From: Dr. Edward P. Sayre ([email protected])
Date: Mon Oct 16 2000 - 12:15:50 PDT
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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