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RJ-45 common mode radiated emissions testing

Title: RE: RJ-45 common mode radiated emissions testing
It is my experience that all you can do is to state in the standard that it must pass the agency requirements. It will be up the equipment maker, Cisco etc, to provide a SYSTEM that includes TESTED subsections that when used together will comprise an EMI approved solution.
 
Every system is different, and like computer systems, just changing the placement of cables during and EMI test can either pass or fail it. The system must also be periodically tested because production variances in components can cause new product to be non-compliant.
 
It is very east to state "this power supply will pass FCC Class B" in a stand alone environment. As a power supply designer I would not stick my neck out to say it would pass when enstalled in the end system.
 
The power supply, even though it may not be the source of emissions, can be a conduit for system noise. In this case the power supply at the source may allow the LAN server to pass its internal noise through it.  Most of my work at Compaq was trying to contain the digital noise from going through the power supply from the system.
 
Power supplies typically have a strong CONDUCTED component at the switching fundamental frequency (100-500khz) and subsequent harmonics that show up BELOW 30mHz.  Broadband energy caused by fast dV/dT in the primary are sometimes conducted out the power cord or output cables, ABOVE 30mHz, but show up as radiated data. The commutation of diodes in the supply can also cause ringing above 30mHz that is conducted out the power cord and show up as radiated.
 
In the 802.3 case, the emissions are being conducted out the CAT5 cable then radiated out. The LAN server should have sufficient metal shielding for the logic to prevent the supply itself from posing a problem.
 
It must be treated as a SYSTEM SOLUTION, not just a power supply problem.
 
EMI is black majic.
 

Ed Walker
Technical Staff
Analog Product Specialist


Texas Instruments Incorporated 
HC66 Box 203 
Mountainair, NM 87036

INTERNET: ed_walker@xxxxxx
WEB SITE: http://www.ti.com
Office = 505-847-0576 

 
-----Original Message-----
From: Rick Brooks [mailto:ribrooks@xxxxxxxxxxxxxxxxxx]
Sent: Monday, September 18, 2000 4:05 PM
To: Jack Andresen
Cc: stds-802-3-pwrviamdi@xxxxxxxx
Subject: RE: RJ-45 common mode radiated emissions testing

Jack,
I was trying for the worst case for common mode radiation.
There were not any differential mode signals generated, except by mode conversion.
The results are ball park in any case.

What you ask for is possible, but not what I was trying to do.
I wanted to add one data point for what common mode noise could be tolerated from the PSE power supply,
given Roger's conclusion that radiated emission requirements are the more restrictive limit for power supply noise.
A conclusion that I agree, by the way, with for frequencies of 30 MHz and higher.

I did this EMC testing as a freebie, I don't have a plan to redo this EMC testing.
If I do any testing in the future, I will keep your request in mind.

thanks,
- Rick