Derek,
the reason for the observed effects is simple:
-1- when using a (foil?) shielded cable the shield of the cable is acting as
an antenna. The RF noise
is injected by coupling of internal circuits to the connector shield the RJ45
connector which is connected
to the cable shield
-2- when using an unshielded cable the cable shield is missing and the RF
currents cannot propagate outside
the equipment enclosure
You have two alternatives:
-1- connect the RJ45 connector shielding to the chassis directly
-2- or use AC-coupling to connect the connector shielding to the chassis
Personally we use direct coupling in our equipment. I have verified that with
foil-shielded Ethernet
and RJ45 connector connected directly to the chassis (using the fingers
provided) on the connector
improves the conducted and radiated immunity by a factor of 2 compared to
unshielded cable.
I have not been able to find a difference between radiated EMI of unshielded
and shielded cable.
Kind regards,
Jan Vercammen
"Derek Walton" <[email protected]>
Sent by: [email protected]
04/10/2006 08:38
Please respond to lfresearch
To: "IEEE EMC Discussion Group" <[email protected]>
cc: (bcc: Jan Vercammen/AMEMV/AGFA)
Subject: Ethernet emissions
HI All,
I have a system on test that has a single very strong emission at about 125
MHz. Is this an Ethernet related frequency?
It appears to jump 20 dB when the receive end, outside the chamber, is plugged
in. Also, with unshielded cables it's not there. With shielded its about 30 dB
up at the same frequency.
Anyone have suggestions as to the source?
Thanks,
Derek Walton
L F Research
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