[time-nuts] Z3801A performance plots

Brian Kirby kirbybq at bellsouth.net
Sat Jan 26 17:59:44 EST 2008

When the 3801A was designed, HP used a 300 or 400 second time constant 
for a SA filter.  Its in one of the early app notes for HP GPSDOs.

Tom Van Baak wrote:
>>> 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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