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

Hi Bala,
  On the line side of the isolation transformer on a PCB you do not need a 
reference plane for Ethernet differential signals. Otherwise, noise on the 
planes could couple to the differential pair and could cause radiated 
emissions. Since Ethernet cables are generally unshielded and are typically 
very long they act as very good antennas. So, removing the reference planes 
(all planes) on the PCB on the line side of the transformer is a good idea. 
Since reference plane is needed on the chip side of the transformer for the 
terminations and ESD capacitors the planes are usually removed at the mid point 
of the transformer package.
   
  Thanks,
  Ram Chundru 
Kbalasubramanian@xxxxxxxxxxxxxx wrote:
  
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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Ram Chundru
  System Level ESD Engineer
  Texas Instruments Inc.
  12500 TI Blvd, MS 8734
  Dallas, TX 75243
  Phone: 214-480-1654
  www.emc-engineer.com
   

 
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