[SI-LIST] Re: off-diagonal resistance and conductance elements
- From: "Dunbar, Tony" <tony_dunbar@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 18 Feb 2003 16:40:46 -0600
For the purposes of clarification:-
Patrick initially stated "The on-diagonal parameters (e.g., L11) are
typically stated to be the self parasitics, ..."
My understanding is that, rather than being purely "self parasitics", they
actually include the effects of coupled neighbors. As a consequence, for
example, L11 will actually be different (lower) than it will be if the same
primary structure existed without any coupled neighbors.
Is that correct?
Thanks,
Tony Dunbar
-----Original Message-----
From: Zabinski, Patrick J. [mailto:zabinski.patrick@xxxxxxxx]
Sent: Tuesday, February 18, 2003 2:42 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] off-diagonal resistance and conductance elements
In a coupled-pair of distributed transmission lines (whether intentionally
for differential or unintentionally with crosstalk), most (good)
EM simulators produce a 2x2 matrix of capacitance, inductance,
resistance, and conductance (C, L, R, & G). The on-diagonal
parameters (e.g., L11) are typically stated to be the self
parasitics, which is quite easy to understand.
For the inductance and capacitance matrices, even the off-diagonal
parastics (e.g., L12, C21, ...) are easy to understand and
well published.
However, I have not been able to find a good description nor
treatment on the off-diagonal resistance and conductance
elements. Can anyone enlighten me a bit?
For example, what does R12 respresent? With the lossless/ideal
case setting R12=0, it cannot represent a resistive element
directly between the two traces. So what is it?
A second yet possibly related question deals with how these
matrices deal with odd- and even-mode using the same matrices.
When looking at any of the common twin-axial cables used
today with Infiniband and other differential protocols, the
two signal conductors are made with "good" (meaning low
loss) materials. In contrast, the outer shield is often
a much lousier (higher loss) material (either through the metallurgy
or thickness).
For odd-mode signals propagating down one of these twin-ax
cables, we believe the return current for one wire is
effectively captured (at least in part) in the other complement
wire, which would result in reasonably low loss. In contrast,
in even-mode propagation, the return current is within the
outer shield, which in turn results in a higher loss than
the odd-mode propagation. The end result (we have plenty
of measurement data confirming this) is that odd-mode
signals propagate reasonably well, but even-mode signals
attenuate and disperse much more significantly. (note:
for many applications, this is a very good thing.)
The question is: how can the LRCG matrices be set up such that you
use one set of matrices (in the form of a W-element if you wish) that
can accurately represent both cases? Does the off-diagonal
R & G matrices play a role?
Thanks,
Pat
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