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

[Bob kb8tq]

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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