Hi You are driving an integrator (the OCXO). You want a very stable voltage on the EFC to get the loop to close. A PWM is as simple a model for a 1 bit D/A as any. One bit A/D's are a feedback on a 1 bit D/A. You do some stuff to move the noise around and to get it all to work.
Bob On Dec 7, 2012, at 2:12 AM, Hal Murray <[email protected]> wrote: > >> The output from a OCXO is divided down and then the phase of the divided >> down 10MHz RF is compared to the PPS and you don't need to even know the how >> far apart they are. All you need to know is "led or lag" just a one bit >> answer. An XOR gate or a flip flop can tell you that. > > Does a 1 bit A/D work? Is there a good web page discussing this aspect? Am > I just confused?What question should I be asking? ... Can somebody give me > a circuit or (pseudo) code so I can simulate things? > > I'm far from a PLL wizard. I think the catch in this case is that the EFC > controls the frequency and what you are measuring is the phase, the integral > of the frequency. > > Suppose you just implement a simple bang-bang control. > > Suppose the EFC is 1 volt and the frequency is correct but the GPSDO phase is > a bit early relative to the GPS PPS. So the FF says early and the software > says go-faster. That keeps happening for a while, the frequency keeps > getting faster and faster. Finally, the GPSDO PPS catches up with the GPS > PPS, but now it's frequency is way fast. The FF says go slower, so the > control software starts dropping the EFC. But the frequency is still way too > high so the error is still increasing. After a while the frequency gets low > enough so the PPS/phase error starts catching up. Eventually the PPS error > crosses over, but by then the frequency offset is way way low. ... Isn't > that cyclic pattern stable? > > Is there a simple tweak to break that loop? Do you first have to recognize > that you are in that mode? If so, how? ... > > I might be able to do fix that in software by looking at the times when > things change state. Suppose it's 193 seconds between the first early and > the last early and that the EFC went from X to Y. I think that's enough info > to work out the crossover point and work back to the desired EFC. > > But that all sounds too complicated. What would hardware-only guys do with a > 1 bit A/D? > > > > -- > These are my opinions. I hate spam. > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
