Hi Bob, thank you for your comments, very helpful. On Fri, Apr 15, 2022 at 2:06 PM Bob kb8tq <kb...@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. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.