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

Tue Oct 27 16:38:19 UTC 2020

On another thread,  G3YSX described this technique :
Taken in conjunction with the Wikipedia entry it begins to make sense here.

...................................
"A note on this thread:  plotting phase against time will tell you a lot
about the effect of propagation on phase, but it is not the technique used
in the synchronisation world to understand the behaviour of oscillators.

They use Allen variance, TDEV and MTIE plots.

Fundamentally what these measurements do is to look at the error every
second, and then cumulatively work out what the error is every second,
every two seconds, every four seconds etc until the limit of the
measurement, but they do it using a sliding window system so in a one
hundred second time (say) they have one hundred one second measurements to
average, 99 two second measurements, 98 three second measurements…. 49 50
sec measurements etc. Then they plot this on a log scale.

This tells you a lot more about what is going on than the phase error vs
time measurements and revolutionised understanding of oscillator and hence
time measurement characteristics. In particular with the more sophisticated
plots developed after Allen initial work, the slope of the curve at various
places along the plot tells you the cause of the error in the system."

Andy
www.g4jnt.com

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On Tue, 27 Oct 2020 at 15:59, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> Indeed this *is* a very fundamental problem in doing this sort of work.
> The test setups and references generally are way harder to set up (and
> more costly) than the design and construction of this or that device.
>
> Indeed, as part of a normal design process on a GPSDO, the testing /
> tweaking phase is much longer and much more expensive than the
> work leading to the first “working” batch of units.
>
> Fun !!!
>
> Bob
>
> > On Oct 27, 2020, at 9:46 AM, Joe & Gisela Noci <jgnoci at gmail.com> wrote:
> >
> > Thanks Bob.   Slowly it penetrates..
> > The 1PPS source is as you say far from being a good reference, so I will
> > cease that route!
> >
> > I think I have to accept what I have - I believe it is good enough for my
> > Ham Radio work as a reference for my radios, at least into the low GHz
> > region.
> > If I manage less than 100Hz error in the 2.4GHz band, I am OK..
> >
> > As I mentioned before, its very difficult for me to obtain or have
> > access to Reference Grade solutions, or for me to buy a handful of
> suitable
> > surplus sources, etc...
> > The usual Ebay sources don't work well to our part of the world, very
> > difficult, very costly, and most Ebay vendors won't even ship toNamibia!
> >
> > Having learned a lot from you chaps, for which I am grateful, I think I
> now
> > at least know the size of the cliff above of me!
> >
> >
> > Thanks to all..
> > regards
> > Joe
> >
> > On Tue, Oct 27, 2020 at 3:29 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> >
> >> Hi
> >>
> >> What are we trying to measure with ADEV?
> >>
> >> On a frequency source, it’s used to measure the noise of that source.
> >> Mostly we
> >> put up plots of ADEV to show how quiet our source is. (Yes, we might
> also
> >> measure
> >> noise floor or amplifier contributions ….).
> >>
> >> That’s the statistics part. Now for the instrumentation:
> >>
> >> There is no device that will directly measure frequency / time / phase
> to
> >> the accuracy
> >> levels we are after. The closest we can get is to measure A against B
> and
> >> look at the
> >> delta. We also could look at A vs B vs C and do some fancy math, that
> >> takes a bit
> >> of setup and has some pretty significant limits. You still are doing a
> >> comparison.
> >>
> >> With an A to B comparison, you need to know that one or the other device
> >> is much
> >> lower noise than the other. Then the plot will (essentially) be the
> noise
> >> of the not so
> >> quiet source. If that’s not true, you have a real tangle. It’s even
> worse
> >> if both our
> >> sources have the *same* noise in them.
> >>
> >> So how does this apply in your case?
> >>
> >> The 1 pps out of your GPS module is *far* from a low noise source close
> >> in. Further
> >> out it will wander more than a little due to ionosphere issues.
> Depending
> >> on how the
> >> OCXO is locked, there  will be a cross over between “free run” and
> >> “follows GPS”.
> >>
> >> The real ADEV of any GPSDO starts out with the free run OCXO noise +
> loop
> >> noise +
> >> GPS noise. Hopefully (but not always) the OCXO noise is the big item
> close
> >> in. As you
> >> get further out, GPS noise becomes the dominant contributor to the
> output
> >> noise. Again
> >> this is a “hopefully” sort of thing. We test our designs because that’s
> >> what shows us
> >> where improvement is needed …..
> >>
> >> What to do?
> >>
> >> You need an independent source or set of sources to compare against. You
> >> need to
> >> feed them into something like a DMTD to measure what’s going on.
> >>
> >> Bob
> >>
> >>> On Oct 27, 2020, at 2:19 AM, Joe & Gisela Noci <jgnoci at gmail.com>
> wrote:
> >>>
> >>> Bob and Tom,
> >>> Thanks again for your time on this.  I understand a lot better, and
> have
