[time-nuts] GPSDO using 100Hz

[Bruce Griffiths]

WarrenS Email wrote:
> This is my first listing so don't know if I'm doing correctly.
>
> Question is: Has anyone done any work using the 100Hz GPS output, instead of the 1 Hz output?
> The reason I ask is because I am in the process of cleaning up my SIMPLE GPS Freq Phase lock tracker breadboard that does about the same as most, but in a different way. By using just a few basic standard  74HCxx IC logic gates and Flip-Flops with RC's I phase lock a low accuracy VCXO to give me a high accuracy 10MHz reference. This is something I built for my own use, to check the accuracy of my 10 MHz freq standard and to check the accuracy of  the 60KHz WWVB signal. 
>
> I found out that the most important trick to keep the GPS tracker simple (GPSDO), fast and low power, and do it without the usual microprocessor stuff,  was to use the 100Hz output instead of the 1HZ output from the Motorola Oncore receiver board.  Basically this allows simple logic circuits to give 10 to 100 times better results, such as phase noise of around 1 ns instead of 100ns from second to second, cold turn on and full accuracy lock of the tracking osc in well under a minute (with GPS left on). It also has the ability to track the GPS phase errors at about a 10 Hz rate which gave me some new interesting insight, such as the 1 sec sawtooth error that the Oncore reports on, that would usually requires the use of a computer to process, is no longer very relevant. 
> I found out the 1 second reported phase error is just an alising artifact of a higher freq phase dither that can be completely filtered out with a 1 second Time constant RC. (i.e RC replaces microprocessor).
>
> It generally takes about an hour Tracking time constant to get good 1e-10 Freq stability using the 1 Second GPS signal, and more like 3 to 24 hours as not degrade a really good Oscillator.
> I am getting such good results with the simple high speed RC PLL, I am now doing some test to see if there are new ways to reduce the Tracking time. 
> It would seem it should be possible to get 1e-10 in more like a 100 seconds instead of an hour. This would then reduce the required stability of the tracking Oscillator by an order of magnitude or better. The simple PLL tracks the 100Hz within about 1 ns and can lock up in 10 seconds, and the GPS signals themselves don't seem to vary by more than about 10 ns with 10 minute and 1Hr average times. (from http://tf.nist.gov/service/gpstrace.htm)
> I have not found any GPS signal accuracy data for shorter average times, so that is what I'm presently plotting.
> One of the other many benefits of fast GPS tracking may be the possibility of a more mobile Disciplined Oscillator.
>
> If anyone has done any work using the 100Hz Oncore GS signal to do frequency tracking,  I would be very interested in further discussions.
>
> Warren
> ********
>   
Warren

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 optimised 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