From: Larry Miller ([email protected])
Date: Mon Oct 16 2000 - 13:36:22 PDT
There are two things related to this that were argued over and settled in
the 100 Mb/s Ethernet (100BASE-TX) standards:
(Yes, that sounds very slow today, but the principles might still hold)
1) Equalization should be done at the receiver end of a link, as opposed to
pre-emphasis at the transmitter. This allows the equalizer to adjust to the
crumminess of the signal after the damage done by the link and also
minimizes EMI. If you don't need EQ (short cable in this case) then you
don't use it. If you have a long cable you need it. Obviously, this puts
extra burden on the noise floor of the receiver. The receiver adapts by
looking at the energy content at a high frequency or by inferring the
impulse response of the link. DSP receivers have gotten very good at this
kind of thing.
2) For differential signals rise and fall time symmetry must be closely
controlled. For 100 Mb/s, the maximum allowed difference is 0.5 ns (out of
an 8 ns cell) and most modern transmitters are really a lot better (0.14 ns
or so). This is essential for minimum crosstalk and again minimum EMI.
Transmitter-end equalization, especially at higher speeds, aggravates the
items you describe. Another thing is that you are limited in the amount of
pre-emphasis you can apply by the amount of driving power available.
100BASE-TX typically has to deal with 25 dB of high frequency loss compared
to low frequencies. With 2-volt nominal signals this would be an impossible
amount of pre-emphasis or pre-distortion.
Is this the kind of thing you are referring to?
> -----Original Message-----
> From: Dr. Edward P. Sayre [SMTP:[email protected]]
> Sent: Monday, October 16, 2000 12:16 PM
> To: [email protected]
> Subject: [SI-LIST] : 2.5 Gbps across a backplane
> 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
> or eye closure (read that too much attenuation) or both, then what
> constitutes an acceptable way to specify the interconnect, be it
> 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
> to be essentially dV/dt noise due to any asymmetries in the geometry of
> 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
> I look forward to folks' ideas on this matter.
> ed sayre
> | NORTH EAST SYSTEMS ASSOCIATES, INC. |
> | ------------------------------------- |
> | "High Performance Engineering & Design" |
> | Dr. Ed Sayre e-mail: [email protected] |
> | NESA, Inc. http://www.nesa.com/ |
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> | Stow, MA 01775 USA Fax +1.978.897-5359 |
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