[time-nuts] Yet another GPSDO project.

[Tobias Pluess]

Thanks for the replies so far.
Unfortunately I used digest mode for this mailing list, so perhaps my answers are not well-formed - please excuse; I have changed my settings to non-digest (even though this is perhaps a bit less convenient).

@Jim Harman
Indeed I know Lars' GPSDO. Can you tell what performance you achieved with one of those?

@ Bob
wow. That's quite a pile of information you put in your post!
Indeed I have some fun with the GPSDO topic, but not yet particularly with my unit, its performance is, in my oppinion, currently way too poor. So I need a better one ;-) Cs or Maser would be awesome, but I don't even have a Rb. I once thought of buying one from the 'bay, but they are often of doubtful pedigree and nobody knows how much life is perhaps left in the Rb valve, if I am unlucky it will only last for a few months, so I think I will not risk that.

Of course I know the PPS from the GPS module is jittery. This is why we want to synchronize our OCXOs, isn't it ;-) 
However the STAR4 GPSDO I have here has also an 1PPS output, which is derived from the OCXO. I used this PPS on my HP 5335A. I am looking for a HP 53131A or 53132A since months for a decent price - this would allow for higher resolution.
I have made an ADEV plot for my GPSDO, but I didn't save it because it was disappointing.

By the way, this is the unit I built:

https://hb9fsx.ch/wordpress/index.php/2018/11/14/my-own-gpsdo-part-i/

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!


@ 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?


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 :-)

Thanks for all your hints
Tobias
HB9FSX