[time-nuts] Re: General discussion of PID algorithms applied to GPSDO control loops (continued 1)

[André Balsa]

Hi Bob, thank you for your comments, very helpful.

On Fri, Apr 15, 2022 at 2:06 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> > For example, the STM32 GPSDO with
> > its very crude FLL algorithm used a fixed interval of 429s to change Vctl
> > (except during the initial calibration). In older GPSDO designs using a
> > purely analog circuit the control variable is applied continuously.
>
> At 429 s, you would need to have a *very* low drift oscillator or put up
> with a lot of noise on the output.


Since the STM32 GPSDO uses an OCXO it does indeed have a very low drift
oscillator.

>
>
> >
> > 12. Consequently the program of an FLL or PLL loop for a GPSDO has two
> > decisions to make every second: a) what is the size of the correction to
> be
> > applied to the control variable iow what is the new value for Vctl ? and
> b)
> > Should the correction that was just computed be applied now, or should we
> > wait and apply a different correction later ?
>
> If the loop only has useful information every 429 seconds, then there
> is not much value in updating any more often. This is one thing that makes
> PLL’s the more common approach.
>
Actually the FLL loop has useful information every second, so we can update
Vctl at any time interval that is a multiple of 1s, or even at varying time
intervals. That is exactly the question that I am asking.


> >
> > 13. The programmer (in this case myself) has to decide whether to use a
> P,
> > PI or PID loop, the optimal values for Kp, Ki and Kd, the use of a fixed
> or
> > variable "time constant" (the delay between changes to the Vctl), and any
> > processing (filetring, averaging, removal of outlying values, etc) of the
> > measurements from the frequency counter or the TIC.
>
> The parameters will be highly dependent on exactly what you have in your
> setup. Large amounts of damping is normally good. A crossover frequency
> for the system that makes sense vs the measured noise is generally the
> next thing on the list.


Indeed, I guess any algorithm I can come up with will have to pay close
attention to the inherent noise/jitter in the readings. Thanks for pointing
this out.

>
>
> >
> > 14. More precisely in my case, there is an extra complicating factor
> > because I am trying to merge the FLL and PLL control loops into a
> "hybrid"
> > FLL/PLL control loop. How to make the best use of the information from
> the
> > two measurement ?
>
> The loop needs to be either an FLL or a PLL. It is not at all unusual to
> switch
> out the entire loop as the GPSDO “warms up”.
>
> Bob
>
> You mean the FLL and PLL are exclusive of each other ? I guess you are
right, but I am trying to think "outside the box" and see if there are any
alternatives.

Again, thanks a bunch for your comments.