I found http://www.leapsecond.com/pages/adev/ being pretty helpful to
get the concept.
Best, Ulli
Am 24.09.2020 um 21:35 schrieb Tom Van Baak:
Hi Hal,
ADEV turns out to be pretty hard to understand. I took me years for it
to sink in. There are web pages, articles, even books. Part of the
problem is matching the explanation with the audience.
It's hard to take a clock phase / frequency / measurement example and
use it to teach people who want to use ADEV for stock market or
climate analysis. It's hard to take quartz and atomic clock examples
and use it to teach a mechanical clock & watch audience. Another
example is applying ADEV to sensor noise [1]. Even though it is
technically correct it's not likely to help explain the guts of a
GPSDO or NTP.
I can point you to a list of references but in my experience they are
too shallow or way too deep. What you want is something specific to
your NTP case. How about ignoring ADEV completely. Maybe just try to
explain why there is polling, why there is a poll interval, how the
poll interval could be less or more, why that may or may not affect
performance, how it might improve performance, or why it might
actually make it worse. That gets the concept of the "cross-over
point" across without all the rest of ADEV baggage.
Another challenge is that an audience needs to unlearn what they were
taught in school. We were told: the more data the better; the more
data the more precision you get; averaging data always gives better
results; get rid of outliers; stuff like that. Those lessons are valid
for measurements of something that doesn't change. But they are not
valid if what you're measuring is changing *while* you are making all
those measurements. Hence the ADEV "V" you mentioned. So maybe try to
explain NTP using words and analogies rather than using heavy math and
statistics; try not mentioning ADEV, slopes, and power law noise at all.
/tvb
[1] https://www.phidgets.com/docs/Allan_Deviation_Primer
On 9/23/2020 5:26 PM, Hal Murray wrote:
I'm looking for something for non-nuts, the general idea without any
complicated math.
Deep in ntpd is logic to adjust the polling interval, attempting to
find the
bottom of the V. It's hard to describe that to somebody who doesn't
know
about ADEV.
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