[time-nuts] Three-cornered hat on timelab?

Bob Stewart bob at evoria.net
Sat Apr 29 18:03:04 EDT 2017

Hi Jim,
I'm not sure you're plotting what you think you are, but perhaps I misunderstood.  The phase error data contains both the position uncertainty of the Adafruit (constellation, ionosphere, etc) and an error caused by correcting the OCXO using that phase error.  IOW, the fact that the phase error puts the OCXO back in phase is problematic.

You might think about disconnecting the EFC from the OCXO and feeding the OCXO directly with a fixed voltage derived from the VRef output of the OCXO, assuming it has one.  Then, carefully adjust the VRef voltage so that the phase error changes very slowly.  Let it cook for a few days and restabilize, then start logging your phase error.  Feed that to Timelab and see what the plot looks like.  Timelab should be able to remove the aging, so that you wind up with a plot that's mostly the Adafruit.  Of course, that depends on which OCXO you're using.  I've had good luck with the Trimble 34310-Ts that are about $20 each depending on the vendor.


      From: Jim Harman <j99harman at gmail.com>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Saturday, April 29, 2017 9:15 AM
 Subject: Re: [time-nuts] Three-cornered hat on timelab?

On Thu, Apr 27, 2017 at 12:48 PM, Bob Stewart <bob at evoria.net> wrote:

So, back to my question:  Where are the large ionospheric phase moves?  This question has been causing me doubt since I started on this project.  Or don't I still have enough data collected for this to happen?


Bob, my test setup is a good deal simpler than yours, but attached is a plot that I think shows the variations you are looking for quite clearly. This is data from my homebrew GPSDO, which uses an Adafruit non-timing GPS module and a run-of-the-mill surplus OCXO. The plot records the phase comparator output over a period of about 1 week. The time constant of the PLL is 1024 seconds and it is plotting the 5-minute average TIC values. 
The full horizontal scale is 24 hours.
The vertical scale shows the data from several days with the traces for successive days offset upwards by the equivalent of 40 nsec.
As you can see there is pretty good correlation of the phase error from day to day and the wiggles migrate to the left a little, corresponding to the 23:56:04 siderial repeat time of the GPS constellation.This is with a pretty good antenna location, under a shingle roof in the attic. I calculate the day-to-day correlation at about 0.8.
Making the time constant larger increases the variations somewhat, because the loop does not adjust as much, and they definitely get worse if I use a less optimal antenna location.


--Jim Harman


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