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

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From: Doug (dmckean@gte.net)
Date: Tue Jun 13 2000 - 09:27:09 PDT


Jan Vercammen wrote:
>
< snipped material >

Listen to Larry. I think he's on target with
the signal source.

> 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?

Not really. The emissions are caused by common mode
currents put onto the cable from the device or devices
to which it's connected. That's not necessarily a
signal/cable issue but a connection issue.

The connection issue is when the continuity of the
shield is in someway disrupted between the cable
and chassis ground. Even in the slightest way.
This causes in some cases the cable to act as
a great antenna structure.

> 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?

Yes.

> How do you test for radiated emission?

Basically all ports must be populated. But, the
labs will allow you to do a crossover or loopback
configuration with the cables.

> Does one disable Ethernet during the
> test (cable left in place)?

No. If it's "normal" operation to have
the Ethernet operating, then you must
make attempts to simulate it during test.

Regards, Doug McKean

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