[time-nuts] GPSDO using 100Hz

[Bruce Griffiths]

WarrenS Email wrote:
> 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
>   
Warren

The optimum loop time constant depends on the quality of the local
oscillator and the GPS timing receiver timing signals.
A time constant of several hours is only useful with a very high quality
OCXO.

The 100Hz output of an M12+T is phase jerked into alignment with the the
second once every second as is the 10kHz output from Jupiter-T GPS receiver.
The variable pulse width of the 100Hz (and 10kHz) outputs do no favours
to an XOR phase detector, its better to use the leading edges of these
signals.
When one uses a low resolution phase detector with dither as in the
Brooks Shera circuit then making 100 or 10,000 measurements of the phase
error every second can, if the dither is of the right form, improve the
effective resolution. However surely the timing quantisation error of
the leading edges of the 100Hz (or 10kHz) outputs limits the potential
improvement?

One can do much better with an inexpensive processor with little
external hardware other than a high resolution DACX (even that can be
implemented in software and hardware within the processor together with
a couple of opamps).


Bruce