a Trimble Thunderbolt, even with a full complement of 
rock-solid CDMA sources.  This may vary somewhat, depending on the 
CDMA equipment in use at any particular location and the diligence of 
the CDMA operator.

Best regards,

Charles