[time-nuts] GPSDO using 100Hz

Tom Van Baak tvb at LeapSecond.com
Tue Nov 25 01:16:26 UTC 2008


> 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?

> 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.

> 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




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