That's a very good question, Hal. If you're talking about stability
testing, GPSDOs are a challenge because they (can) work well over a wide
range of measurement intervals -- you can have the short term stability
of a good OXCO, the mid-term stability of a mid-range Rb, and the long
term stability of a Cesium standard.
When we measure one quantity (the DUT) relative to another (the
reference), you want the reference to be significantly better than the
DUT, or you don't know what you're actually measuring. A rule of thumb
seems to be that the reference should be 10 dB better than the DUT. Any
smaller difference increases the reference's contribution to the noise.
So, ignoring for a minute the resolution/noise of the measuring device
(I'll come back to that), to measure the short term stability you need
an OCXO. Since a good GPSDO doesn't compromise the short term
performance of its internal OCXO, that means if the GPSDO has a pretty
good OCXO, to measure it you need a *very* good OCXO as reference,
ideally with ADEV an order of magnitude better.
For midterm stability, the GPSDO is at its worst because of the "hump"
where the drift of the OCXO meets the noise of the GPS. The worst ADEV
in a typical GPSDO occurs around 1000 seconds, and the ADEV might be in
the high parts in 1e12. The telecom Rbs are in the low 12's to high
13's at 1000 seconds, so they will work reasonably well. Or a really
good OCXO may still be good enough at that tau.
Long term stability is actually the easiest measurement because you can
use a GPS PPS signal and time interval counter, or even oscilloscope, to
get good results at tau >10K seconds -- it's easy to compare the GPSDO
PPS with the GPS PPS.
At short and mid term, the capability of the measurement device is a
factor. Few time interval and timestamping counters have a
resolution/noise floor better than high parts in 1e11 at one second,
improving at one order of magnitude for each order of magnitude tau. So
unless the GPSDO is pretty awful, the time interval counter method is
the limiting factor for tau out to to somewhere between 100 and 1K seconds.
To do better than that, you can buy an expensive test set like the
TimePod, or build a "dual mixer frequency difference" test set or the
"tight PLL" measurement system that's been described here.
So the bottom line is that to fully characterize a GPSDO's stability,
you probably need three references -- OCXO, Rb, Cs -- (unless you have a
maser) and two measurement systems -- one for short term, the other for
mid and long term.
Or... if you build two GPSDO that perform very similarly, you can
measure one against the other and take the square root of two to get the
average performance between them. That might or might not be absolutely
correct, but as long as the two have similar stability, it will get you
into the ballpark.
What you *can't* do, though, is rely on any internal measurement, like
plotting the EFC voltage in the correction loop, to get absolute
results. How much the EFC wiggles is meaningless if the wiggles keep
the frequency where it should be. It's the output that counts and you
have to measure that against an external source.
And all this is what happens when you become a time-nut. :-)
John
----
On 2/20/22 3:41 AM, Hal Murray wrote:
Let's change the discussion a bit. Assuming I have a GPSDO, home built or
eBay, how can I test it with a limited budget?
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