[time-nuts] Re: Trimble Thunderbolt temp coefficient

[Bob Camp]

Hi

Unless you have a *very* stable temperature environment, 1000 seconds is long for the typical TBolt. 

Any time you tweak this stuff you have a compromise involved:

	Longer TC gives better ADEV (up to some point …)
	Shorter TC gives better time accuracy 

That’s just how it’s going to work. Long control loop response is not great for staying exactly on target. Fast control loop response lets a lot of noise through. 

Bob



> On Feb 18, 2024, at 6:58 PM, Ben Gelb via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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
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