[SI-LIST] : radiated emissions from Ethernel twisted pair

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From: Jan Vercammen (jvercamm@roam.agfa.be)
Date: Tue Jun 13 2000 - 06:07:04 PDT


I have a somewhat EMC oriented question. I hope this does not interfere with the more
usual signal integrity stuff.

I have done radiated emission measurements and conducted immunity testing on one of our
printers. The printer is connected to Ethernet, which is the main point of my questions.

I have tested the Ethernet interface with cat. 5 shielded (STP) and cat.5 unshielded twisted
pair (UTP) running at 10Mbit/s or 10baseT (though I can not exclude 100baseT because
the printer Etherent hardware features auto-negotiation). The printer is in an anechoic room
were it communicates with a labtop outside. At the feedthrough point their is a big ferrite
absorbing clamp. The cable length inside the shielded room is 5m. We use the
ping command to transmit small packets between the printer and the laptop.

Neglecting the printer emissions we found that there are emissions due to the Ethernet
cable itself. The emissions disappear if we diconnect or shutdown the external laptop.
The emissions are not due to the external laptop, which we checked by powering down the

For our setup the Etnernet radiated emissions are concentrated between 30 and 55MHz, with two
characteristic humps at about 35MHz and 43MHz.

The STP cable performs about 6 to 8dB better than the UTP, that is, the radiation is
lower with the STP. We also performed RF conducted immunity test on the Ethernet cable.
The result is similar: the STP cable is about 2 to 3 times less susceptible.
The UTP cable has about 3-4dB margin with class A CISPR 22, the STP cable more than 8dB
for radiated emissions.

These are the facts. The question is the following: if the emission is due to the
Ethernet cable and we have not made any errors with regard to the layout (of the PCB
with the Ethernet circuits) then can we conclude that we are measuring the performance
of the twisted pair cable and the Ethernet signaling? What I find disturbing is the low
margin (or no margin at all) with respect to the class B limit. Does anyone had similar
experiences? How do you test for radiated emission? Does one disable Ethernet during the
test (cable left in place)?

Thanks in advance.


Jan Vercammen
EMC/PCB Engineering
Agfa-Gevaert NV, Belgium


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