[time-nuts] Re: Validating GPSDO control loop with simulation and measurement. Is this amount of difference between measurement and simulation to be expected?

[Erik Kaashoek]

After solving various problems ranging from GPS antenna location to 
elimination of a rounding problem in the DAC calculation the performance 
of the GPSDO starts to improve.
There are still many things to check and much longer measurements to be 
done but at least it is going in the right direction. Mostly thanks to 
the help of the good people on this list
Attached the ADEV plot, phase plot and TIM files of raw PPS phase and 
GPSDO frequencies
Erik.


On 26-2-2022 17:59, Bob kb8tq wrote:
> HI
>
>> On Feb 26, 2022, at 11:36 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
>>
>> Magnus, Bob,
>> Thanks for the replies.
>> @Magnus, I need to study your input a bit longer. What I currently fail to understand is : Should there be a low pass filter with a 1/100 s corner frequency in series with the Vtune when the ADEV of the VC-TCXO and the PPS intersect at 100 s? Why not have a Kp of 0.01 instead of an additional low pass filter? I understand how the low pass filter can help reduce noise from the DAC if needed  or is there another reason?
> Another way to look at damping is to call it phase margin. The more phase
> shift you have in your loop the less damping you get. The less damping you
> have, the more peaking there is. Stuffing a filter inside the loop is very likely
> to add phase shift to the loop ….
>
> What sort of phase margin? It is not uncommon to look at “ideal” as 90 degrees
> in this sort of loop. Targets like > 89 degrees do indeed get used.  That’s not
> much added phase shift …..
>
>> @Bob, the reason for not converging may be some infrequent disturbances of the control loop caused by something not yet identified. Need to do some investigation into the cause.
> One way to track that is to look at the actual phase plot rather than the ADEV.
> If you have a bunch of step functions then … yup .. there’s something else. You
> still will need a longer data run to get enough of a view of what’s what.
>
> One additional gotcha: The gain and phase that let you achieve “lock in” from
> a cold start may be way beyond what you want to use once locked. It is very
> common to scale things as the GPSDO process moves along. Just how you do
> this and how well it works is yet another rabbit hole ….
>
> Bob
>
>> Erik.
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