# [SI-LIST] Re: Guard traces for differential pairs

*From*: Chris Cheng <chris.cheng@xxxxxxxxxxxx>*To*: "'Duane Takahashi'" <duanet@xxxxxxxxxxxxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx, Chris Cheng <chris.cheng@xxxxxxxxxxxx>*Date*: Thu, 2 Oct 2003 16:18:50 -0700

Let me ask this one last time. What has that to do with guard trace on differential pairs (which is the original discussion of this thread) ? Why would the guard trace affects any of those package trace inductance on the net crosstalk. You have the funniest explanation on crosstalk I've ever seen in my career and I certainly want to learn. It will be helpful if you explain what exactly you mean by inductive xtalk appear in the lab. And I have a suspicion it will end up being sso noise. -----Original Message----- From: Duane Takahashi [mailto:duanet@xxxxxxxxxxxxxxxxxxxxxx] Sent: Thursday, October 02, 2003 3:56 PM To: si-list@xxxxxxxxxxxxx; chris.cheng@xxxxxxxxxxxx Subject: [SI-LIST] Re: Guard traces for differential pairs Hi Chris: Nice to hear from you again. I wasn't talking about a non homogeneous tline model or SSO noise. I was trying to explain the conditions under which I've seen inductive xtalk appear in the lab. Duane > Can you explain what does the presence of guard trace for differential > pairs > has any thing to do with driver impedance and load impedance ? You are > mixing up a non-homogeneous transmission line model with SSO noise and > Lij/Lii ne Cij/Cii has nothing to do with inductive or capacitive xtalk but > due to the non-homogeneous dielectric boundary. If the driver is > differential, the differential switching noise cancels each other out. > Don't > keep throwing terms out just to confuse people. > > -----Original Message----- > From: Duane Takahashi [mailto:duanet@xxxx] > Sent: Tuesday, September 30, 2003 9:25 AM > To: damonjbowser@xxxx; si-list@xxxx > Subject: [SI-LIST] Re: Guard traces for differential pairs > > > Hi Damon: > > I don't think you can look at the L and C matrices, and determine if the > xtalk will be capacitive or inductive. As Scott points out, they move > together. > > IMHO, you need to take into account the driver's impedance and the > load's impedance. To me, packages look inductive: bond wires with > traces over perforated reference planes. Flip chip helps a lot, but you > still have the perforated planes. > > When the driver fires, the inductance forces the voltage wave to lead > the current wave. This phase relationship travels down the line. To > the load, the line's impedance is no longer purely real, and has an > imaginary component to it. > > In this case, the capacitive and inductive forward xtalk components do > not cancel, and the inductance xtalk dominates. > > Duane > > > For TEM and quasi-TEM transmission line systems, the Inductance and the > > Capacitance matrices are related. For lossless and low loss systems, > > the inductance matrix is: > > L = mu * epsilon * (C inverse) > > > > They are linked as long as the magnetic and electric fields remain > > orthogonal. Thus, whenever you change the capacitance of a system of > > traces, you also change their inductance. > > > > A good quasi-static field solver will compute the correct L and C > > matrices for any given trace configuration. In fact, many of the > > currently available field solvers assume that the conductors are perfect > > and therefore only solve for the capacitance matrix. The inductance > > matrix is computed using the expression above. Clayton Paul has a good > > discussion of this in his book, "Analysis of Multiconductor Transmission > > Lines." > > > > As you change capacitive coupling, you also change inductinve coupling. > > > > regards, > > > > scott > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

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