FreeLists / si-list / [SI-LIST] Re: RLGC matrices of a differential pair line

["Dima Smolyansky" <Dima.Smolyansky@xxxxxxxxxxxxxx>]

Hello, Hassan:

You can not compute R and G from characteristic impedances, but you can
compute L and C. My guess is that the confusion is because L and C are
normally computed from even and odd mode impedances, and different people
define differential and common mode impedances differently. The industry
standard way is (for a symmetric line pair):
Zdiff = Zodd * 2, hence, Zodd = Zdiff / 2
Zcommon = Zeven /2, hence, Zeven = Zcommon *2

You need to first figure out whether your differential and common mode
impedances were obtained using these definitions or other ones. An easy way
to tell this will be as follows. For a typical transmission line on the
board, with a target 100 Ohm differential impedance and in presence of
coupling, you should expect common mode impedance value to be 25-40 Ohm
(which would make your even mode impedance about 50-80, and odd mode
impedance about 50). In any event, odd mode impedance can not be higher than
even mode impedance (but will be equal to even mode impedance in absence of
coupling). If your common mode impedance in the given case is in 50-80 ohm
range, then the assumption used is probably that Zeven=Zcommon.

See our appnote on differential interconnect characterization for equations
relating Zodd and Zeven with L and C,
http://www.tdasystems.com/library/appnotes/diff1199.pdf, from which you can
easily obtain equations for L and C. More appnotes can be found under
http://www.tdasystems.com/support/applicationnotes/.

Thanks,

===================
Dima Smolyansky
TDA Systems, Inc.
4000 Kruse Way Place
Blvd. 2, Suite 300
Lake Oswego, OR 97035
(503) 246-2272
(503) 246-2282 (fax)

Download new IConnect 3.0 Quick Tour at www.tdasystems.com/products/  !




----- Original Message -----
From: "Hassan O. Ali" <hassan@xxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Thursday, February 06, 2003 8:44 AM
Subject: [SI-LIST] RLGC matrices of a differential pair line


>
>
> Hi All,
>
> Does anyone have formulas for computing the terms of R, L, G, and C
matrices for a
> differential pair line using its differential and common-mode
characteristic impedances?
> There seems to be a confusion of those formulas on the Net.
>
> Anyone knows the preferred structure of the equivalent circuit of a
differential pair
> line of unit length in terms of the per-unit R, L, G, C, parameters? I'm
particularly not
> sure of where to place the mutual terms in such a network?
>
> Thanks.
>
> Hassan.
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