>> Here I put the two runs in color on the same plot:
>> http://www.leapsecond.com/museum/z3801a/z3801a-free-lock.gif
> 
> That's an interesting graph.
> 
> I find it strange that the free-running system is better than the locked 
> system in the range from 50 to 5000 seconds.

Hi Hal,

All GPSDO, by definition, will have a range of tau where the
free-running LO is better than the locked system. That is the
compromise you accept for a hybrid system. And even when
perfectly tuned, any GPSDO is worse than sqrt(2) of GPS and
LO performance at or near the cross-over point.

If you use a GPSDO as your house reference a case could
be made, depending on your needs, to let the GPSDO free-run
during the day, and then catch-up overnight.

You pay a medium performance penalty to track GPS short-term.
Of course, it all depends on the quality of the LO; but it's not
uncommon for a good GPSDO to use a loop filter on the order
of many hundreds or a few thousand seconds.

So that's why those free vs. locked plots look they way they do.

Note also for long-term, high -accuracy UTC measurements
(against Cs or maser) I don't use a GPSDO. Instead I use
a plain, non-disciplined OEM GPS board or something like a
CNS II (http://www.cnssys.com/cnsclock/CNSClockII.html).

High-end, short-term, or low phase noise, you always use
quartz; long-term you always used GPS. The cool thing about
a GPSDO is that you get (almost) the best of both in one box.

> Does that mean the filter needs a longer time constant?  Or is there 
> something fundamental about systems like this that makes graphs with that 
> sort of pattern?

Yes, in that example, with that particular S/N of Z3801A, in my
particular hands-off, temperature controlled lab, and my well-placed
GPS antenna, a longer time constant would have been more ideal.

But that's a lot of if's. When HP ships a product they have to make
sure it works to spec; into the wild cell-tower near Fairbanks or in
my home lab.

In addition, some surplus GPSDO randomly have better LO than
others so the disparity between free-run and locked will appear
to be greater.

> How much did they understand about things like this graph when the Z3801A was 
> shipped?

Based on conversations with some of the SmartClock engineers
years ago, and from reading the published papers, I would judge
it was all perfectly calculated, in the usual HP way.

/tvb


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