From: Chris Rokusek (email@example.com)
Date: Tue Jun 13 2000 - 09:31:43 PDT
It might also be "fun" to disconnect the signal wires from the STP connector
leaving only the shield connection to help insure that ground/chassis noise
conducted onto the shield is not a problem.
> -----Original Message-----
> From: firstname.lastname@example.org
> [mailto:email@example.com]On Behalf Of Jan Vercammen
> Sent: Tuesday, June 13, 2000 6:07 AM
> To: firstname.lastname@example.org
> Subject: [SI-LIST] : radiated emissions from Ethernel twisted pair
> 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
> 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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