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

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.


> 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
> 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
>  > 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
>  > Lines."
>  >
>  > As you change capacitive coupling, you also change inductinve coupling.
>  >
>  > regards,
>  >
>  > scott
>  >
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