RE: [SI-LIST] : 2.5 Gbps across a backplane

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From: Knighten, Jim L (JK100005@exchange.SanDiegoCA.NCR.COM)
Date: Mon Oct 16 2000 - 13:11:09 PDT


Ed,

Addressing the last of your questions, i.e., can active equalization help
from an EMI point of view, it is by opinion that the answer is no.

The work that we have done indicates the following:

* The common-mode component of the differential signal is what causes
radiated EMI from the signal itself (as opposed to other noise that hops a
ride on the differential transmission line). Of course, the
differential-mode component is what the circuit designer intended to exist
on the transmission line. This is probably old news to many readers

* The common-mode is created by imbalance in the differential signal.
The most important type of imbalance is "delay skew," or a timing, or path
length difference between the two sides of the differential signal. We have
published data on this in the IEEE EMC Symposia. Other types of imbalance
also produce a common-mode signal, but not nearly as effectively.

* In my experience, active equalization doesn't contribute to the
delay skew in the line, hence it is not an important part of the EMI
equation.

Regards,

Jim
________________________________________________________
Dr. Jim Knighten e-mail: jim.knighten@ncr.com
<jim.knighten@ncr.com>
Technical Consultant - Design
NCR
17095 Via del Campo
San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com>
Tel: 858-485-2537
Fax: 858-485-3788

                -----Original Message-----
                From: Dr. Edward P. Sayre [mailto:esayre@nesa.com]
                Sent: Monday, October 16, 2000 12:16 PM
                To: si-list@silab.eng.sun.com
                Subject: [SI-LIST] : 2.5 Gbps across a backplane

                Folks:

                A subtle issue came up in the recent T11 (Fibre Channel)
meetings. How do
                you spec an interconnect when adaptive equalization parts
will be used to
                dig out the signal? If you have really good eye-diagrams
response then do
                you need equalization? Conversely, if you need equalization
because of
                excessive deterministic jitter (read that insufficient rise
time bandwidth)
                or eye closure (read that too much attenuation) or both,
then what
                constitutes an acceptable way to specify the interconnect,
be it backplane,
                cable or connector?

                Now, the question of cross talk corruption of the signals is
another
                matter, especially when equalization is involved. Cross talk
can be of no
                consequence or it can be of terrible consequence, especially
if there is a
                requirement for byte (a 4 bit nibble) aligned serial data
streams. Our
                work indicates that connector cross talk in differential
connectors appears
                to be essentially dV/dt noise due to any asymmetries in the
geometry of the
                connector or interconnect (vias count here!). Although
small, and
                generally in the (1 - 4%) range, it is obviously a problem
when many lines
                are switching and especially if the time alignment is not
within a small %
                of the rise time between individual differential components.
If the signal
                is broken into its even (common mode) and odd (differential)
mode
                components, it can be shown that very soon the common mode
cross talk
                dominates the cross talk.

                So, when is cross talk an equalization issue from a
differential eye
                diagram point of view; and can equalization help from an EMI
point of view?
                 I look forward to folks' ideas on this matter.

                ed sayre

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