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 <[email protected]> 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. :) > > 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
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