[time-nuts] Expected 10 MHz offset from a GPSDO?

Bob Stewart bob at evoria.net
Fri Aug 19 15:07:42 EDT 2016

Hi Tim,
If I understand your post correctly, you're using a frequency counter to measure the accuracy of your GPSDO.  Assuming there's nothing wrong with the GPSDO, what you're actually doing is measuring the frequency accuracy of your counters.  Unless you have something that is known to be more accurate than your GPSDO, such as a *calibrated* cesium standard or hydrogen maser, then the GPSDO is the most accurate frequency reference you own.  At this point, other than work on improving your antenna, all you can do is compare your GPSDO to other GPSDOs, or a cesium standard or hydrogen maser, in order to quantify the phase error of your GPSDO.  Frequency error, except down at the milli-Hertz level or smaller, is no longer an issue.  



      From: Tim Lister <listertim at gmail.com>
 To: time-nuts at febo.com 
 Sent: Friday, August 19, 2016 1:20 PM
 Subject: [time-nuts] Expected 10 MHz offset from a GPSDO?
Hi all, I am starting to dip my toes into time-nuttery after being
interested in precise time/position for quite a while (I'm an
astronomer IRL). I have acquired a used OCXO and LPRO-101 rubidium,
along with a HP5334B (with Option 10) and a HP5370B from ebay and
almost done assembling a TADD-2 kit from TAPR.

I recently acquired one of the re-packaged Trimble UCCM GPSDO's (it
doesn't seem to be one of the bg7tbl ones) from ebay and have it setup
with the supplied puck antenna outside (it's not in a great location
(close to the house) but I have set the elevation mask to 20 degrees
in the GPSDO) and it's been running for 2 days. The results of the
SYST:STAT? command indicate it is in position hold and tracking 3-4+
satellites.  Connecting the 10 MHz output to the Channel A input (on
the 5334B) or the frequency input (on the 5370B) and setting 50 Ohm
termination and AC signals but without any other input or external
reference results in a frequency that is off by about 0.1-0.2 Hz. Is
this amount of offset expected or a sign something is not right with
the GPSDO?

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