[time-nuts] Re: Can the ADEV of a GPSDO output ever be lower than the minimum of the ADEV of the internal oscilator and the ADEV of the GPS PPS?

[Erik Kaashoek]

Thanks for confirming something is still wrong. :-(
I've extended the simulation to contain a full Kalman filter working 
with 2 state parameters: phase and frequency.
The biggest impact I can see is when increasing Kp above the optimal 
value the PPS noise normally starts to impact the output phase and the 
ADEV at tau 1 becomes worse
The Kalman filter seems to be able to filter the noise from the PPS 
better so with equally high Kp the ADEV at tau =1 is about a factor 4 better
Unfortunately the high Kp of 0.1 is far from optimal and setting Kp to 
0.01 gives overall a better performance and the Kalman filter no longer 
seem to have a visible impact.
Octave code for the simulation and the used data files are attached.
Also 3 plots are attached showing optimal Kp, high Kp with no filter and 
high Kp with Kalman filer
I'm still seeing some weird stuff in the ADEV plots.
Erik.

On 29-4-2022 16:53, André Balsa wrote:
> Hi Erik,
> Mathematically, no, a GPSDO cannot have a lower uncertainty (ADEV) than the
> minimum observable uncertainty (ADEV) of the combined oscillator
> (disciplined clock) and PPS (disciplining clock) from the GPS receiver.
> Unless there is some magic trick to remove the uncertainty in a clock that
> I am not aware of. ;)
>
> On Thu, Apr 28, 2022 at 10:03 PM Erik Kaashoek <erik at kaashoek.com> wrote:
>
>> I'm doing some simulations to understand the impact of a filter between the
>> TIC measurement and the PI controller steering the Vtune of the OCXO.
>> With a well tuned PI controller without filter the best ADEV I can get is
>> just above the minimum ADEV of an actual measured  OCXO and an actual
>> measured GPS PPS.
>> When I add an alpha-beta filter, similar to a first order Kalman filter
>> with a manually tuned Kalman gain, and using similar Kp, Ki, the overall
>> performance does not change (much)
>> However with the filter its is possible to increase the Kp, Ki with a
>> factor 10 and when I use in the simulation instead of a measured PPS an
>> artificial PPS created from noise with the same ADEV as the GPS PP but with
>> a very constant phase (different from the varying phase of a GPS PPS)  the
>> ADEV of the GPSDO output in my simulation seems to drops below the ADEV of
>> the PPS. Am I correct to assume this is a hint there is still something
>> wrong in the simulation or was my initial assumption about the possible
>> range of the GPSDO ADEV wrong?
>> Erik.
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