Hi
If on the same graph you plotted the “low pass filter” response of your sample / average process, it would show how much / how little impact there likely is. It’s not any different than a standard circuit analysis. The old “poles at 10X frequency don’t count” rule. No measurement we ever make is 100% perfect, so a small impact does not immediately rule out an approach. Your measurement gets better by some number related to the number of samples. It might be square root of N, it could be something else. If it’s sqrt(N), a 100 sample burst is getting you an order of magnitude better number when you sample. You could go another 10X at 10K samples. A very real question comes up about “better” in this case. It probably does not improve accuracy, resolution, repeatability, and noise floor all to the same degree. At some point it improves some of those and makes your MADEV measurement less accurate. ===== Because we strive for perfection in our measurements, *anything* that impacts their accuracy is suspect. A very closely related (and classic) example is lowpass filtering in front of an ADEV measurement. People have questioned doing this back at least into the early 1970’s. There may have been earlier questions, if so I was not there to hear them. It took about 20 years to come up with a “blessed” filtering approach for ADEV. It still is suspect to some because it (obviously) changes the ADEV plot you get at the shortest tau. That kind of decades long debate makes getting a conclusive answer to a question like this unlikely. ===== The approach you are using is still a discrete time sampling approach. As such it does not directly violate the data requirements for ADEV or MADEV. As long as the sample burst is much shorter than the Tau you are after, this will be true. If the samples cover < 1% of the Tau, it is very hard to demonstrate a noise spectrum that this process messes up. Put in the context of the circuit pole, you now are at 100X the design frequency. At that point it’s *way* less of a filter than the sort of vaguely documented ADEV pre-filtering that was going on for years and years ….. (names withheld to protect the guilty …) Is this in a back door way saying that these numbers probably are (at best) 1% of reading sorts of data? Yes indeed that’s an implicit part of my argument. If you have devices that repeat to three digits on multiple runs, this may not be the approach you would want to use. In 40 years of doing untold thousands these measurements I have yet to see devices (as opposed to instrument / measurement floors) that repeat to under 1% of reading. Bob > On Jul 31, 2015, at 5:04 PM, Poul-Henning Kamp <[email protected]> wrote: > > -------- > >> If you look at the attached plot there are four datasets. > > And of course... > > Here it is: > > -- > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > [email protected] | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by incompetence. > <allan.png>_______________________________________________ > 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.
