[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?

[Markus Kleinhenz]

Hi Bob,

thanks for the heads-up. The only other source I had was a paper
claiming similar things, but they had only done so in simulations.
So I thought the manual of the FS740 would be the more trustworthy of
the two.

Greetings
Markus

Am 02.05.2022 um 13:38 schrieb Bob kb8tq:
> Hi
>
> If you have a FS740 and measure it’s performance …. you likely will 
> take anything the manual says a lot less seriously ….. Their ADEV 
> performance in the real world is a bit underwhelming.
>
> Bob
>
>> On May 2, 2022, at 1:42 AM, Markus Kleinhenz via time-nuts <time-nuts at lists.febo.com> wrote:
>>
>> Hi Erik,
>>
>> I just found a hint that what you are seeing may be correct after all:
>>
>> The Manual of the Stanford Research Systems FS740 says:
>>
>>    /*Predictive Filtering*//
>>    //The superior short term stabilities of the OCXO and Rb timebases
>>    enable the usage of//
>>    //predictive filtering to improve the stability of the FS740 by up
>>    to 3 times over traditional//
>>    //methods. Predictive filtering uses state space methods to predict
>>    the phase of the local//
>>    //timebase relative to GNSS. The technique is quite similar to
>>    Kalman filtering. The//
>>    //benefit is that the FS740 can average the GNSS signal much more
>>    effectively, resulting//
>>    //in a significantly more stable signal with a much shorter time
>>    constant than would be//
>>    //possible with traditional filtering./
>>
>> And has the ADEV Plots I attached. The GPS curve they printed is in the
>> realm of a sawtooth corrected M8T (<1e-12 @ tau=10ks) [See the Plot from
>> John Ackermanns Ublox evaluation]. But especially the Rb option seems to
>> surpass the reference in a parallel fashion.
>>
>> My two cents on simulated 1PPS Signals:
>>
>> One has to be careful when only using ADEV as the only characteristic
>> for modeling the 1PPS Signal as it combines White PM and Flicker PM in
>> one slope. So you may create an artificial signal which is pure WPN and
>> in turn is best predicted by something like the kalman filter.
>>
>> Regards,
>>
>> Markus
>>
>> Am 29.04.2022 um 19:24 schrieb 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.
>>>>>
>> <FS740_ADEV.PNG><UBLOX_QERR_ADEV.PNG>_______________________________________________