[time-nuts] Tuning a Trimble Thunderbolt

[Charles Steinmetz]

Pete wrote:

>when using the default
>parameters it doesn't meet the holdover specifications of +/- 1 us
>over 2 hours with a maximum of +/- 15C temperature change: it will
>drift at least 20 us over 2 hours in holdover.

Executive summary -- you are expecting way too much, way too soon.

>2. Is it typical for an oscillator in holdover to drift in a
>non-linear way?

Yes

>Can this non-linearity be corrected through the judicious choice of
>tuning parameters or some other means?

Probably not, but it should calm down significantly after the unit 
has been on continuously for several months.

>That document says the units under test
>were standard Thunderbolts (not Thunderbolt E's) and were on for three
>days and had a "training period" of two hours prior to the test.

I don't even know what "been on for three days and had a two hour 
training period" means.  Tbolts are training whenever they are locked 
and operating normally.

>My
>Thunderbolt has been on for a week and had been locked to the GPS
>signal for at least 12 hours prior to the test.

Quartz crystals are creatures of habit.  They need to be on 
continuously for months before they settle down to their best 
behavior.  And the Kalman filter needs to train long enough to see 
significant temperature changes and long periods of steady (that is, 
settled-down) drift.  Thinking in terms of days and hours is orders 
of magnitude shorter than any realistic expectation of good 
behavior.  If a Tbolt has been running locked and normally for 
months, and is turned off for a day and then turned back on, it may 
settle down in a day or three.  But if you bought it surplus, it may 
have been off for years, and banged around considerably (or much 
worse) between the last use and you receiving it.  In that case, you 
may well need to leave it alone for months before the crystal settles 
down and the Kalman filter is well-trained.

>3. Is it normal for there to be "spikes" in the phase and frequency
>error when the number of satellites being tracked changes?

Yes.  The better your survey is, the smaller they will be.  But even 
with the best possible survey, you will see 10nS plus or minus when 
the constellation changes.  And the poorer your antenna location is, 
the more often the constellation will change.

>I observe
>changes of ~100ns and 100-200ppt whenever there's a change in the
>number of satellites. Can this be smoothed out?

That's quite a lot.  I suspect you do not have an accurate survey, or 
you have a marginal antenna location (poor carrier to noise ratio 
and/or multipath problems), or both (the latter pretty much 
guarantees the former).  If you have multipath problems, a good 
choke-ring antenna can help substantially.

Find a good location for the antenna, find a good, out-of-the-way 
location for the Tbolt, and shield it from drafts (a simple cardboard 
box is a huge improvement).  [Check the archives for other 
solutions.  I have posted about my experiences with "cast aluminum 
box"es, "thermal mass," and "thermal capacitance," and many others 
have posted lots and lots of other ideas.]  Then, let it run for 3 or 
4 months without playing with it.  Don't change any parameters, don't 
go into manual holdover, don't do ANYthing.  THEN see how it 
works.  Until then, you're just chasing your tail tracking the 
crystal as it settles down.

Best regards,

Charles