[SI-LIST] Re: Even/Odd mode impedance

  • From: "Michael Allen" <mallen@xxxxxxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 12 Jun 2001 16:11:49 -0400

Alexis,
        The odd mode impedance should be approximately equal to half of
the differential impedance and the even mode should be as high as
possible.  TDA systems has a very good paper on the subject.

Mike Allen
Principal Engineer, Signal Integrity
TeraConnect, Inc
Nashua, NH


> -----Original Message-----
> From: Alexis LESTRA [SMTP:alestra@xxxxxxxxxx]
> Sent: Tuesday, June 12, 2001 11:58 AM
> To:   si-list@xxxxxxxxxxxxx
> Subject:      [SI-LIST] Even/Odd mode impedance
>=20
>=20
> Dear all,
>=20
> I am new in the field of signal integrity and would like to submit a=20
> basic problem. I am currently designing a  backplane for differential=20
> signals at 2.5  Gbit/s. The receiver is differential, with 100 Ohm=20
> termination. For coupled transmission lines, which characteristic=20
> impedance should match 50 Ohm, the even-mode impedance or the
> even-mode=20
> impedance ? After reading some documentation on the topic, I am=20
> confused. On one  hand, I read that differential signals propagate in=20
> odd-mode since  coupled transmission lines are driven with equel=20
> magnitude and 180=B0 out  of phase ("High-speed Digital System =
Design",=20
> S.H. Hall, G.W. Hall, J.A.  McCall, p.53) . On the other hand, I read=20
> that since differential  signals are launched from silicon devices,
> the=20
> gates that driver these  signals are both referenced to ground,=20
> resulting in even-mode coupling =20
> (http://www.amp.com/simulation/files/papers/98060A 5.pdf   p.19).
> Can anyone help me to clarify this point.
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> Thanks in advance.
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> Alexis
>=20
> Alexis Lestra
> Tachys Technologies
> Les Algorithmes, Batiment Aristote A
> 2000, route des Lucioles
> F-06901 Sophia-Antipolis Cedex
>=20
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