Hi > On Aug 20, 2018, at 5:46 AM, Nicolas Braud-Santoni <nico...@braud-santoni.eu> > wrote: > > On Mon, Aug 20, 2018 at 01:15:10AM +0200, Attila Kinali wrote: >> On Sun, 19 Aug 2018 23:26:03 +0200 >> Nicolas Braud-Santoni <nico...@braud-santoni.eu> wrote: >> >>>> Yeah. u-blox isn't as nice as they used to be to small customers :-( >>> >>> Ah, that's a pity. :( >>> >>> FWIW, I'm going to try going through a ublox reseller that says they have >>> that timing GNSS IC available, ask whether I can purchase in small >>> quantities >>> and whether I could have the datasheet. >> >> The online shop still works pretty well, as far as I am aware of. >> If you order more than 10-20 LEA/NEO modules, I recommend going >> through the u-blox sales. They might not resopond to you as quickly >> as one would like but they should still give you a decent offer. > > They do not have the UBX-M8030-KT-FT in the shop, but perhaps I should try > and modify a NEO module as you suggested. > >>>> 1) use the timing of the PPS to deduce what the phase relation between >>>> your clock and the internal oscillator of the LEA is. >>>> In principle, this is possible, but I have not worked out the math, >>>> so I cannot say for sure. >>> >>> I've considered that, and it ends up being mostly equivalent to what I'm >>> currently doing. Part of the issue is that I don't want to wait ~1 month >>> for a PLL lock, but I also need/want an integration time about that long, >>> as that's about where the GPS becomes more stable than my local XO. >> >> One month? If you are not using a Cs beam standard, then having >> an integration time of a month is pretty pointless. > > The issue is that I only get a measurement every second, with a fairly-large > amount of noise, so it takes a while to get enough samples until the noise > averages down to below the oscillator's own noise. > (Hence why I want to move to carrier-phase measurements; a better receiver > would definitely help, though) > > I mis-spoke, though, one month was how long it took, with a particular set of > parameters, to get a PLL lock from a cold start (i.e. without the Kalman > filter > being seeded with previous measurements); I guess that what I get when writing > late at night. :) >
This is why many GPSDO’s use multi stage filters. Most of the code involved is aimed at getting the transitions between the filter stages to work properly. Bob > >>> I was able to work around the problem in part, by dynamically adjusting the >>> constants of my IIR (and so the integration time), and it works pretty OK >>> despite being highly non-linear, but there is only so much one can do when >>> fixing hardware deficiencies in software. :( >> >> Hmm? What kind of problems? > > “The problem” in question was the trade-off between precision and the time it > takes for the PLL to lock. > > >>>> 2) replace the internal oscillator with one phase locked to your OCXO. >>>> The internal clock of the LEA is derived from a single TCXO. You can >>>> easily unsolder it and feed your own signal in. >>> >>> That seems pretty much equivalent to using a “naked” GPS IC, as the part >>> I care about is clocking it with my XO and getting phase measurements >>> (wrt. the time-code and the carrier) out. >> >> No it's not. As you can relate the phase measurements of the GPS module >> to your clock. > > Yes, that's the goal of the whole thing. > > >>> I indeed low-key considered rolling my own GNSS receiver, as there are now >>> some RFSoCs that would make it not too bad, but I decided against it as: >>> >>> - Trying to make a good GPSDO is hard enough as-is ;) >> >> Not really. You just need to understand what the limitations of the >> components are and how to design a proper control loop. That's why >> Trimble GPSDOs or the Star-4 work so well. They were designed by >> people who know these things. > > I'm keenly aware; I was saying that I don't have the same expertise as > Trimble's engineer, so there is a bunch of learning involved. > > >>> - I would need anyway to be able to validate that the PLL works correctly >>> and gives the expected accuracy, with a known-good GNSS receiver. >> >> If you build a GPSDO using a GPS module, you still have to vialidate >> it works correctly by comparing it to stable sources. At the minimum >> against another GPSDO of a different design and a Rb standard. > > Yes, I currently validate my design against a commercial GPSDO driving a Rb > standard; some friends at a local university's timing lab offered to measure > ADEV and phase noise with their equipment (they use a Cs beam as a reference, > which is itself synced by GPS common-view measurements), but I'm holding off > on > that until my design stabilises more. > > > Best, > > nicoo > _______________________________________________ > 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.