[time-nuts] Trimble Thunderbolt temp coefficient

[Ben Gelb]

Hi all -

First time poster, hopefully this is on point for this list. I have
recently come into an early 2000s era Trimble Thunderbolt and have
been enjoying fooling around with it and trying to learn more about
how to understand its behavior and assess its performance. But still
quite green. Probably not qualified as a "nut" yet by any stretch, but
maybe on the way...

Though I don't currently possess the instruments to measure it myself,
the internet (and this list) has brought me to understand the default
TC and Damping parameters are rather conservative, and the best freq
stability behavior is observed w/ a longer time constant (1000 sec
appears to be a common choice). Intuitively I understand this to be
effectively that the control loop is being too heavy handed in
steering the OCXO and that gps jitter starts to dominate the short
term stability (where the raw OCXO is probably better than GPS).

So I have been experimenting w/ TC=1000sec, Damping=0.707, and
watching the behavior.

I notice the vco tuning voltage moves much more slowly and less
noisily (as expected), but also notice large excursions in average PPS
error (up to several 10s of ns) sometimes lasting hours at a time.
These seem to be highly correlated with the diurnal temperature
rise/fall in my garage, as reported by the thunderbolt.

For example, today between 8am and 2pm, the temp (as reported by
tbolt) rose by 2.125C before ~stabilizing. The average PPS error began
around 0 at 8am, rose steadily to a peak of around 40ns at 12:30pm,
then finally came under the control of the control loop and was
brought back to near 0 again by 2pm. The DAC offset from start to
finish was about -800uV, which is about 3e-3Hz (using the LH-measured
-3.733Hz/V VCO gain) or 3e-10 parts/C.

I think my main question is just whether this is ~normal temp
stability for the tbolt OCXO? I don't have a good intuition for what
"normal" is for these devices, so I guess that's my first question.
And I guess the second is whether the rough analysis above passes
muster (or is a completely wrong way to think about it?).

W/o really fully being able to intuit how this temp variation would
show up on an ADEV plot (I'm really just getting my feet wet here)
this temp effect seems like it is going to show up as a high order
bit?

I did find this old thread on this list:
https://www.febo.com/pipermail/time-nuts/2012-December/072823.html

Which seemed sort of related but did not see any firm conclusion.

Thanks in advance for any feedback/pointers!

Ben