Re: [SI-LIST] : 100Base-TX board SI issues

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From: jrbarnes@lexmark.com
Date: Mon Nov 27 2000 - 06:44:51 PST


Ellis,
We have done a number of 10BASE-T/100BASE-TX designs in the last four years.
Several items that we have found to be very critical in layout are:
* Minimize the distance from the PHYceiver to the connector-- we have found
that just 1/2 inch increase in the length of
   the traces from the connector (via common-mode-choke and transformer/filter)
to the PHYceiver can reduce the
   maximum length of cable we work with by 30%.
* Keep *everything* away from the Ethernet signals-- cut a hole, in every
layer, that extends from the signal pins of the
   RJ-45 connector to halfway between the rows of pins of the
transformer/filter. Make this hole as wide as the
   transformer/filter and common-mode choke. Do not permit anything except the
Ethernet components and traces in
   this area.
* Try to have only those components and traces directly associated with the
Ethernet signals between the
   transformer/filter and PHYceiver, although we do permit power and ground
planes in this area.
* Run the Ethernet traces parallel from the connector to the
transformer/filter. If they must be twisted to match the
   pinouts, do so between the transformer/filter and the PHYceiver.

For Radiated Emissions, we design our boards with a four-winding common-mode
choke between the transformer/filter. Because of their cost, we prefer not to
use them if we don't have to. So, for our first-pass boards, we either:
* Short the windings with traces on the topside of the card-- which can be
cut, then a common-mode choke populated if
    needed.
        OR
* Place 0603 0-ohm topside resistors in parallel with the windings--- which can
be removed, and the common-mode
   choke populated if needed.
                                   John Barnes Advisory Engineer
                                   Lexmark International

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