[time-nuts] Interpreting and Understanding Allen Deviation Results
cubecentral at gmail.com
Wed Nov 15 11:12:22 EST 2017
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
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
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
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
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
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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