Hi
> On Mar 3, 2022, at 3:36 PM, Erik Kaashoek <e...@kaashoek.com> wrote: > > The GPSDO I'm building started with frequency locking but now I'm adding > phase locking so the time stamping counter can be on GPS time. > A first version works with a PI controller setting the vc-tcxo Vtune DAC > based on the phase difference of the 10 MHz with the PPS phase. Due to > tolerances the tcxo frequency range is big and is set by a 16 bit DAC where > 1 bit is about 2e-11 frequency change. Take a look at just how good the LSB ( or maybe even LSB’s … ) on your DAC really are. Some are not all that great and others are quite impressive. Cost is indeed part of that. Summing a pair of lower resolution parts often turns out to be pretty cost competitive. With 2^16 steps a LSB of 2x10^-11 (and perfect linearity) you would get a 1.3 ppm tune range. If it’s symmetric you would get +/- 0.65 ppm. That probably is not “enough” to compensate for much aging on a low cost TCXO….. > Once the DAC value is close to the correct frequency the loop catches > nicely but if the setting is far off catching takes a long time. > A possible solution is to use the frequency error to set the DAC close to > the optimal frequency for catching. > Speed of catching is important as the design is intended to only be > switched on when needed. > Does anyone have pointers to info on how to do quick catching in such a > control loop? The “normal answer” is to use a sequence of control loops. You start off with a fast / wide band loop and then once it gets “good enough” you go to a somewhat narrower loop. Your state monitoring tells you when it’s got to an adequately settled point and you step again. It is not uncommon to have anywhere from a few to a dozen or so of these steps. Bob > Erik > _______________________________________________ > 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. _______________________________________________ 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.