Hi 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. Bob > On Apr 20, 2015, at 5:39 AM, Pete Stephenson <p...@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 > _______________________________________________ > 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. _______________________________________________ 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.