Re: EMI Sensitivity of Coax and Twisted Pair wires
- From: glen herrmannsfeldt <gah@xxxxxxxxxxxxxxxx>
- Date: Tue, 27 Jun 2006 16:47:48 -0700
Al Dykes wrote:
Ralf Koenig wrote:
(snip)
I'm looking for a comparison that quantifies the effects of EMI
(electro-magnetic interference) on the following media:
(snip)
The EMI issue began when Ethernet over UTP was really new and I've
never known it to be anything but theoritical. I don't even recall
"CAT3" as a specification when I first put Cabletron hubs in service.
At the time somebody did some practial test, wrapping loops of what we
would call CAT3 cable around zerox machines,arc welders, small power
tools, etc, while running BERT tests. As I recall, the BERT error
level was ZERO. I don't think I've ever heard of EMI-induced errors in
commercial use of UTP. If I were installing a network near an MRI
machine or near an electric arc furnace I'd use fibre, but I'd have
lots of other issues, too.
Well, one is that it depends much on the transformers at each end
of the cable. It is very easy to pick up a common mode (the same
voltage or current on both wires of a pair) signal with UTP cable.
The transformer at each end keeps that signal away from the rest of
the electronics. There has been discussion that UTP ethernet should
work with a 240VAC 60Hz common mode component on the cable. I have thought about testing this, but still haven't done it.
The twisting of the pairs keeps out most external differential mode
signals. Signals with a wavelength shorter than the twist pitch, and
localized so they don't average out over the cable length could get
through. With the usual twist, that might be about 10GHz, and at
fairly high power to couple in enough to interfere with the signal.
Coax grounded at no more than one point is, for similar reasons,
pretty good against common mode signals. If you ground at more than
one point it is fairly easy to get large currents through the shield
(especially at 60Hz) with no compensating current in the center conductor. Ethernet transceivers for coax are electrically isolated
(with a transformer coupled DC to DC converter, and transformers
on the AUI signals).
The physics of the coupling is different for different cable types,
and it depends very much on the isolation (especially transformers)
used. There is also some frequency dependence. For all those reasons,
it can't be given as a property of the cable alone.
-- glen
Even if there were an occasional noise shot, A TCP/IP network would
resend the packet very nicely, thank you.
UTP cable is twisted pairs of wire. One byproduct of that is that any
external EM field that would induce voltage in one wire induces an
identical, opposite voltage in the other wire of the pair. They cancel
out. For perfectly symetrical pairs EMI induces zero net voltage.
As I understand things, UTP wire, in each step from CAT3 to CAT 6 has
improved the exactness of the twists. This makes CAT6 much more
EMI-resistant than CAT3, especially what was in use when the tests I
recall were done.
.
- References:
- EMI Sensitivity of Coax and Twisted Pair wires
- From: Ralf Koenig
- Re: EMI Sensitivity of Coax and Twisted Pair wires
- From: Dmitri(Cabling-Design.com)
- Re: EMI Sensitivity of Coax and Twisted Pair wires
- From: Al Dykes
- EMI Sensitivity of Coax and Twisted Pair wires
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