[time-nuts] Trimble "UCCM" GPSDO adevs

John Ackermann N8UR jra at febo.com
Thu Feb 23 19:49:49 EST 2017

This is very, very cool, Mark.  I've been concerned for a long time that 
people take way too seriously what you properly call "bogo-ADEV" that's 
self-generated from the GPSDO.  Integrating a counter that compares 
against an external source is a huge win.

But I want to remind everyone that as neat as the TICC is, like any time 
interval counter it still has limited performance for short-term ADEV. 
The ADEV floor is typically a bit under 1x10e-10 at one second, and goes 
down at a slope of -1 (so, a bit under 1x10e-13 at 1000 seconds averaging).

If you're comparing a pair of decent sources, results at measurement tau 
of less than at least several hundred seconds are likely showing counter 
jitter, not real performance.  It's important always to keep that in mind.


On 02/23/2017 05:27 PM, Mark Sims wrote:
> I just added the capability of Heather to monitor and control a GPS device while simultaneously calculating ADEVs from an external time interval counter like the TAPR TICC  (it can also work with the time interval analyzer only).  This mode lets Heather calculate true adevs instead of "bogo-adevs" derived from the GPSDO self-reported control loop statistics.
> As a test.  I connected Heather to a Trimble "UUCM" GPSDO and a TICC.  The TICC channel A is the UCCM and channel B is a Oscilloquartz Star-4 GPSDO.  The TICC reference is a ERC-130 rubidium (uses an LPRO-101.  Attached is a screen dump of a 12 hour run.  The UUCM beats the Star-4 at tau<20 seconds.
> There are a few variants of the UCCM receivers out there... one by Symmetricom and at least two (UCCM and UCCM-P) by Trimble.  They run off of 6VDC.   Performance is not too shabby for a small $70-$120 GPSDO.   Search Ebay for "Trimble 65256",  "Trimble GPSDO", or "Symmetricom GPSDO".   Be aware that there are sellers sending units that were damaged in salvage.   I'd probably go with a "tested" unit.
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