Hi To further add to the mess, you don't *know* if the pop or jump came from the OCXO or the GPS. Most of the time, it's coming from the GPS….
Bob On Sep 16, 2012, at 7:15 PM, Magnus Danielson <mag...@rubidium.dyndns.org> wrote: > On 09/16/2012 11:51 PM, Poul-Henning Kamp wrote: >> In message<ad054298-f656-477f-9fb1-5d48c1b07...@gmail.com>, Dennis Ferguson >> wr >> ites: >> >>> If you >>> are using a PLL in both cases, however, then the problems are >>> essentially the same. >> >> Well, not quite: Depending on the stiffness of your PLL, you can >> minimize phase error at the cost of frequency error or frequency >> error at the cost of phase error, and either is a valid engineering >> decision depending which of the two are more important to you. >> > > Sometimes such compromises is the only way to go, but sometimes you may > consider to raise your system complexity. One such thing is to increase the > PLL degree. There are many tools in the toolbox. > > Another example is the OCXO oven control. A typical OCXO oven tries to > quickly steer back the temperature. During the little temperature trip, the > oscillator will have the wrong frequency, but as the oven settles again, it > will be more or less back where you started. Trouble is, often you have only > gone above or below frequency, so the integral of that frequency error is a > phase-shift. oups. Hope your application wasn't phase-stability sensitive... > I have seen only one vendor address this issue, complete with graphs showing > the phase-creep over several temperature cycles, and yes... a typical oven > shifts phase with a residual error after a full temperature cycle of ambient > temperature, since the errors doesn't cancel completely. > > While this example may not be spot on to the point Poul-Henning is making, it > can be used as a good illustration that frequency stability goal and phase > stability goals isn't necessarily the same. > > Going back to the PLL, with a tight PLL, you track in errors quickly. This > looks good as you then track in phase errors and the time error as it > accumulates doesn't become large. On the other hand, when doing this you need > to steer your frequency wider in order to more quickly track in that phase > error. A looser PLL will track in errors more sluggishly, and hence will use > less frequency deviations for track-in, but with the downside that the > frequency errors will remain longer and the time error will become larger. > These are the systematic reactions to phase and frequency steps and ramps. > The degree of the system will also change these parameters. > > It is also important to remember that changes in the reference and changes > within the loop gets low-passed and high-passed (respectively) by the loop > bandwidth. A temperature shift on the locked oscillator will be a typical > in-loop effect which gets high-passed. > > Then there is the background noise processes to consider, but we spend so > much time on them already. > > Cheers, > Magnsu > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
