[time-nuts] Re: Optimizing GPSDO for phase stability

[Magnus Danielson]

Dear Erik,

On 2022-05-27 18:02, Erik Kaashoek via time-nuts wrote:
> The GPSDO/Timer/Counter I'm building also is intended to have a 
> stabilized PPS output (so with GPS jitter removed).
> The output PPS is created by multiplying/dividing the 10MHz of a 
> disciplined TCXO up and down to 1 Hz using a PLL and a divide by 2e8. 
> No SW or re-timing involved.
> The 1 PPS output is phase synchronized with the PPS using a SW control 
> loop and thus should be a good basis for experiments that require a 
> time pulse that is stable and GPS time correct.
> As I have no clue how to specify or evaluate the performance of such a 
> PPS output I've done some experiments.
> In the first attached graph you can see the ADEV of the GPS PPS (PPS - 
> Rb) and the 1 PPS output with three different control parameters (Tick 
> - RB)
> As I found it difficult to understand what the ADEV plot in practice 
> means for the output phase stability I also added the Time Deviation 
> plot as I'm assuming this gives information on the phase error versus 
> the time scale of observation.

The ADEV plot is the frequency stability plot, so it can be a bit 
challenging to use it for phase stability.

The TDEV plot is the phase stability plot, so it is more useful for that 
purpose.

There is a technical difference between these beyond the difference of 
frequency vs phase stability, and that is that ADEV is the frequency 
stability for a Pi-counter where as TDEV is the phase stability for a 
Lambda-counter, where MDEV is the frequency stability for the 
Lambda-counter. There is no standardized phase-stability for Pi-counter. 
For a nit-pick like me it is significant, but for others it may be 
mearly a little confusing.

> Lastly a plot is added showing the Phase Difference. All plots where 
> created using the linear residue as the Rb used as reference is a bit 
> out of tune.
> Also the TIM files are attached
> The "PPS - RB" and "Tick - RB Kp=0.04" where measured simultaneously 
> and should show the extend to which the GPS PPS is actually drifting 
> in phase versus the Rb and how this impacts the output phase of the 
> stabilized output PPS.
> My conclusion is that a higher then expected Kp of 0.1 gives the most 
> stable output phase performance where the best frequency performance 
> is realized with a Kp = 0.04
> I welcome feedback on the interpretation of these measurements and the 
> application of output phase stabilization.

Since Kp is proportional to the damping-factor, this is completely 
expected result for me. As the damping factor increases, the jitter 
peaking decreases, and thus the positive gain at the loop resonance 
frequency.

What I seem to notice is that the resonance seems to move with Kp 
shifts, rather than having a peak of fixed frequency/tau. Doing 
phase-noise plots of the data in Stable32 should be a way to see if this 
is an actual shift or just an apparent shift.

The details of the PI-loop control may be relevant to correct for if the 
f_0 shifts as consequence of changing Kp rather than changing Ki.

The trouble one faces with a PLL is that optimum phase stability and 
optimum frequency stability comes at different PLL bandwidth settings. 
Keeping the damping factor high to keep jitter peaking low is however a 
common optimization.

Cheers,
Magnus