Hi Consider that your VCXO has a minimum tune range of 20 ppm. There are no typicals or or max limits shown. Just to toss out a number, say it’s 30 ppm in some cases. They list linearity at 5%. That’s likely per 55310, so it’s a box spec. Say that gets you to a 1.5:1 slope ratio. Your most sensitive part of the curve now would equate to 45 ppm.
That would give you a 0.7 ppb LSB *if* the DAC is perfect. If it’s good to what many are (they are monotonic), you have steps at or above your 1 ppb limit. That *assumes* the loop only steps 1 step at a time. With a FLL at 100 seconds, that’s very unlikely. Any time you “pop” the frequency by 1 ppb, you go out of your accuracy limit. That without the VCXO moving with a standard deviation at 1 second of 1 ppb. That error would add on top of your steps. One alternative - find an oscillator with a *lot* less EFC range. That part is designed to hold 4.6 ppm forever and to guarantee lock to another source that is also at +/- 4.6 ppm forever. There are a few other bits and pieces involved so it really needs to be > +/- 11 ppm EFC at ship. In your case, an EFC that corrects the aging of the part (4.6 ppm) is plenty good enough. Bob > On Aug 17, 2015, at 5:37 PM, Nick Sayer via time-nuts <[email protected]> > wrote: > > >> On Aug 17, 2015, at 12:49 PM, Nick Sayer via time-nuts <[email protected]> >> wrote: >> >> >>> On Aug 17, 2015, at 12:07 PM, Attila Kinali <[email protected]> wrote: >>> >>> You really should read the wikipedia article on the PID loop and implement >>> a simple PI loop (no need for the D part). That's not more effort than what >>> you already did, but gives you better stability. >> >> I’ve done PID before (for a reflow oven controller), but thought that the >> current code was easier to understand. I’m going to try the GPSDO simulator >> and see how it matches up. It’s entirely possible that an improvement could >> be made in the time-to-lock, but the steady state performance appears to my >> eyes to be as close to optimal as I could envision. But I’m new at this, so >> it’s entirely possible that I’m not looking at it correctly. >> > > I thought some more, and in principle, I could use the 100 second sample > error as the proportional and 1000 second cumulative error as the integral. > What I wanted to insure with my hand-coded decision making was that the > system was not completely insensitive to momentary excursions in the steady > state, but that it didn’t overreact. I suppose that could just mean that Kp = > 1 and Ki =~ 2 or 3. > _______________________________________________ > 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.
