Re: [time-nuts] Sawtooth Error Correction, WAS: Capturing NMEA and TICC timestamp data in time-correlated way?
Hi The TBolt does code phase to lock up the OCXO. Effectively its best guess error is zero once the control loop closes. That also means the average error is also zero. Since it’s yanking the OCXO *and* guessing at the error, separating those two numbers can get a bit tangled up. One of the reasons a “normal” receiver leaves the TCXO alone is that it does not have to deal with two moving targets at once. Indeed there is a static offset due to cable delay and similar issues. If you have a “perfect” receiver to compare to, you could adjust that out with the normal commands. For full zeroing you also would need to know your antenna delay. Bob > On Sep 15, 2019, at 10:57 AM, Dana Whitlow wrote: > > Assuming that the T'bird works as you stated, you might still want to know > where in that > sawtooth the supposedly "fixed" error falls. Maybe this is unimportant if > all you want is a > stable reference frequency source, or very important if you actually want > to know what > time it is. However, the always somewhat noisy phase lock means that the > "fixed" error > is not constant, merely less variable than otherwise. What I don't know is > whether or > not the hardware can even be aware of this variability- I kind of suspect > not. > > Dana > > > On Sun, Sep 15, 2019 at 11:03 AM Alberto di Bene wrote: > >> On 2019-09-15 14:12, Forrest Christian (List Account) wrote: >> >>> Most GPS receivers with 1PPS outputs are only able to align the 1PPS to >> the >>> nearest edge of some internal clock. In addition, in most GPS >> receivers, >>> that internal clock is not synchronized to the 1PPS signal in any way. >> >> Am I right in saying that the Trimble Thunderbolt should not require any >> sawtooth error correction, as the processor >> clock is the same 10 MHz crystal that is disciplined by the GPS signal ? >> Or something is escaping me (it happened >> before...) ? >> >> 73 Alberto I2PHD >> >> >> ___ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Sawtooth Error Correction, WAS: Capturing NMEA and TICC timestamp data in time-correlated way?
Assuming that the T'bird works as you stated, you might still want to know where in that sawtooth the supposedly "fixed" error falls. Maybe this is unimportant if all you want is a stable reference frequency source, or very important if you actually want to know what time it is. However, the always somewhat noisy phase lock means that the "fixed" error is not constant, merely less variable than otherwise. What I don't know is whether or not the hardware can even be aware of this variability- I kind of suspect not. Dana On Sun, Sep 15, 2019 at 11:03 AM Alberto di Bene wrote: > On 2019-09-15 14:12, Forrest Christian (List Account) wrote: > > > Most GPS receivers with 1PPS outputs are only able to align the 1PPS to > the > > nearest edge of some internal clock. In addition, in most GPS > receivers, > > that internal clock is not synchronized to the 1PPS signal in any way. > > Am I right in saying that the Trimble Thunderbolt should not require any > sawtooth error correction, as the processor > clock is the same 10 MHz crystal that is disciplined by the GPS signal ? > Or something is escaping me (it happened > before...) ? > > 73 Alberto I2PHD > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Sawtooth Error Correction, WAS: Capturing NMEA and TICC timestamp data in time-correlated way?
Precisely. On Sun, Sep 15, 2019 at 6:03 PM Alberto di Bene wrote: > On 2019-09-15 14:12, Forrest Christian (List Account) wrote: > > > Most GPS receivers with 1PPS outputs are only able to align the 1PPS to > the > > nearest edge of some internal clock. In addition, in most GPS > receivers, > > that internal clock is not synchronized to the 1PPS signal in any way. > > Am I right in saying that the Trimble Thunderbolt should not require any > sawtooth error correction, as the processor > clock is the same 10 MHz crystal that is disciplined by the GPS signal ? > Or something is escaping me (it happened > before...) ? > > 73 Alberto I2PHD > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Sawtooth Error Correction, WAS: Capturing NMEA and TICC timestamp data in time-correlated way?
Let me see if I can give this a go. Some of this will likely be oversimplified and not exactly accurate. Consider this a simplified 101 level class taught by an assistant and not the professor. Most GPS receivers with 1PPS outputs are only able to align the 1PPS to the nearest edge of some internal clock. In addition, in most GPS receivers, that internal clock is not synchronized to the 1PPS signal in any way. For instance, if you had a crappy GPS receiver which only had a 1MHz clock, the GPS receiver could only align the 1PPS output to the nearest uS (1 / MHz), and since the timing signal from the Satellites are much better than this, the 1PPS won't actually be aligned as closely as the GPS receiver knows it could be. For instance the GPS might pick a particular edge of the 1Mhz clock, but it's also going to know that it really wanted to output a certain amount of time before or after that specific pulse. This data is what is being output as correction data. So, the GPS receiver in effect is saying 'I did the best I could outputting the pulse, but if I could have, I would have outputted the pulse this amount of time earlier or later'. Not all GPS receivers support providing this information, but at least some that do send it in NMEA between the pulses. Note that at least some of the GPS receivers also tell you in advance of the errant pulse so one could correct it in real time.(I'm hedging a bit about how many receivers do this, as I don't have enough experience with enough different receivers to know how many do or don't). The term 'sawtooth' comes from how the uncorrected error looks when you measure the 1PPS signal. Generally the error will start out really low (or negative) then grow until it has reached a point where one clock worth of phase drift has accumulated, then the GPS will say 'oh, it's better to use the next pulse', at which point the error will immediately return to the low point since all of the accumulated phase error has been removed by outputting on the next clock. Thus a sawtooth... slope from low to high, then an immediate drop. Note that the phase drift can also go the other way, so it starts high and then goes low. And, sometimes you'll see the direction of the phase drift change as the GPS clock changes frequency slightly. That's what causes the hanging or suspension bridges. If you find a graph of this online, you'll see the sawtooths are going one way on one side of the bridge, and the other way on the other side. On Sun, Sep 15, 2019 at 5:21 AM Andrew Kohlsmith (mailing lists account) < akli...@mixdown.ca> wrote: > I’m interested in learning more about how this sawtooth comes about, how > it’s predicted (so you can correct for it) and so on. Are there > approachable documents/sites/source with this information? I’m still a > time-nuts newb. > -- - Forrest ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
