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

[Bob kb8tq]

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.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> To unsubscribe, go to and follow the instructions there.