[time-nuts] Mentorship needed in learning about Allan Deviation and variation.

Mon Oct 26 14:53:39 UTC 2020

Hi again Bob - Seems I am destined to hog your time!

If my understanding and my questions appear foolish, sorry - I do have a
lot to learn to just understand the basics.

Maybe my setup is in fact comparing itself with itself?..!
I'll try to explain my hardware concisely -

The GPS 1PPS (with hardware saw-tooth compensation, not relevant here I
think) sets a flip-flop. The OCXO 10MHz is divided by 10, and that 1MHz
drives the reset of the flip-flop.
So the 1PPS rising edge sets the FF, the first rising edge of the OCXO
derived 1MHz clock resets it. The length of the pulse on the Q output of
the FF is now directly related to the stability of the 1MHz clock edge to
the 1PPs edge.
The max length is 1us ( the 1MHz clock period). It is this pulse length
that is measured (The TIC) and logged. The logged data is time stamped by
the falling edge of the GPS 1PPS. This data contains the essential Phase
difference between the PPS signal edge and the 1MHz clock edge, with a
regular jitter from the inherent PPS jitter from the GPS.
I then used TimeLab to read in that file, with sample interval set to
1second, and plot the Adev chart.
I understood that in my setup as described, my GPSDO , or the OXCO 10MHz
output, is now being compared to the GPS 'accurate' 1PPS - I am aware of
the PPS jitter, etc, but I also understand that that will be smoothed out
in the Adev derivation.

So, how much of my understanding of the above is bad?!

Secondly, I have not grasped the meaning of :

*As a practical point, 100 samples for an ADEV plot with Tau = 100,000 is a
prettylong data run.  Most of us don’t have ~4 months to do that sort of
run*

Could you explain what the 100 samples are, and what Tau=100,000 means?  I
am trying to put that in context with my data file -
eg, I have a file logged for maybe 32hours, with 145,000 logs, each one
second apart. Is that 145,000 samples? or just 145,000 data points? Or
is it Tau=145,000?
When I plot the Adev chart, it shows on the x axis that my 145,000 data
points plots as nearly 20,000 seconds - is that the 'samples' you mean?
And how did you equate that to needing ~4months?  Forgive my doltness - I
need to understand the relationships and terminology before it clicks!

I have gone through many examples and read heaps, but have not found a real
101 explanation of terms and data relationships yet..

Thank You!
Joe

On Mon, Oct 26, 2020 at 4:21 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> Any practical measure you feed into an ADEV computation will be a look at
> “device A” versus “device B”. In this case one of them is your GPSDO. What
> is the other device? ( = your TIC has a DUT input and a REF IN, it compares
> one to the other ….).
>
> If you feed your measurement system with the same signal on both inputs,
> you get a “noise floor” measurement. This *does* have value since it
> represents
> the best numbers you should ever see out of your system. Checking the
> noise floor is highly recommended …..
>
> ======
>
> One of the wonderful things about going to conferences is the ability to
> interact
> with folks face to face. The question of “how many samples for ADEV?” was
> one
> of my favorites. The range of answers turned out to be all over the place.
> On one
> end, the practical answer of “I plot what I’ve got”. On the other end the
> statistician's
> answer of 100 to 250. Both answers generally were also tagged with a lot of
> “that depends” sort of stuff.
>
> As a practical point, 100 samples for an ADEV plot with Tau = 100,000 is a
> pretty
> long data run. Most of us don’t have ~4 months to do that sort of run.
> Indeed one
> of the drivers for various “other” dev measurements has been to improve
> the
> confidence with a lower number of samples.
>
> Lots of zigs and zags ….
>
> Bob
>
> > On Oct 26, 2020, at 8:16 AM, Joe & Gisela Noci <jgnoci at gmail.com> wrote:
> >
> > Hi to all,
> > Very new and green at this ..
> > I am trying to understand more about Allen Deviation, a subject about
> which
> > I , for practical purposes know 'nothing'.. I have spent a lot of time
> > digging on internet and have read many articles , inc W.J Wriley
> > publications, the Stable32 files and user manuals, dug in the TimeLab
> > files, etc.
> > I understand the concepts somewhat, but some of my practical results are
> > beginning to seem too good to be true.
> >
> > I built a GPSDO with an HP 00105-6013 OCXO, covered in some previous
> mails
> > on this forum so I won't go into the detail, but I have finally got the
> > thing working well, 'stable' or so I believe/thought.
> >
> > I log the output of the TIC, in nanoseconds, and use that file to
> generate
> > an ADEV plot.
> > After 10-12 hours, it looks good - was approaching 10 minus 12, and that
> is
> > good enough for my purposes. So I left the unit running and logging -
> after
> > 32 hours, it approaches 10 minus 13,....
> > The line continues downwards - I do not see much indication of the
> upwards
> > turn I see in all other ADEV plots. I have not managed to understand the
> > mechanism behind this upturn..
> > The attached  - Rub_Adev.gif is just an example of what I mean - this
> image
> > courtesy of LeapSeconds.com, of a rubidium source.
> > A plot of my oscillator is in JN_1e-13.gif.   That show that after approx
> > 142K 1sec samples, the Adev is around 1E minus 13...!! and not showing
> > signs of turning up yet.
> >
> > This is surely not true?  If not true, what should I be looking at to
> > understand what is going on?
> > My TIC measurement resolution is for all intents, around 0.25ns, but I
> > suspect noise makes it no better than 2ns.
> > Why does my plot keep going down, below minus 13?
> > Why is it going down that far? My GPSDO cannot be that good?
> > Is there a means of determining the ( minimum?)  number of phase samples
> > needed to give a sensible indication of the Adev value?
> >
> > Before I ask more questions, I need to discover the extent of what I
> don't
> > know, so I don't ask too many foolish questions!
> >
> > Thank you!
> > regards
> > Joe
> > <Rub_Adev.gif><JN_1e-13
> GPSDO.gif>_______________________________________________
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