[time-nuts] Interpreting and Understanding Allen Deviation Results

Thu Nov 16 18:18:45 UTC 2017

Hi

The exact software command used depends on the GPSDO you have. How you send the command to 
the GPSDO depends a bit on the driver software you are running. Not all GPSDO’s or modules have 
cable delay. All the ones I run do have the feature. Normally it comes set to some random default setting
like 60 ns. I typically take one GPSDO and deliberately make it the odd one out of the group. By making 
the offset large (say a microsecond or two) it’s easy to spot the “false ticker” in the group if things get 
mixed up. 

Assuming you set the odd GPSDO to be early (which could be cable delay + or cable delay -) it would 
go to input A on the counter. That would be the start channel. Input B would come from any normal 
pps signal and it would be the stop channel. You can of course do this other ways that work just as
well. 

Again - this is for looking at a pps relative to a GPSDO using a 5335 or similar counter. 

Bob

> On Nov 16, 2017, at 12:05 PM, Jerry <jsternmd at att.net> wrote:
> 
> Bob,
> 
> I am also a time newbie... how do you adjust this in software?  For time interval as discussed below, the unaltered GPSDO output goes to A and how do you create the GPSDO delay for B without a physical coax delay?  Any change in GPSDO cable delay setting will affect A and B the same.  Sorry if this is a stupid question
> 
> Jerry, NY2KW
> 
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Bob kb8tq
> Sent: Thursday, November 16, 2017 11:15 AM
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Subject: Re: [time-nuts] Interpreting and Understanding Allen Deviation Results
> 
> Hi
> 
> Yes, that’s exactly what I’m saying. You just use the software rather than dragging around a big hunk of coax. It makes it easy to get one pps into the “that’s way more than I need” range.
> With the coax approach, is 50NS enough? Might 100NS be needed? Is there a 231NS case?.
> I’ve spent a *lot* of time finding those cases in the middle of long data runs ….
> 
> Bob
> 
>> On Nov 16, 2017, at 10:37 AM, Jerry <jsternmd at att.net> wrote:
>> 
>> Bob,
>> 
>> Do you mean then you do not need to put a physical long length of cable for the delay, just do it in software or do you do both?
>> 
>> Jerry, NY2KW
>> 
>> -----Original Message-----
>> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Bob 
>> kb8tq
>> Sent: Thursday, November 16, 2017 9:58 AM
>> To: Discussion of precise time and frequency measurement 
>> <time-nuts at febo.com>
>> Subject: Re: [time-nuts] Interpreting and Understanding Allen 
>> Deviation Results
>> 
>> Hi
>> 
>> One “cute” thing to do when looking at GPSDO’s or GPS modules is to use the “cable delay” setting. It will allow you to move the pps of one unit relative to the pps of the other one. You then can be sure of which pps happens first. That makes the A to B measurement much easier to analyze. 
>> 
>> Short intervals also can lessen the impact of the time base accuracy in the counter ( you always are measuring a microsecond or so to a nanosecond resolution). Indeed there are other issues (like jitter) that still are an issue. 
>> 
>> Bob
>> 
>>> On Nov 16, 2017, at 4:10 AM, Azelio Boriani <azelio.boriani at gmail.com> wrote:
>>> 
>>> As already stated here, the best measurement mode is the 
>>> time-interval mode. The 5335A is a 2ns single-shot resolution 
>>> counter. Use the PPS output from the GPSDO, route it to the A (start) 
>>> input and to a coaxial cable used as a delay line (10m, 50ns, should 
>>> be enough). The other end of the cable into the B input (stop), 
>>> select the time interval mode TIME A -> B. Let the internal reference 
>>> clock the counter. Set trigger levels and the various parameter to 
>>> get stable readings and collect your data.
>>> 
>>> On Thu, Nov 16, 2017 at 3:59 AM, Mike Garvey <r3m1g4 at verizon.net> wrote:
>>>> Could you post some phase plots?  The data you show is not 1/tau and very likely not white phase noise.
