[SI-LIST] Re: symmetry vs length matching for diff signals

One quick way to determine loose or close coupling is to look at the single
ended and differential impedances. If the differential impedance is much
less than twice the single ended impedance for the same track width, (i.e.
50 single, 90 differential), you have closely coupled lines. If the
impedance is slightly less (i.e. 50 Ohms single, 97-100 differential, you
are loosely coupled). The loosely coupled pair is easier to route with
minimal differential impedance discontinuities if you have to go around vias
or other obstructions by breaking up the diff pair.



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Rick Brooks
Sent: Tuesday, November 30, 2004 6:34 PM
To: Cortex.Chen@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: symmetry vs length matching for diff signals


I'm not sure this is what you are looking for or not...

There is a description in Ron Poon's book: Computer Circuits Electrical
Design, pg 198.
He defines a magnetic coupling coefficient km and a capacitive coupling
coefficient kc
with regard to coupled transmission lines

km = Lm/(SQRT(L1*L2))
kc = Cm/(SQRT(C1*C2))

Lm and Cm are mutual inductance and mutual capacitance per unit length
L1 and L2 are self inductance and capacitance per unit length.

As a rough guideline: if km and kc are less than 0.2, the lines are said to
have "loose" coupling.


I hope it helps,
- Rick






At 07:30 PM 11/30/2004 +0800, Cortex Chen wrote:
>Dear Abe Riazi,
>How to determine the tightly and loosely coupled pairs?
>
>Regards,
>
>Cortex
>
>
>
>
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