[SI-LIST] Re: Negative Deterministic Jitter
- From: vinod ah <ah.vinod@xxxxxxxxx>
- To: ransom@xxxxxxxxxxxxxxxx
- Date: Thu, 4 Jul 2013 10:58:55 +0530
Thanks all for in-depth analysis. It did give lot of info how these
jitter processing tools works and what to take into consideration and
what not to take during jitter analysis :-)
On Thu, Jul 4, 2013 at 3:02 AM, Ransom Stephens <ransom@xxxxxxxxxxxxxxxx> wrote:
> On dual-Dirac as an approximation:
> Since the RJ-dominated tails are the only part of the distribution that are
> relevant to calculating TJ from
> TJ(BER) = DJ(dual-Dirac) + Q_BER * (rms RJ)
> it doesn't matter what the actual DJ distribution looks like.
>
> Something to keep in mind when using that expression for TJ(BER): It's
> important that the model-dependent, dual-Dirac DJ be used in the equation.
> Generally, DJ(peak-to-peak) >= DJ(dual-Dirac)
>
> The exception is when the tails of the distribution are *not* dominated by
> RJ. This occurs when DJ is big enough that the tails are not smooth and RJ
> dominated. The worst case is when something like crosstalk puts a bump in
> the bathtub plot toward the center of the eye.
>
> Also, we saw at DesignCon in Marty Miller's paper, that at high data rates
> (10+ Gb/s) with long test patterns, like PRBS31, DJ becomes
> indistinguishable from RJ. You can take this as confusion between what we
> think of as RJ and DJ, "a large sum of small effects" --> RJ even if many of
> those small effects are DJ, in which case you can't measure DJ with any
> accuracy, but it's okay because your RJ sucks it up and it's *really* acting
> like RJ so the extra Q_BER factor is warranted. Or you can take it as
> evidence that the whole business of separating jitter into different buckets
> is corrupt.
>
> Either way, it's worth keeping in mind that there doesn't exist equipment
> that can reliably measure or estimate TJ with accuracy better than 10% -- so
> don't worry too much about it.
>
> _____________________________
> Ransom W. Stephens, Ph.D.
> Science & Technology - Content & Analysis
>
> www.ransomsnotes.com
>
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>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>> On Behalf Of Joseph.Schachner@xxxxxxxxxxxxxxxxxx
>> Sent: Wednesday, July 03, 2013 6:02 AM
>> To: ah.vinod@xxxxxxxxx
>> Cc: SI-LIST
>> Subject: [SI-LIST] Re: Negative Deterministic Jitter
>>
>> I've seen SigTest do that sometimes. Clearly it is a flaw in SigTest that
>> it ever shows negative Rj. It has to do with how they decompose jitter.
>> Here's roughly how it works. It's relatively easy to measure the total
>> jitter, SigTest does have to pick RX Eq and PLL settings, but after that
>> TJ is just the jitter that there is on the data relative to the recovered
>> clock.
>> That leaves the problem of how we divide TJ into RJ and DJ.
>>
>> SigTest (and many real measuring instruments) use a spectral
>> decomposition. To do that you take the jitter values as a function of time
>> and FFT them. You get a spectrum, showing jitter magnitude vs frequency.
>> Now, the assumption is that anything that forms a peak is DJ and the
>> background level, broadband noise, is RJ. The sticky details that cause
>> the problems here is the algorithm that decides what is a peak and what is
>> just background.
>>
>> Note: RJ is a sigma, which specifies the width of a Gaussian distribution.
>> Since 14.02 * Rj + DJ is supposed to equal TJ according to the
>> dual-dirac model that thinks DJ just makes two Gaussian RJ distributions
>> centered at values separated by the DJ, if we make that assumption, this
>> is an over-constrained problem since we would have values for all three
>> and yet they are not independent. I am going to guess, with good
>> confidence, that SigTest uses the RJ number it gets from spectral
>> decomposition and computes DJ like this: DJ = TJ = 14.02 * RJ
>>
>> Now, just consider for a moment. Suppose SigTest is just a little too
>> reluctant to attribute energy to peaks in the spectrum, thus leaving a
>> little more energy in what it considers "background". That will raise RJ.
>> Having just slightly too high RJ, after it's multiplied by about 14, can
>> make DJ go negative. -0,53 / 14 is -0.038, which means RJ needs to be
>> probably about 38fs higher. That difference is all it would take to make 0
>> DJ turn into -0.53ps of DJ.
>>
>> It would be reasonable for SigTest to never show a negative DJ. It might
>> be reasonable for it to show 0 DJ and add what is required back into RJ to
>> make that work out.
>>
>> One more thing I just have to say, is that as Einstein said, a model
>> should be as simple as possible but NOT simpler. I believe that the dual
>> dirac model for jitter, although it is widely used, is often too simple.
>> Because It is widely used it does give numbers that can be compared
>> between instruments that use the same model. However, in terms of
>> producing true estimates of Rj and Dj, the requirement that DJ form
>> exactly two identical distributions is not realistic in too many cases. It
>> is possible to analyze the jitter and determine the effect of DJ much more
>> honestly, and then normalize for that. If there really are just two
>> separated distributions it works exactly like Dual Dirac. But if the
>> situation is not that simple it is not misled. When the Dual Dirac model
>> is not appropriate it is likely that using the Dual Dirac model to
>> determine Rj and Dj will result in RJ too high, and therefore negative DJ.
>>
>> Here's an article from EDN about Normalized Q-scale, the more flexible
>> model:
>> http://www.edn.com/design/test-and-measurement/4314553/Normalized-Q-scale-
>> analysis-Theory-and-background
>>
>>
>> --- Joe S.
>>
>>
>>
>> From: vinod ah <ah.vinod@xxxxxxxxx>
>> To: SI-LIST <si-list@xxxxxxxxxxxxx>
>> Date: 07/03/2013 05:44 AM
>> Subject: [SI-LIST] Negative Deterministic Jitter
>> Sent by: si-list-bounce@xxxxxxxxxxxxx
>>
>>
>>
>> Hi all,
>>
>> I am measuring Rj, Dj & Tj for a saved .wfm format waveform of PCIe3
>> compliance pattern (modified PRBS-11 pattern). When i feed the pattern
>> to SIGTEST software available for PCISIG.com, i see Tj of 17.34ps, Dj
>> of -0.53ps & Rj of 1.27ps rms.
>>
>> I am unable to understand on negative result of Dj. Is it possible to
>> have negative jitter ??
>>
>> In clock jitter measurements, edge moving ahead is considered as +ve
>> jitter while edge moving behind is considered as negative jitter, but
>> how is that applicable to a PRBS sort of pattern.
>>
>> Regards
>> Vinod A H
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