> >>> just one issue I would like to 'harp' on a little, if you would allow..
> >>> In the simplistic example of a freq counter measuring its own
> reference,
> >> It
> >>> it easy to grasp and understand the incestous nature of the
> measurement.
> >>>
> >>> I am having difficulty extending that concept to what I have though.
> For
> >>> the purpose of understanding this concept, lets ignore jitter, and all
> >>> other 2nd order effects for now.
> >>>
> >>> My OCXO is phase locked to a GPS 1PPS . The same GPS 1PPS is 'locked'
> to
> >>> a very accurate , very stable  (Cesium?) reference within the SAT
> >>> constellation.
> >>> I would say that if I use the GPS 1PPS ( which is the same as the
> Cesium
> >>> reference, in my example..) as my reference, then when I measure the
> >> phase
> >>> delta between the OCXO and that 1PPS
> >>> I am in fact measuring the phase delta between the OXCO and the
> accurate
> >>> Cesium reference.
> >>> I realise I am in fact measuring how well the OCXO is phase locked to
> the
> >>> Reference ( 1PPS , derived from the Cesium reference..) , but that
> should
> >>> still show what the frequency and phase offset is
> >>> between the OCXO and the Reference.
> >>> I am having difficulty seeing that this is in fact not independent -
> the
> >>> underlying raw reference for the measurement is the Cesium reference
> and
> >> I
> >>> can't get better than that.
> >>> Substituting a separate, equally good Cesium reference from which I
> >> derive
> >>> a 1PPS, is surely no different?
> >>>
> >>> To simplify my confusion, I have attached a PDF block diagram - this
> >> shows
> >>> a 'perfect' 10MHz reference oscillator - perfect in accuracy, drift,
> >> phase
> >>> errors, etc - just perfect.
> >>> It is the reference for a PLL with the OCXO being controlled. The
> perfect
> >>> osc is divided down to present a 1PPS to the TIC. The OCXO is divided
> >> down
> >>> to present a 1MHz signal to the TIC.
> >>> The resultant phase delta is logged and used to plot Adev - basically
> >> what
> >>> I described above, but a perfect Osc instead of the GPS.
> >>> This surely is comparing the OCXO phase to the perfect osc phase,
> >>> regardless of what is controlling or steering the OCXO?
> >>>
> >>>
> >>> Tom, I am not sure what you mean by -
> >>>
> >>> *The Trimble Thunderbolt (aka TBolt) GPSDO has this
> >>> disable-discipliningfeature. Note it's not "holdover"; that's something
> >>> else entirely.  *
> >>>
> >>> I assume 'holdover' to be when the OCXO EFC voltage is just held fixed?
> >>> If so, I do not understand how disciplining can be disabled without the
> >> EFC
> >>> voltage just being held to a fixed value?
> >>> Can you explain the difference between 'disciplining-disabled' and
> >>> 'holdover' please?
> >>>
> >>> Chaps, thank you for indulging me on this - the basic concepts are the
> >>> formative grounding for beginning to understand this subject even a
> >> little
> >>> and I appreciate your assistance
> >>> and guidance in this!
> >>> Regards
> >>> Joe
> >>>
> >>> On Tue, Oct 27, 2020 at 1:51 AM Tom Van Baak <tvb at leapsecond.com>
> wrote:
> >>>
> >>>> Joe,
> >>>>
> >>>>> I log the output of the TIC, in nanoseconds, and use that file to
> >>>> generate an ADEV plot.
> >>>>
> >>>> Good. That's what you need. During normal operation those readings are
> >>>> bounded by the PLL. So it's essentially a measurement of how well the
> >>>> PLL is working, how aggressive the OCXO is steered, etc. ADEV isn't
> the
> >>>> best way to process that kind of data because it's a boring, even
> >>>> misleading, straight line going down forever.
> >>>>
> >>>>> Maybe my setup is in fact comparing itself with itself?..!
> >>>>
> >>>> Yes. Oops. But, here's an idea for you.
> >>>>
> >>>> One useful technique is to have your GPSDO running fine and then
> >>>> *disable* the disciplining. If you designed the GPSDO you'll know the
> >>>> exact spot in the h/w or s/w to do this. From this point forward your
> >>>> OCXO is still running, your GPS/1PPS receiver is still receiving, the
> >>>> TIC is still comparing, and you are still logging TIC readings every
> >>>> second. But now the DAC is frozen and the OCXO is free-running.
> >>>>
> >>>> When you plot this data you will see phase slowly wandering away from
> >>>> zero, you may see a slight drift in frequency, and mostly what you
> will
> >>>> see is the "bathtub" ADEV shape that you were looking for. This method
> >>>> works because as soon as your disable disciplining your OCXO becomes
> >>>> independent of GPS and so the ADEV plot will be a measurement of an
> >>>> oscillator instead of a measurement of a PLL.
> >>>>
> >>>> This is not something you would do everyday, but especially now that
> you
> >>>> are understanding how a GPSDO works and playing with Allan statistics
> >>>> it's a educational exercise.
> >>>>
> >>>> The Trimble Thunderbolt (aka TBolt) GPSDO has this
> disable-disciplining
> >>>> feature. Note it's not "holdover"; that's something else entirely.
> >>>>
> >>>> /tvb
> >>>>
> >>>>
> >>>> _______________________________________________
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> >>>>
> >>> <Block Diagram Adev
> >> measurement.pdf>_______________________________________________
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