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 <e...@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 <e...@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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