[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?

[André Balsa]

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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