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

Mon Oct 26 22:30:13 UTC 2020

Hi

To be really clear:

ADEV can be used to measure anything and done any way you wish. 

To have an ADEV that is a valid measure of a device, you need to do a
comparison to another *totally independent* device. That independent 
device needs to have a better ADEV over the range of Tau that you are
after over that range of Tau.

Anything that tries to get ADEV out of a single device or cross connected
setup will run into problems.

Bob

> On Oct 26, 2020, at 3:36 PM, Joe & Gisela Noci <jgnoci at gmail.com> wrote:
> 
> 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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