Yo Achim! On Tue, 28 Aug 2018 22:56:37 +0200 Achim Gratz via devel <devel@ntpsec.org> wrote:
> Gary E. Miller via devel writes: > >> So do I, in fact I run between 140ns to 240ns of "average jitter" > >> pretty consistently across five machines (three different rasPi and > >> two TinkerBoard). > > > > I was talking Standard Deviation, a bit better measure of the > > jiter. > > As you should know, the jitter distribution is not normal, Yes, which is why ntpviz provides skewness and kurtosis so you can tell how the distribution differs from a standard distribution. ntpviz also supplies a number of other bits about the data set: mean, average, min, max, etc. > so strictly > speaking the standard deviation you talk about doesn't exist. Nope. Standard Deviation applies to ALL data sets. https://en.wikipedia.org/wiki/Standard_deviation "In statistics, the standard deviation (SD, also represented by the Greek letter sigma σ or the Latin letter s) is a measure that is used to quantify the amount of variation or dispersion of a set of data values." People are taught in college how Standard Deviation looks for a "normal distrubution', so they assume that one denotes the other. > You can > however obtain a similar measure of distribution width that converges > to the standard deviation for normal distributions under fairly loose > conditions. Which is useless for us, because, as you rightly noted, we have nothing at all that looks like a normal distribution. The skewness and kurtosis was added to ntpviz to make this clear. > > Can you post your ntpvix URL? > > No, it's an internal network and I won't change that. Not a requirement to post ntpviz data. Many people upload their ntpd from their stratum one to another server to do the computations. > Besides, as you > well know, I don't run ntpviz. Thus my request that you do. > I'm pretty sure I posted my plots > before however, I can check later this week to point you at the > archives. Which would be OLD data. I'm looking for LIVE data. > > Yes, studies have shown the consumer GPS can be stable in the 10 to > > 20 ns range, but the offset from "true" GPS can be much worse. > > Again, I wasn't talking about that number. The GPS module's internal > time representation, for all we know, is the ground truth towards > we're supposed to coverge. No, we are trying to converge on UTC(USNO). > > They are all we have. If you can create some more meaningful > > numbers then please send patches to ntpviz. > > You need some independent measurement to get there. The ntpviz plots > will never show an independent view. Yup, which is why we are working to get the PPS-out patch into the kernel. That coupled with my TAPR-TICC and Rb will give us that external confirmation. So hopefully 'never' is this year. > > For now we can use our numbers as arbitrary goodness numbers to > > compare two configurations. > > Yes, but you keep making statements that go far beyond that confidence > interval. We'll have to disagree on that. My data is open to all to see, in real time. I'm open to suggestions on how to improve it more. > > Yup, got that. I think my Rb is 10e-14. At least that is what the > > calibration papers said two months ago. > > Read it again. You're not getting that performance at tau=1s, not > even close. I did dig it out, my mistake. I'm "only" getting 1.5e-11 at tau=1s. I guess I'll have to settle for that. > In fact you're unlikely to get that performance even for > larger tau unless you discipline it via GPS (in which case you get > there maybe at around 2…4 weeks). At which time you are measuring the GPS, not the Rb. And a time scale that is useless for measuring the short term jitter we are discussing. And, to answer the next question. Yes, I am discipling my Rb against a GPS. The additional accuracy is very small. Maybe another 10x. RGDS GARY --------------------------------------------------------------------------- Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703 g...@rellim.com Tel:+1 541 382 8588 Veritas liberabit vos. -- Quid est veritas? "If you can’t measure it, you can’t improve it." - Lord Kelvin
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