[time-nuts] Tuning a Trimble Thunderbolt

Bob Camp kb8tq at n1k.org
Mon Apr 20 18:08:55 EDT 2015


Expecting that unit to meet holdover after only being locked for 12 hours is not 
a reasonable thing. 

Let it run for a week. Let it lock up for at least 4 or 5 days and get a good survey 
on the location. It should run < +/- 5ns one sigma with a good survey.


> On Apr 20, 2015, at 5:39 AM, Pete Stephenson <pete at heypete.com> wrote:
> Hi all,
> I recently acquired a 2004-era Trimble Thunderbolt with firmware 3.00
> from eBay. It looks essentially identical to the one sold in the
> TAPR/Time Nuts 2009 group buy[1]. A sticker says it's the "Rev E"
> (it's not a Thunderbolt E, just revision E of the original
> Thunderbolt).
> I don't use it for anything critical, just local timekeeping and hobby
> stuff (I'm beginning to get into ham radio, so a good frequency
> reference will be handy). I've been really impressed with it, but I'm
> interested in tuning it for even better performance. The list archives
> here have been useful, as have other outside resources[2], but I have
> a few questions for the gurus here if that's not too much trouble.
> I don't have a time-interval counter or local reference clock; all my
> data is from Lady Heather. The Thunderbolt is resting in the shade on
> a foam block on a table in my living room, which is not actively
> temperature-controlled but is well-insulated and typically within a
> few degrees of 20C. It's been running for about a week with
> uninterrupted GPS signal, though I typically enable manual holdover
> for a few hours a day for testing.
> The default tuning parameters keep the phase and frequency error
> within the published specifications[3] of +/- 20 ns (1 sigma) and
> ~10^-12 over the course of a day, respectively, so long as the
> receiver is locked to the GPS signal. However, 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.
> 1. Is there some preferred, step-by-step method for manual tuning?
> I'm familiar with the Ziegler–Nichols method for tuning PID
> controllers and that method works reasonably well for adjusting PID
> controllers used for temperature control in the lab at my workplace.
> Is there some method that's comparable? As a general example, would
> "Adjust time constant until the phase error starts to oscillate but
> frequency error is stable and low. Reduce damping value until phase
> error stabilizes." be sensible?
> I ask because although Lady Heather's "autotune" function works well
> at setting the gain and DAC values, the time constant (500 seconds)
> and damping constant (1.00) is hard-coded into the source and want to
> know how to adjust those parameters for my particular Thunderbolt.
> 2. Is it typical for an oscillator in holdover to drift in a
> non-linear way? For example, with a bit of tuning my Thunderbolt
> drifted to a PPS offset of 1 us after 126 minutes. However, at 160
> minutes the offset was 2 us, at 190 minutes it was 3 us, and so on.
> After about five and a half hours (330 minutes) the offset was 11 us.
> Can this non-linearity be corrected through the judicious choice of
> tuning parameters or some other means?
> 3. Although my attempts at tuning have improved the holdover
> performance over the default parameters, I'm nowhere near the
> performance reported by [4]. 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. 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.
> Lady Heather shows "N/A" for the Kalman filter (the PV, static, and
> altitude filters are on). This appears to be normal, as screenshots
> from others [2] show the same thing. Is this expected? Is there still
> some internal Kalman filter?
> 3. Is it normal for there to be "spikes" in the phase and frequency
> error when the number of satellites being tracked changes? I observe
> changes of ~100ns and 100-200ppt whenever there's a change in the
> number of satellites. Can this be smoothed out?
> Many thanks. I apologize if I'm duplicating an earlier discussion, but
> my search-fu didn't turn up answers to these questions in the list
> archives. If this has been discussed before, pointers to the earlier
> discussion would be most appreciated.
> Cheers!
> -Pete
> Bern, Switzerland
> [1] http://www.leapsecond.com/pages/tapr-tbolt/
> [2] http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:thunderbolt_damping
> [3] http://trl.trimble.com/docushare/dsweb/Get/Document-10015/
> [4] http://www.w8bapdstar.info/library/PrecisionClocking/Trimble%20Thunderbolt/Thunderbolt%20Holdover%20Document-8428.pdf
> -- 
> Pete Stephenson
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