Great dialog, One thing I have seen is the Allan intercept almost always has a "knee". If you wanted the best possible GPS quartz reference developing a variable Allan intercept would allow this knee to be moved and then mathematically removed during a gated measurement. Allowing to effectively see behind he knee offering lower uncertainty in this important area.
Thomas Knox > Date: Sun, 16 Sep 2012 19:20:24 +0200 > From: mag...@rubidium.dyndns.org > To: time-nuts@febo.com > Subject: Re: [time-nuts] GPSDO control loops and correcting quantizationerror > > On 09/16/2012 05:47 PM, Poul-Henning Kamp 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. > > Well, what is being used is phase-noise intercept. Conceptually a > similar intercept point will be available in Allan variance. However, as > you shift between noise-variants, the Allan (and Modified Allan) > variance has different scaling factor to the underlying phase noise > amplitudes. The danger of using the Allan variance variant is that you > get a bias in position compared to the phase-noise plots cross-overs. > However, the concept is essentially the same, and the relative slopes is > the same. You get in the right neighbourhood thought. > > The concept has been in use in the phasenoise world of things, so you > would need to search the phase-noise articles to find the real source. > It's been used to generate stable high-frequency signals. > > The analysis of PLL based splicing of ADEV curves is tricky, and I have > not seen any good comprehensive analysis even if the general concept is > roughly understood. The equivalent on phase-noise is however well > understood and leaves no magic too it. > > > 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. > > You need to treat the data as loose and tight PLL measure, depending on > what you look for. There is loads of calibration issues, covered in > literature. > > > 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.) > > > > It's a complex field, and things like temperature dependencies helps to > confuse you. > > Cheers, > Magnus > > _______________________________________________ > 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.
