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

Mon Oct 26 19:36:38 UTC 2020

Bob, thanks for the very detailed explanation. It starts to make much more
sense.

To answer your question, yes, the 10MHz OCXO I am measuring is part of the
GPSDO and is steered to GPS 1PPS.
The process I explained ( the 1PPS and the 1MHz clock from the OCXO / 10
into the flip flop, etc) and the TIC, OCXO in question, and GPS 1PPS are
all part of said GPSDO.

It is then the same GPS 1PPS cycle which time stamps the sampled and
logged  phase data from the TIC, ie, there is not a second GPS 1PPS or
anything.
The 1 second phase output value from the TIC feeds a 32 tap IIR filter,
which then drives the EFC control DAC. At the same time that same 1 second
cycle phase value is logged, each second.
Does that make sense, and am I sampling appropriately doing it this way?

I hope what I have done is not the same as trying to measure a 10Mhz signal
on a frequency counter, while using the same 10MHz signal as the reference
clock for the frequency counter timebase...!!

I am concerned about your statement :
* If you have 145,000 data points spaced at 1 second apart *and* TimeLab or
Stable32*

*shows it as 20,000 seconds … you have a problem with your data file.
Something aboutthe time tags isn’t in line with what the program is
expecting.*

The Timelab plot I attached in my first post shows on the bottom -
Input Freq = 10Mhz
Sample Interval = 1second
duration = 1day 15hours
Acquired points = 141879  ( this is the number of points in the file)
So 141879 seconds is 1day 15hours, but the plot X Axis shows 20000
seconds...

I cannot find anything wrong with the sample data - no missing samples, etc.
Is this not representing the averaging process?

Thank You again for your explanations!
Joe

On Mon, Oct 26, 2020 at 9:03 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> > On Oct 26, 2020, at 10:53 AM, Joe & Gisela Noci <jgnoci at gmail.com>
> wrote:
> >
> > 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.
>
> Is the OCXO part of a GPSDO? In other words, is it steered to GPS?
>
> >
> > 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?
>
> In terms of ADEV, Tau is the time between phase samples. We talk about “1
> second
> ADEV”. In that case Tau is 1 second. You take phase samples at clock tick
> 0, then
> again one second later at tick 1 and so on until you have a full data set.
>
> If you look at other data spacing’s you get a different number for ADEV.
> 10, 100,
> and 1,000 seconds are pretty typical. Going out to a bit over a day
> between samples
> gets you to a tau of 100,000 seconds.
>
> > 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?
>
> Tau is 1 second if you analyze that data for 1 sec ADEV. You will have
> 145,000 samples
> in the data set.
>
> If you go for Tau = 1000 seconds, you throw out 999 out of every 1000
> samples. That turns
> your data set into 145 samples. Same data, same “ADEV”, just at a
> different Tau.
>
> > 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?
>
> If you have 145,000 data points spaced at 1 second apart *and* TimeLab or
> Stable32
> shows it as 20,000 seconds … you have a problem with your data file.
> Something about
> the time tags isn’t in line with what the program is expecting.
>
> > And how did you equate that to needing ~4months?
>
> If you have points 100,000 seconds apart and want 100 points, it will take
> you 10,000,000
> seconds to get that data. That’s 115.74 days of data collection. Four
> months would be 120
> days …
>
> > 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..
>
> A lot of this is simply getting the data into a form that this or that
> program “likes”. It’s
> just a lot of tweaking and seeing if the result comes out right.
>
> Bob
>
>
>
>
> >
> > 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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> >>
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