[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]

André

I've completed a comparison between simulation and measurement.
In the simulation [1] you see the GPSDO ADEV pushed below the OCXO and 
PPS ADEV around the intersection of the PPS ADEV with the OCXO ADEV.
In an actual measurement [2] with identical controller and controller 
parameters I see the same.
So there is no need to worry and the behavior is understood
Erik

[1] http://athome.kaashoek.com/time-nuts/GPSDO_simulation.JPG
[2]http://athome.kaashoek.com/time-nuts/GPSDO_measurement.JPG

On 30-4-2022 5:41, André Balsa wrote:
> Hi Eric,
> I am probably missing something, but I don't see anything wrong with the
> ADEV plots, to me they make sense and look exactly as expected: with an
> optimal Kp the GPSDO's ADEV combines the OCXO ADEV with the PPS ADEV at
> around the point where these curves intersect which is indeed an optimal
> behavior. And with non-optimal Kp values this is not achieved. In other
> words, exactly what can be expected.
>
> What am I missing?
>
> On Fri, Apr 29, 2022 at 11:43 PM Erik Kaashoek <erik at kaashoek.com> wrote:
>
>> 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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