[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?

Mon May 2 21:32:06 UTC 2022

Hi Bob, Markus,

although we do regularly rely on FS740s for synchronization of our 
distributed RF measurements, I have to agree with Bob. While the 
predictive filtering sounds great, I have reason to believe that it 
actually is detrimental to the common view time transfer we use the 
FS740 for.

I've attach results of a measurement similar to our paper I've cited 
before. It visualizes the time difference between two FS740 spaced 1.25 
km apart. Both FS740s' GNSS antennas have almost unobstructed view of 
the hemisphere. The gray scatter plot is the current time error between 
GNSS and internal timebase as logged by both FS740s. From the way I 
understand the manual [1] this refers to the actual raw GNSS PPS. The 
solid blue line is a low-pass filtered version of the gray scatter plot.

The orange scatter plot is the time difference of arrival (TDoA) of the 
line of sight (LoS) path between an RF emitter and two RF receivers 
synchronized to the FS740s' 10 MHz output. It's intermittent, as we only 
use it for occasional calibration with a stationary emitter. Only these 
calibration measurements with stationary emitter at guaranteed LoS 
locations are shown. The TDoA is corrected for propagation delay. As you 
can see, the raw TDoA estimations (orange) are subject to a ~5 ns 
long-term "wiggle".

In post-processing, we use the low-pass filtered GNSS PPS time error 
(blue line) to correct the wiggly raw TDoA, yielding the green scatter 
plot. That brings the standard deviation down from 1.27 ns (orange) to 
0.48 ns (green), presumably by fighting the state space filtering and 
going for an old-fashioned low-pass filter on the jittery PPS.

Best regards,
Carsten

[1] https://www.thinksrs.com/downloads/pdfs/manuals/FS740m.pdf#page=53

On 02.05.22 14:06, Markus Kleinhenz via time-nuts wrote:
> 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 itâ€™s performance â€¦. you likely will
>> take anything the manual says a lot less seriously â€¦.. 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, AndrÃ© 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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-- 
M.Sc. Carsten Andrich

Technische UniversitÃ¤t Ilmenau
Fachgebiet Elektronische Messtechnik und Signalverarbeitung (EMS)
Helmholtzplatz 2
98693 Ilmenau
T +49 3677 69-4269
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