[time-nuts] Interpreting and Understanding Allen Deviation Results

Mike Garvey r3m1g4 at verizon.net
Wed Nov 15 21:59:12 EST 2017

Could you post some phase plots?  The data you show is not 1/tau and very likely not white phase noise.

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of CubeCentral
Sent: Wednesday, November 15, 2017 11:12
To: time-nuts at febo.com
Subject: [time-nuts] Interpreting and Understanding Allen Deviation Results

Greetings, time-nuts!

After reading [ http://www.leapsecond.com/pages/adev/adev-why.htm ] I felt that I better understood how an Allan Deviation is calculated and endeavored to try an experiment.  It should be noted that I have a hobbyist-level understanding of the concepts described and tools used below.  If my thinking or test methodology is incorrect, please let me know so that I might learn something.

A GPSDO with a 10MHz output was run into the EXT TIME BASE input on the back of an HP5335A.
Then, the TIME BASE OUT on the back was run to the A input on the front of the HP5335A.
My intention was to characterize the performance of the HP5335A counter itself so that I might understand better future plots involving other GPSDO and the counter's internal clock (which was bypassed for this test).

The settings of the HP5335A were as follows:
Gate Mode: Normal
Cycle: Normal

A Input ------------------------------
Trigger Adjust: Full left to 'Preset' detent
Z select  =  in   =  50ohm
x10 ATTN  =  in   =  x10 ATTN   (should have been out/off?)
Slope     =  out  =  up
AC        =  in   =  AC coupled
COMA      =  out  =  Not ComA
AutoTrig  =  out  =  Not Auto Tiggered (should have been in/on?)

(Tangentially, if someone has a good 'primer' or how-to resource detailing Universal Counter operation, showing when/why/how to set the knobs in certain situations it would be welcome!)

I then set the Time Lab V1.29 software to repeatedly acquire data for 12 hours, starting the next test as soon as I could.  This means that, normally, a test was run during the day for 12 hours, and then overnight for
12 hours.

The results are shown here:  [ https://i.imgur.com/0sMVMfk.png ]  The associated .TIM files are available upon request.

So, now we get to the heart of the matter and the questions this test and results have raised.
I am trying to understand what the data is telling me about the test, and therefore the character of the counter.

1)  Why are the plots a straight line from ~0.25s until ~100s?
2)  Why, after falling at the start, do the plots all seem to go back up from ~100s to ~1000s?
3)  What do the "peaks" mean, after the plot has fallen and begin to rise again?
4)  Why is the period from ~1000s to ~10000s so chaotic?
5)  The pattern "Fall to a minimum point, then rise to a peak, then fall again" seems to be prevalent.  What does that indicate?
6)  Why does that pattern in question (5) seem to repeat sometimes?  What is that showing me?

And finally, some general questions about looking at these plots.
a)  Would a "perfect" plot be a straight line falling from left to right?
(Meaning a hypothetical "ideal" source with perfect timing?)
b)  Is there some example showing plots from two different sources that then describes why one source is better than the other (based upon the ADEV plot)?
c)  I believe that if I understood the math better, these types of plots would be more telling.  Without having to dive back into my college Calculus or Statistics books, is there a good resource for me to be able to understand this better?

Lastly, thank you for your patience and for keeping this brain-trust alive.
I am quite grateful for all the time and energy members pour into this list.
The archives have been a good source of learning material.

	-Randal (at CubeCentral Labs...)

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