Hi > On Oct 25, 2014, at 4:04 PM, Magnus Danielson <[email protected]> > wrote: > > Bob, > > On 10/25/2014 08:15 PM, Bob Camp wrote: >> Hi >>> On Oct 25, 2014, at 12:19 PM, Magnus Danielson <[email protected]> >>> wrote: >>> >>> Bob, >>> >>> On 10/25/2014 02:02 PM, Bob Camp wrote: >>>> >>>>> On Oct 24, 2014, at 6:25 PM, Magnus Danielson >>>>> <[email protected]> wrote: >>>>> >>>>> Tom, >>>>> >>>>> On 10/24/2014 11:31 PM, Tom Van Baak wrote: >>>>>>>> ADEV most certainly does change with time, even for short tau's. >>>>>>> >>>>>>> Can you elaborate? >>>>>>> Such as when, why, what kind of change, how much change, >>>>>>> at how short of tau's, over how long of time, >>>>>>> and using what type Oscillators? >>>>>>> Do you know what in the freq or Phase plot is causing the ADEV to >>>>>>> change? >>>>>> >>>>>> I'm happy to let Bob answer his own claim here. I'm curious as well. >>>>>> Unless he's talking about thermal noise, in which case I now believe him >>>>>> 100%. >>>>>> >>>>>> OTOH, for time intervals of minutes to hours or days, the plotted ADEV >>>>>> can often vary. When in doubt, enable error bars in your ADEV >>>>>> calculations or use DAVAR in Stable32, or use "Trace History" of TmeLab >>>>>> to expose how little or much the computed ADEV depends on tau and N. >>>>>> >>>>>> In general, never do an ADEV calculation without visually checking the >>>>>> phase or frequency time series first. >>>>> >>>>> You should make sure that you remove all forms of systematic effects >>>>> before turning the residue random noise over to ADEV. >>>>> >>>>> If you have random noise being modulated in amplitude, you need to >>>>> measure long enough for the averaging end not to have a great impact on >>>>> the result. >>>> >>>> Is days long enough for a 1 second tau? If you define 1,000 x tau as “long >>>> enough” you are being way more >>>> conservative than just about anybody out there. My claim is that rather >>>> than telling everybody to run for 10,000 or >>>> 100,000 x tau, simply accept that ADEV does / may change. >>> >>> I did not say that you need to do 1000xtau, that was what someone else >>> said. If you paid attention I said that the number of samples N and the >>> tau0-multiple m for a particular dominant noise (of that tau) creates a >>> certain degree of freedom for a particular ADEV estimator algorithm. >>> Discussing the length of the measure without discussing which estimator >>> algorithm you're using and what confidence interval you aim to reach is >>> just taking a single value and run with it without thinking about it. >>> >>> For ITU-T telecom standards, the measurement length is 12 times the maximum >>> tau, using the overlapping estimator (see O.172, §10.5.1 for limit and >>> G.810 §II.3 for TDEV algorithm). That was chosen to ensure comparability >>> between different implementations for the same type of measure. See O.172 >>> for other relevant details on limits for implementation, tau0 has an upper >>> limit, so does bandwidth. Naturally, these limits is for this specific >>> purpose, algorithms etc. which may not fit the needs of other needs or >>> choices. >> >> >> If you are using under 100 samples for the test (overlapping or not), your >> confidence is not as high as it might be. >> You can see ADEV “drift in” over a period of days, even with a lot more than >> 10 samples. > > Yes. One needs to look at what happens to judge when you can trust the > values. In the standard case, there would be a lot of samples, with a minumum > of 360 for the extreme-case. > >>>> Yes indeed you can find FCS papers with all sorts of interesting >>>> “adjustments” or no processing at all. The consensus >>>> seems to be that if you go past drift correction, you really should have a >>>> footnote. >>> >>> When you do not make a drift compensation, and that line shows up, you >>> better explain that too. >>> >>> In the end, ADEV is overused to represent things for which it is not a good >>> tool. You will need other tools in the tool-box to build a good estimation >>> of how that oscillator will behave at some tau. >> >> Except that ADEV is used by many as an acceptance test on systems and >> oscillators. Saying it’s OK to pull data out >> of a test run makes for a very interesting test design. We certainly use >> ADEV (without subtractions) here on the list >> to compare things like GPSDO’s at the system level. > > I use ADEV, TDEV, phase-plot and frequency plot to best illustrate and > understand what is happening. Would be using FFT for long-term if only > TimeLab would support it for normal counter measures. Would be using > phase-noise more if I had a TimePod at work.
I would suggest adding the Hadamard deviation to that list. It highlights some things that the others do not. Bob > > Cheers, > Magnus > _______________________________________________ > 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.
