[SI-LIST] Ethernet magnetics question
- From: Joel Brown <joel@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 05 Apr 2006 13:59:37 -0700
The following is from an Intel Ethernet controller app note:
*The magnetics module chassis or output ground (secondary side of
transformer) should be
separated from the digital or input ground (primary side) by a physical
separation of 100 mils
minimum. Splitting the ground planes beneath the transformer minimizes
noise coupling between
the primary and secondary sides of the transformer and between the
adjacent coils in the magnetics.
This arrangement also improves the common mode choke functionality of
magnetics module.
Figure 9 illustrates the split plane layout for a discrete magnetics
module. Capacitors are used to
interconnect chassis ground and signal ground.
Figure 10 below shows the preferred method for implementing a ground
split under an integrated
magnetics module/RJ-45 connector. The capacitor stuffing options (C1
through C6) are used to
reduce and filter high frequency emissions. The values of the capacitor
stuffing options may be
different for each board. Experiments will need to be performed to
determine which values provide
best EMI performance.
The placement of C1 through C6 may also be different for each board
design (in other words, not
all of the capacitors may need to be populated). Also, the capacitors
may not be needed on both
sides of the magnetic module.
*I sent this app note to our EMI consultant he said it does not work.
Plane splits cause EMI problems, they not solve them.
Adding capacitors across the plane split effectively creates an RF short
across the plane split which defeats the isolation that is trying to be
achieved in the first place.
Our EMI consultant recommends using a solid continuous plane underneath
the Ethernet magnetics. I have also seen this plane split recommendation
for other types of I/O connectors. Does anybody know the origin of this
idea and if it has any practical validity?
Thanks - Joel
*
*
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