[SI-LIST] Re: off-diagonal resistance and conductance elements
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 18 Feb 2003 14:19:56 -0800 (PST)
Patrick-
Ansoft published an app-note in sept. 2000 entitled
"Mutual Resistance in Spicelink (authored by Eric Bracken) which
covers the topic pretty well.
Downloadable at:
ftp://ftp.ansoft.com/techsup/download/web/ftproot_inet/products/2dextractor/app_
notes/mutualresistanc
e.pdf (if you are a customer)
-Ray Anderson
Sun Microsystems Inc.
>
>
>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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