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 > > > > Hi Duane, > > > > So how does one determine if it is magnetic or > > capacitive crosstalk that is dominating in a > > particular configuration? > > > > Let's say I get two matrices from a field solver that > > look like this: > > > > C11 C12 > > C21 C22 > > > > and > > > > L11 L12 > > L21 L22 > > > > Do I just look at how C12/C11 compares to L12/L11 and > > how C21/C22 compares to L21/L22 ? > > > > Thanks > > Damon > > > > > >>X-Original-To: si-list@xxxx > >>Delivered-To: si-list@xxxx > >>From: Duane Takahashi > >><duanet@xxxx> > >>User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; > >>en-US; rv:1.4) > >>Gecko/20030624 Netscape/7.1 (ax) > >>X-Accept-Language: en-us, en > >>MIME-Version: 1.0 > >>To: si-list@xxxx > >>Subject: [SI-LIST] Re: Guard traces for differential > >>pairs > >>Content-Transfer-Encoding: 8bit > >>X-archive-position: 8392 > >>X-ecartis-version: Ecartis v1.0.0 > >>X-original-sender: duanet@xxxx > >>X-list: si-list > >> > >>It also depends on the type of crosstalk: electric > >>or magnetic ( > >>capacitive or inductive ). If it's magnetic, guard > >>traces won't help. > >>If it's capacitive, guards will help but will leave > >>you with the > >>'magnetic remainder' of the xtalk. > >> > >>I've seen high impedance lines, more than 60 ohms, > >>exhibit inductive > >>xtalk. My guess is guard traces would not have > >>helped in this case. > >> > >>Regards, > >>Duane > >> > >> > >>>Sogo, > >>> > >>>It is true that guard traces can INCREASE > >> > >>crosstalk, but > >> > >>>this can be avoided (and crosstalk in fact can be > >> > >>REDUCED) > >> > >>>if the via stitching is frequent enough. Between > >> > >>each > >> > >>>stitching, there is a half-wavelength resonator, > >> > >>with a > >> > >>>lowest resonance frequency of fres=1/(2*tpd), > >> > >>where > >> > >>>tpd is the propagation delay between two stitching > >> > >>vias. > >> > >>>As long as the bandwidth of the signal on the > >> > >>structure > >> > >>>is safely below this resonance frequency, there > >> > >>should be no > >> > >>>problem with signal integrity. > >>> > >>>Regards, > >>> > >>>Istvan Novak > >>>SUN Microsystems -- Duane Takahashi phone: 408-720-4200 Greenfield Networks fax: 408-720-4210 255 Santa Ana Court email: duanet@xxxxxxxxxxxxxxxxxxxxxx Sunnyvale, CA 94085 * MOVING! 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