[time-nuts] Interpreting and Understanding Allen Deviation Results

[Azelio Boriani]

As already stated here, the best measurement mode is the time-interval
mode. The 5335A is a 2ns single-shot resolution counter. Use the PPS
output from the GPSDO, route it to the A (start) input and to a
coaxial cable used as a delay line (10m, 50ns, should be enough). The
other end of the cable into the B input (stop), select the time
interval mode TIME A -> B. Let the internal reference clock the
counter. Set trigger levels and the various parameter to get stable
readings and collect your data.

On Thu, Nov 16, 2017 at 3:59 AM, Mike Garvey <r3m1g4 at verizon.net> wrote:
> Could you post some phase plots?  The data you show is not 1/tau and very likely not white phase noise.
> Mike
>
> -----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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