>>>> Mike
>>>> 
>>>> -----Original Message-----
>>>> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of 
>>>> CubeCentral
>>>> Sent: Wednesday, November 15, 2017 11:12
>>>> To: time-nuts at febo.com
>>>> Subject: [time-nuts] Interpreting and Understanding Allen Deviation 
>>>> Results
>>>> 
>>>> Greetings, time-nuts!
>>>> 
>>>> After reading [ http://www.leapsecond.com/pages/adev/adev-why.htm ] I felt that I better understood how an Allan Deviation is calculated and endeavored to try an experiment.  It should be noted that I have a hobbyist-level understanding of the concepts described and tools used below.  If my thinking or test methodology is incorrect, please let me know so that I might learn something.
>>>> 
>>>> A GPSDO with a 10MHz output was run into the EXT TIME BASE input on the back of an HP5335A.
>>>> Then, the TIME BASE OUT on the back was run to the A input on the front of the HP5335A.
>>>> My intention was to characterize the performance of the HP5335A counter itself so that I might understand better future plots involving other GPSDO and the counter's internal clock (which was bypassed for this test).
>>>> 
>>>> The settings of the HP5335A were as follows:
>>>> Gate Mode: Normal
>>>> Cycle: Normal
>>>> 
>>>> A Input ------------------------------ Trigger Adjust: Full left to 
>>>> 'Preset' detent
>>>> Z select  =  in   =  50ohm
>>>> x10 ATTN  =  in   =  x10 ATTN   (should have been out/off?)
>>>> Slope     =  out  =  up
>>>> AC        =  in   =  AC coupled
>>>> COMA      =  out  =  Not ComA
>>>> AutoTrig  =  out  =  Not Auto Tiggered (should have been in/on?)
>>>> 
>>>> (Tangentially, if someone has a good 'primer' or how-to resource 
>>>> detailing Universal Counter operation, showing when/why/how to set 
>>>> the knobs in certain situations it would be welcome!)
>>>> 
>>>> I then set the Time Lab V1.29 software to repeatedly acquire data 
>>>> for
>>>> 12 hours, starting the next test as soon as I could.  This means 
>>>> that, normally, a test was run during the day for 12 hours, and then 
>>>> overnight for
>>>> 12 hours.
>>>> 
>>>> The results are shown here:  [ https://i.imgur.com/0sMVMfk.png ]  The associated .TIM files are available upon request.
>>>> 
>>>> So, now we get to the heart of the matter and the questions this test and results have raised.
>>>> I am trying to understand what the data is telling me about the test, and therefore the character of the counter.
>>>> 
>>>> 1)  Why are the plots a straight line from ~0.25s until ~100s?
>>>> 2)  Why, after falling at the start, do the plots all seem to go back up from ~100s to ~1000s?
>>>> 3)  What do the "peaks" mean, after the plot has fallen and begin to rise again?
>>>> 4)  Why is the period from ~1000s to ~10000s so chaotic?
>>>> 5)  The pattern "Fall to a minimum point, then rise to a peak, then fall again" seems to be prevalent.  What does that indicate?
>>>> 6)  Why does that pattern in question (5) seem to repeat sometimes?  What is that showing me?
>>>> 
>>>> And finally, some general questions about looking at these plots.
>>>> a)  Would a "perfect" plot be a straight line falling from left to right?
>>>> (Meaning a hypothetical "ideal" source with perfect timing?)
>>>> b)  Is there some example showing plots from two different sources that then describes why one source is better than the other (based upon the ADEV plot)?
>>>> c)  I believe that if I understood the math better, these types of plots would be more telling.  Without having to dive back into my college Calculus or Statistics books, is there a good resource for me to be able to understand this better?
>>>> 
>>>> Lastly, thank you for your patience and for keeping this brain-trust alive.
>>>> I am quite grateful for all the time and energy members pour into this list.
>>>> The archives have been a good source of learning material.
>>>> 
>>>>      -Randal (at CubeCentral Labs...)
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
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