[SI-LIST] Re: Reference ground plane below ethernet differential pairs - good or bad?

The energy in your differential pair is differential -- and couples from
one conductor to the next as you note. As a result, the return current
is close to the transmitting conductor. This gives a small loop area, or
small effective antenna. This can also be characterized by a low
impedance between the paired conductors.

In contrast, noise on both sides of the signal pair is common mode.
Since it is common to both sides, it returns via any coupling path it
can, creating a large loop area and high emissions. Micro-amps of common
mode current can cause you to fail, while milli-amps of differential
current can still pass.

So you need to look at all places where energy could couple to both
sides of your differential pair. If they run over a noisy plane this
will be a problem. If the output (line) side of the transformer is clear
of all other traces and planes it helps. Then energy needs to couple
across the inter-winding capacitance of the transformer to get onto the
line.=20

                                                                Curt

Curt McNamara // senior electrical engineer=20
Logic Product Development
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436.5178
F // 612.672.9489
www.logicpd.com=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Kbalasubramanian@xxxxxxxxxxxxxx
Sent: Tuesday, March 27, 2007 2:55 AM
To: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Reference ground plane below ethernet differential
pairs - good or bad?



Dear Experts,
      EMI radiated emission is failing in our current ethernet product
design .All the ethernet harmonics are appearing in the emission list. I
am told that removing the reference plane below the ethernet
differential pair signals (RX & TX) will solve this issue. For
differential signals reference plane is not required since the return
current flows through the other conductor. But will the presence of
reference plane have negative effect?

Sincerely

K.Balasubramanian
Project Leader - Hardware.

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