[time-nuts] GPSDO using 100Hz

[WarrenS]

 Warren Answers and comments to /tvb  below in the text.

Besides asking if anyone is using the 100Hz output, I would like to ask 
>> why don't the generally available GPSDO use the 100Hz, which can give 
>> about 1 ns of certainly with a simple PLL and analog RC filter, instead of 
>> the using the 1 sec which has more like 100 ns of uncorrected uncertainty 
>> in it and must use a processor?

>Where do you get your "1 ns" and "100 ns" figures? Have
>you actually measured these values? Also, over what time
>average (tau) are you assuming this level of resolution?

1 ns and 100ns are measured Values of the nominal phase Jitter over a few seconds.
Long enough to let the Phase cycle thru a couple of its sawtooth cycles, 
and short enough so as not to include the added GSP signal errors.
The values are pretty much independent of time, as long as the time is not too long or too short.
The GPS signal errors that I see are around 10 ns over about a 10 + seconds period, 
with short excursions (no more than a few seconds) of up to an additional to 30 ns.  


> Also I should comment that on  LeapSecond.com 
> http://www.leapsecond.com/pages/gpsdo/
> you stated "where the ADEV for various GPSDO remains below 1E-11 
> over the Tau range of [0.1s, 100,000s]. This Tau has NOTHING to do with 
> the tracking time constant that the GPSDO is set to, which is usually 
> recommended to be set for a TC of several hours for optimal results.

>For ideal results start with the TC = the tau where the ADEV
>of the GPS engine & phase detector crosses the ADEV of the
>OCXO. However there are other practical considerations.

Yes we agree on this, and this is somewhere between an hour and a day for good osc.
(and as short as a few seconds for some special applications).

> BTW most of what they have plotted is the results of NOT setting the GPSDO 
> tracking TC slow enough. This is why the Allan Deviation increases in the tracking 
> mode at mid averaging times. The 1 second GPS tracking signal is adding noise, 
> which pretty much makes my point that 1 PPS signal is not so good to use 
> if you want good fast results.
> 
> Warren

At short averaging intervals when there is a phase difference
it is easy for the GPSDO to assume the OCXO is stable and
the GPS engine has noise. So you average more samples.

At long averaging intervals when there is a phase difference
is it easy for the GPSDO to assume the GPS engine is more
correct and the OCXO has drifted. So you steer using EFC.

At the tau of the ideal TC, the GPSDO sees an average phase
difference but can't totally blame either the OCXO or GPS for
the error. By definition at this tau, half the noise is due to each
subsystem. Hence you nearly always see an ADEV hump that
goes above where you'd really like it. Best case sqrt(2).

The ADEV hump itself is not indication of a mistuned TC; it is
an indication that the GPSDO is working correctly.

You can move the hump left and right and distort its shape by
changing the TC.

In practice I suspect most commercial GPSDO have a TC that
appears too low for your liking. I can discuss why if you wish.


/tvb

Thanks, Good information to remember.
Note that in many of the units shown,  the tracking hump is more like 4 to one instead of 1.5 to one.
I would love to know the  'can Discuss more' reasons

Warren