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

Attila Kinali attila at kinali.ch
Sun Aug 19 23:15:10 UTC 2018


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.
>From above 30 pcs you get quite decent prices to work with.
The big part of the price seems to be handling, thus going for
larger numbers helps a lot.


> > 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. For an Rb standard
anything over 1M seconds is pointless, and for an OCXO, even for an
exceptional one, more than 10k and the OCXO's noise and drift will dominate.
For a simple XO... a few minutes at most.

 
> 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?

> > 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.
 
> 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 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. 


			Attila Kinali
-- 
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.




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