[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?

[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>_______________________________________________
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