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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
