… I need to go back to school…


The Thunderbolt Trimble GPSDO I ordered finally arrived, after all you can’t possibly have _too_ many time sources.   Of course the first thing I did was set it up, run the position survey and plug it into my HP 5335 Universal Counter which was recently calibrated to a rubidium standard.

Lo and behold after warming up the counter still read:

9.999 999 71 +6   (i.e. 9.999 999 71 * 10E6)

Ok, the trimble will take a while to run the Allan Deviation, I’ll leave it go and come back later.   24 hours later I read:

9.999 999 71 +6

No change, so what’s going on here as I expected exactly (or close to) 10 MHz and I’m reasonably sure the HP Counter is accurate.

I switch to Period mode:

100.000 002 9 -9 (i.e. 100.0000029 *10E-9

Which matches the observed frequency, at 10MHz the period should be exactly 100 nanoseconds…  what’s going on here, lots of fiddling and trying to work out why it wasn’t right, is my counter wrong or there a problem with the Trimble…

What I didn’t facter in is that the ‘002 9′ is in fact ‘2.9 Femtoseconds’ or 2.9*10E-15, the rated accuracy of the GPSDO is 1.16*10E-12.

Aka… timing resolution achieved, and I can forgive the 2.9 femto seconds in either the counter or gpsdo.  In other words, its reasonably accurate as is my counter, and that’s probably as accurate as I’m likely to have… at least until I get my hands on a cesium standard 😉

All I need to do is sit, wait, confirm the results once the GPS has reached its rated resolution, and then use that as my frequency reference.  This will probably take a week or so, but for now it’s not bad.