[SI-LIST] Corrections of typing errors in my previous message.
- From: Joseph.Schachner@xxxxxxxxxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 3 Jul 2013 09:24:47 -0400
Two corrections:
1) in the first paragraph: "...ever shows negative Dj." I had
incorrectly typed "negative Rj"
2) end of the 4th paragraph: "which means RJ needs to be probably about
38fs lower. That difference is all it would take to make -0.53ps DJ turn
into 0ps of DJ."
This is the new wording. When I re-read the old wording I realized even I
didn't think it expressed what I wanted, and I had just written it. Sorry.
--- Joe S.
----- Forwarded by Joseph M. Schachner/NEWYORK/LECROY on 07/03/2013 09:25
AM -----
From: Joseph.Schachner@xxxxxxxxxxxxxxxxxx
To: ah.vinod@xxxxxxxxx
Cc: SI-LIST <si-list@xxxxxxxxxxxxx>
Date: 07/03/2013 09:05 AM
Subject: [SI-LIST] Re: Negative Deterministic Jitter
Sent by: si-list-bounce@xxxxxxxxxxxxx
I've seen SigTest do that sometimes. Clearly it is a flaw in SigTest that
it ever shows negative Dj. 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 lower. That difference is all it would take to make
-0.53ps
DJ turn into 0ps 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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