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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Aug 18 16:13:49 EDT 2018


Hi,

On 08/18/2018 10:01 PM, Bob kb8tq wrote:
> Hi
> 
> 
> 
>> On Aug 18, 2018, at 2:25 PM, Attila Kinali <attila at kinali.ch> wrote:
>>
>> On Thu, 16 Aug 2018 17:45:30 +0200
>> Nicolas Braud-Santoni <nicolas at braud-santoni.eu> wrote:
>>
>>> The main issue I'm running into is that most timing GPS modules will
>>> happily give you carrier phase measurements... for their internal
>>> oscillator, and the few ICs I can find that would possibly do the job, like
>>> ublox's UBX-M8030-KT-FT, do not have publicly-available datasheets & docs.
>>> (I tried contacting ublox to ask, and never got an answer...)
>>
>> Yeah. u-blox isn't as nice as they used to be to small customers :-(
>> There are two ways to proceede: 
>>
>> 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. 
>>
>> 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. It should be relatively
>> easy to reverse engineer what you need. If you don't want to reverse
>> engineer it and if you get hold of a u-blox representative, you can get
>> the required information from them. I got the data from them a couple
>> of years ago. Unfortunately, I was explicitly asked not to share this
>> information :-(
> 
> If you go this route, the close in phase noise of your signal needs to be in the
> “pretty good TCXO” range. There are indeed some sources out there that
> are way to noisy ….

Indeed. You want both pretty good wideband noise and close-in.
Nowdays GPS-receiver TCXOs is their own class of TCXOs for a reason, and
it's pretty impressive stuff for their size and cost.

It's always good to keep an external ref input available to servo up the
TCXO from.

Cheers,
Magnus



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