[time-nuts] Re: What GNSS module to buy for a good time reference?

[Thomas Abbott]

> if you take the lid off a ZED-F9T module, you will see among the
> three or four ICs there a separate TCXO.
>
> in theory you could remove that oscillator (I think it's ~60 MHz) and
> feed in an external reference there.  I don't know anyone who's
> tried that, and I don't know if the software has the right hooks

Someone tried that with a NEO-6M, it is messy but quite beautiful:
- HP10811 10 MHz oscillator,
- 26 MHz synthesizer from this
- uBlox NEO-6M with internal crystal removed
- microcontroller gently steering the master oscillator
- using standard messages from the GPS - first the reported freq accuracy
to get it close to the right frequency, and then the TPQerr for phase lock.

Detailed write-up here:
http://lea.hamradio.si/~s57uuu/mischam/gpsy/index.html#self

In the end I don't think its frequency stability will be better than a
regular GPSDO, it just needs no TIC. This guy wasn't so interested in
absolute time, but if you added one more PPS input/output for once-off
sync, the clock data from the NRCAN PPP would now accurately reflect the
time error of the oscillator.

You could also run the whole thing off your master reference, without
steering it, and the same would apply. You'd have to once find the
relationship between GPS time and one of your 10 MHz edges, maybe using the
PPS output. The post-processing would tell you the clock error over time. I
suppose it would work with the F9T.
I found the F9T timepulse was degraded from ~1 ns RMS (short term) to ~10
ns, if the temperature was changing quickly, or if there was any vibration.
So it might perform even better with a more stable reference.


Thomas


On Sun, 2 Jul 2023 at 13:04, John Ackermann N8UR via time-nuts <
time-nuts at lists.febo.com> wrote:

> I'm not aware of anything in the ZED-F9T price range that accepts an
> external reference.
>
> The closest I know of is the Septentrio Mosaic-T, but the last I heard
> the evaluation board was >$1K and I don't think you can buy the bare
> modules in one-off quantities.  You can buy a bunch of ZED-F9Ts for
> that, but it is thoroughly modern and works really well.  Any completely
> packaged receiver with similar capabilities will cost several time that.
>
> As Bob mentioned, there are some used dual frequency receivers that can
> do this.  The ones likely to be <$1K are the Trimble NetRS and the
> Ashtech Z12 or variants.  The NetRS works pretty well and are easy to
> configure and (relatively) easy to get raw observations out of.
>
> The Ashtechs are really, really ancient and very proprietary, and a lot
> of them don't work anymore.  I got a couple going, and wrote some python
> to convert their serial data stream into RINEX, but I can't really
> recommend anyone go down that road these days.  The NetRS is a much less
> painful choice.
>
> Now, for those who have are really nuts (time or otherwise), if you take
> the lid off a ZED-F9T module, you will see among the three or four ICs
> there a separate TCXO.  Don't ask me how I know this; it's not pretty.
>
> Anyway, in theory you could remove that oscillator (I think it's ~60
> MHz) and feed in an external reference there.  I don't know anyone who's
> tried that, and I don't know if the software has the right hooks to do
> anything useful with it.
>
> The Navspark dual-freq is designed for portable RTK applications and
> probably does pretty well at that, but from the not-that-great
> documentation, its timing performance looks to be pretty basic.  I could
> be wrong, but I don't think it supports a 0-D timing mode.
>
> John
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