Hi By far the most common approach to optimizing these is the "measure it and see" approach.
1) measure the noise out of the GPS ( must be done no matter what) 2) measurer the noise of the specific OCXO (again must be done) 3) *guess* at a cross over 4) try it and measure the result. 5) step and repeat 3 and 4 until exhaustion sets in Indeed converting the data to phase noise rather than ADEV helps the guess process. You can go a bit crazy with math to get a better first guess. Unless you measure what you get, you won't find all the silly little things you forgot to put into your math model. If you simply try a dynamic tune approach, you never really get to an optimum point. You need a "better than" reference to let you know where you are. You can keep pushing out the time constant and watching with just a GPS and OCXO, but you never really know when to stop. Bob On Sep 16, 2012, at 11:47 AM, Poul-Henning Kamp <p...@phk.freebsd.dk> wrote: > In message <5C52FBDBA5084AD4A36300FBA73BEF5E@pc52>, "Tom Van Baak" writes: >>> Yes, timing accuracy has been my main focus and in general I have been >>> using integration times on the low side of 10000 seconds for that, >>> but it depends a lot on the OCXO/Rb and environment. >>> >>> The PLL in NTPns is a (by now) old attempt to make a self-tuning PLL >>> for optimal time stability, and it does a surprisingly good job at it. >> >> Are there papers that talk about how to optimize for best timing or best >> frequency or (no free lunch) some compromise combination of the two? > > The only writings I am aware of, is what Dave Mills has written and > the PLL code in NTPns, but I havn't followed this closely in the last > 10 years, so do check for newer writings. > > Dave Mills coined the term "allan intercept" as the cross over of > the two sources allan variances and it's a good google search for > his relevant papers. > > I'm not entirely sure his rule of thumb for regulating to that point > is mathematically sound & precise, but the concept itself is certainly > valid, even if you have to compensate for the timeconstant of the > PLL you use to regulate to that point. > > I spent a lot of time with the code in NTPns, to try to get that PLL > to converge on the optimum, and while generally good, it's not perfect. > > The basic problem is that the data you have available for autotuning, > is the allan variance between your input and your steered source. > > If you also have the allan variance between the steered source and > a 3rd, better, source, the task is pretty trivial: Minimize the > area below that curve. > > But if you do that on the curve you have, you don't optimize, you > pessimize, since the lowest area, is with a timeconstant of zero. > > Going the other direction and maximizing the area is no good either > and trying to balance the area around some pivot related to the > present PLL timeconstant does not converge in my experience. > > What I did instead was to (badly) reinvent Shewarts ideas for testing > if the phase residual is under "statistical process control": > > I increase the timeconstant if the phase residual has too frequent > zero-crossings and loosen it if they happen too seldom. > > Having read a lot more about statistical process control, since I > built those NTP servers for the Air Traffic Control 10 years ago, > I would leverage more of the theory and heuristics developed in > process control. (3sigma violations, length of monotonic direction > etc. etc.) > > -- > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > p...@freebsd.org | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by incompetence. > > _______________________________________________ > 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.
