[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


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.


More information about the time-nuts mailing list