[time-nuts] Yet another GPSDO project.

[Achim Gratz]

Tobias Pluess writes:
> By the way, this is the unit I built:
>
> https://hb9fsx.ch/wordpress/index.php/2018/11/14/my-own-gpsdo-part-i/

Nice build.  Minor nit: Whatever foam you used to isolate your OCXO
further is probably not SAGEX, but looks to be either cross-linked PE
foam or (more likely) PU foam.  Isolating the unit like that shifts down
the maximum usable ambient temperature and may confuse the temperature
controller if you overdo it.

> I have also a homemade antenna for it, since I read somewhere that the
> quadrifilar helix antennas are better than patch antennas, I made one
> by myself. A choke ring antenna would perhaps be even more awesome,
> but this is for a later project!

That's a whole 'nother rabbit hole, search this list (and elsewhere) for
"phase center".  These days even the professional geodesic antennas are
mostly using patches.  The only Helix antennas you can still get seem to
be wherever the original SAGEM have ended up getting owned.

> @ Achim.  Yes indeed, I encounter the problem that my measurement
> resolution is only 100ns. So it takes ages until the frequency locks
> and it takes longer and longer, the closer the frequency is... and I
> exactly assumed what you have pointed out: perhaps my steering becomes
> so sparse in time, that the OCXO's aging rate is faster.  2e-11 at 1s
> is not bad, but hoped for something with e-12 or even better - I think
> this should be possible, not?

Yes.  Even keeping everything the same as you have now, if you get
better resolution on your phase difference it will imporve the
performance considerably.  What I was trying to tell you is that you can
use the M8T to measure the phase difference to considerably better than
100ns and that you can (in addition, if your module has both EXTINT
inputs available) use a divided-down frequency input to the module to
more closely align its internal oscillator to your superior OCXO from
the start.  You can still add a TIC to go into sub-ns territory later
on.

> So I think I would really like to go the PLL route, I hope to achieve
> faster lock time and a bonus would be that the PPS pulses derived from
> my OCXO would be aligned to those of the GPS module.  But how to
> interface the PFD to a microcontroller and implement the loop filter
> is still a mystery for me :-)

You'll end up with some FLL/PLL hybrid most likely.  Anyway, start with
getting better PD measurements and take it from there.


Regards,
Achim.
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