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

[Bob kb8tq]

Hi

> On Aug 20, 2018, at 5:46 AM, Nicolas Braud-Santoni <nicolas at 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 <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. :)
> 

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
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