[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.


> 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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