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

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Tue Jun 13 2000 - 06:54:45 PDT


All your symptoms seem to point to you having a 100 Mb/s link-- the 30 and
55 MHz bumps, the fact that you get significant emissions even when you are
not sending data (100TX sends a scrambled idle stream as noisy as data,
10-BASE-T only sends little link test pulses at 60/sec).

The difference in emission levels between UTP and STP sounds pretty right,
too. UTP is, after all, UNshielded. And, yes we have a hard time meeting
Class B. UTP walks the edge.

So why do people use UTP?

The answer will be found when you go to buy a significant amount of cable.
(Hint: $$$$$$$)

Larry Miller

> -----Original Message-----
> From: jvercamm@roam.agfa.be [SMTP:jvercamm@roam.agfa.be]
> Sent: Tuesday, June 13, 2000 6:07 AM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : radiated emissions from Ethernel twisted pair
>
> hello,
>
> 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
> printer.
>
> 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.
>
> Regards,
>
> Jan Vercammen
> EMC/PCB Engineering
> Agfa-Gevaert NV, Belgium
>
>
>
>
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