[SI-LIST] Re: Negative Deterministic Jitter
- From: Hank Z <deltisme@xxxxxxxxx>
- To: ah.vinod@xxxxxxxxx, SI-LIST <si-list@xxxxxxxxxxxxx>
- Date: Wed, 3 Jul 2013 11:40:49 -0400
This usually happens to PRBS pattern.
I suggest you to use compliance measurement mode (CMM) pattern.
HTH,
Hanqiao
Intel
On Wed, Jul 3, 2013 at 9:02 AM, <Joseph.Schachner@xxxxxxxxxxxxxxxxxx> wrote:
> 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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