[time-nuts] Choosing a GPS IC for carrier phase measurements

Nicolas Braud-Santoni nicolas at braud-santoni.eu
Mon Aug 20 05:46:15 EDT 2018

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 <nicolas at 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. :)

> > 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.


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