[time-nuts] GPSDO using 100Hz

[WarrenS Email]

Bruce Griffiths answered:

Its difficult to make much useful comment as you provide few measured
results.

With an M12+T or equivalent the ADEV of the PPS output (without sawtooth
correction) goes below 1E-10 at  Tau > 200 sec or so.
Thus with an optimized GPSDO it wont take an hour or so to achieve 1E-10
stability.
However a single shot phase error measurement system resolution of
around 1ns or so is usually required.
Take a look at the GPSDO ADEV plots at:

http://www.leapsecond.com/pages/gpsdo/

Where the ADEV for various GPSDO remains below 1E-11 over the Tau range
of [0.1s, 100,000s]

The achievable performance also depends a great deal on the quality of
the OCXO used in the GPSDO.

If the "digital" phase error measurement techniques you have been
comparing your system with have inadequate resolution it will tale
longer for the measured ADEV to fall below 1E-10.
If the OCXO used has a relatively high ADEV at low values of tau then it
may well required averaging over very long time intervals to achieve an
ADEV below 1E-10.

More detail is required before an analysis of the performance of your
system is possible.

Bruce

*********************

Bruce, Thanks for the feedback, Good information to know, 
but you seemed to missed my point and question.

Yes I am comparing to SIMPLE, and home built type, non optimized, 
trackers like  Brooks Shera's "10 Mhz GPS Frequency Standard". 
I understand, and even wrote, NIST reports about  a 10ns uncertainty 
with a 10 minute average, which would give a 1e-8 / 600, or  1.5 parts in 1E-11 
possible in 600 seconds, 1e-10 in 100 seconds.  Its safe to assume NIST is 
not using Shera's unit, which I believe adds an additional  24 ns or is 
it 41.7 ns uncertainty to each 1 second reading. 
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?

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.
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