[time-nuts] Simple questions on GPSDO

Joseph Gray jgray at zianet.com
Sun Sep 27 07:20:14 UTC 2009


Four nights ago, I hooked up one of my Z3801A's for the first time in
over three years. When I last had things up and running, I was just
getting into trying to understand how these devices work. Since then,
I have forgotten much of what little I learned back then. Knowing the
level of expertise here on the list, I would ask your indulgence while
I ask some simple questions.

Here is a plot from GPSCon Pro of the Z3801A that I got running almost
four days ago: http://www.zianet.com/jgray/temp/Z3801A.png. Forget
about the huge excursions at the beginning. I powered the unit on some
hours before I hooked it up to a PC and told it to do a position
survey (I moved across town from my previous location). Upon powerup I
did initially see the GPS Lock LED, but then it went to Holdover mode
until the survey was in progress.

>From looking at the EFC graph, I assume that even after almost 48
hours, the GPSDO hasn't yet reached it's best performance (correct?).
The PU at this time 3.4uS. I understand that this is a prediction for
the next 24hrs if GPS lock is lost. The question is, what does the
3.4uS apply to? I assume it is the PPS. Since the PPS and the 10MHz
are phase locked, then the 10MHz drift when unlocked should be
proportional, right? What do the numbers for the EFC correspond to? I
don't know what units are being used for these numbers. I understand
that LPF is Low Pass Filter, but what exactly is that number telling
me?

On the TI graph, are those data points showing me the corrections made
to the internal clock, based on GPS data? According to the graph, over
the past 48hrs, the average difference of this GPSDO from UTC is
14.77nS, correct? How does this compare with most other Z3801A's?

Sorry for so many questions. Any additional expanations are encouraged
and welcome.

Joe Gray
KA5ZEC




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