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

Sat May 28 08:29:29 UTC 2022

Hi Magnus,

I've insufficient understanding of PLL's to grab the full meaning of 
your remark on "shift of the resonance"
Attached are the 3 phase PSD plots from stable32. Is that what you where 
looking for?
Tick_01 is for Kp=0.1, Tick_004 is for Kp=0.04, etc...
With Kp=0.01 there seems to be a peak at 3e-3Hz, for the other Kp it 
seems to be less evident if there is a resonance peak in the phase.
Also attached are the Frequency PSD plots (Freq_001, Freq_004, etc...)Â  
and these show a clear shift of the peak.
Does this shift imply the loop is not yet tuned optimal?
Erik.

On 27-5-2022 21:30, Magnus Danielson via time-nuts wrote:
> 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
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