RadioBanter - View Single Post - Ferrite noise supressors on sale at Parts Express

March 10th 05, 11:00 PM

On 10/03/2005 3:36 AM, wrote:
I get those clamp on ferrites for a dollar or two at the electronics
fleamarket.

Has anyone ever figured out if it is kosher to put a ferrite on an
ethernet cable. Those cables are real noise sources. I guess they don't
shield them to be cheap or to keep the capacitance low.

Ethernet is actually a number of twisted-pairs. The most common format
is CAT-5E which minimizes most crosstalk (especially near-end variety.)
I think this will almost guarantee some harmonics being radiated.

However, UTP (unshielded twisted pair) cables can radiate *less* than
shielded TP cables. The shield can act as an antenna. There is
industrial stuff wrapped in aluminium that works more like coax or
ladder-line.

How much it radiates is based on frequency, and CAT5E is rated to
350MHz, I think. EMI is actually a problem for these cables. It is
recommended to not run stranded, unshielded UTP cable within 12in of an
EMI source. At any rate, running data through 100baseTx apparently does
not have a real noticeable spike in the noise -- the noise is spread out
quite a bit.

And it's hard to filter EMI on ethernet. A ferrite at each connector
end will not be as useful as on a 60Hz power line. Ethernet is
something like 1v nicely round waveforms. I'm not sure there is even a
line filter available for the frequencies that 100Base-T is expected to
run at.

There are supposed to be filters and/or chokes on specific leads in
switches and NICs, but I'm guessing that they may leave a lot to be
desired. This will only minimize noise coupled to the +DC and GND lines
from the power supply, I suppose.

The rules of thumb I've read a

- keep your runs short as possible
- keep your turns no less than 10x the diameter of the cable
- get or make cables with really good connectors. You don't want any
stretched or untwisted ends -- this is where a lot of the noise is.
- kinks crimps or stretch cables will generate a lot more EMI
- If you have a switch or hub of some sort, but it in a metal enclosure

--
clvrmnky

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