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 <[email protected]> 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 <[email protected]> wrote: > >> Hi >> >>> On Oct 26, 2020, at 10:53 AM, Joe & Gisela Noci <[email protected]> >> 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 <[email protected]> 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 <[email protected]> >> 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>_______________________________________________ >>>>> time-nuts mailing list -- [email protected] >>>>> To unsubscribe, go to >>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>>>> and follow the instructions there. >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>>> and follow the instructions there. >>>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
