[time-nuts] GPSDO time constant

Tom Van Baak tvb at LeapSecond.com
Thu Jan 8 08:02:44 UTC 2009


Here are ADEV plots and interesting results from a recent
experiment on varying the time-constant of a GPSDO:

http://www.leapsecond.com/pages/tbolt-tc/

There was a thread recently where Warren suggested that the
loop time constant (TC) of GPSDO was less than ideal. He is
correct. There are a couple of reasons for this, if I may guess.

1) Some GPSDO, like the surplus SmartClock designs from HP,
were designed to meet spec even when S/A was still in effect.
With the much greater wander in civilian GPS timing during
those years, the TC needed to be less than what you can get
away with today.

2) If you are a manufacturer and have a GPSDO spec to meet,
you need to make sure the TC is valid for all OCXO that you ship,
not just the average one. The way to do this is to be conservative
and to ship the units with a TC that is short enough that even the
parts on the lower end of the bell curve still meet your spec.

The other alternative is to individually measure (days, weeks?)
every OCXO and individually burn a default TC into each unit
shipped.

3) Most commercial GPSDO need to work over a fairly wide
temperature range. This might require a tighter TC. If the user
has a more controlled environment they can probably tolerate a
longer TC that what the manufacturer dare ship as a default.

4) A GPSDO should still work reliably in the face of phase or
frequency jumps in the OCXO. Although the timing of these is
not predictable, their typical magnitude is probably something
that the designer can learn. The TC needs to be short enough
so that the GPSDO gracefully handles these jumps. If one is
too aggressive the GPSDO will wander out of spec instead of
more closely tracking GPS.

5) I'm not sure it's possible to optimize for both time stability
and frequency stability at the same time. A long TC will help
avoid too sudden frequency changes in the GPSDO; a short
TC will help the 1pps stay close to UTC. I suppose a GPSDO
might be optimized more for one application than the other;
this would affect the choice of default TC.

6) Most OCXO demonstrate much better drift rates after they
have been in operation for weeks or months. The drift rate has
a some impact on the choice of TC. It's probably not a good
business model to ship a GPSDO with a TC optimized for how
the unit might eventually run a year from now. It has to work
out of the box. So this cause the default TC to be set shorter
than ideal.

7) There may also be a SV, sky-view, or latitude dependence.
Someone enjoying all 32 SV today, with a clear 360 degree
view of the sky at mid-latitude will probably enjoy slightly better
performance than someone a few years ago when there were
less operational SV in orbit, or with mountain, forest, building
obstructions, or at extreme latitudes. If you have much better
than average reception you could probably move the TC out
further.

So for these reasons (more like guesses), it would not surprise
me if most GPSDO have the TC set on the low side. Let me
know if you have additional info on this topic.

The good news is that if you, the time-nut, have the gear and
the time to measure the stability of the OCXO in your GPSDO,
and know your environment well, then you can probably safely
lengthen the TC and achieve much better mid-term stability out
of your GPSDO as shown in the plot above.

/tvb





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