[time-nuts] Is there a good web page introducing ADEV?

[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.
>
>