100BaseT EMI ?
James.P.Lux at jpl.nasa.gov
Wed Mar 27 09:45:09 PST 2002
At a first glance, commercial products will have to meet FCC Part 15
requirements, giving you an upper bound.
There are two issues to worry about:
1) radiation from the ethernet signals themselves...Shielding may not make
all that much difference, inasmuch as the signals themselves are on twisted
pairs and don't radiate all that much to begin with.
2) A bigger issue will be EMI from "inside the PC box" coming out on the
surface of the wires. Shielding might(!) help here, but it is a highly
idiosyncratic thing, and highly "workmanship" dependent. Small changes in
how things are assembled can make a big difference.
What frequencies (ranges, in general) are you concerned about? kHz, MHz,
The shielding strategy, and EMI/EMC techniques, vary quite a bit. Is
liberal application of copper foil tape (while testing) a feasible strategy?
Optical interconnects are readily available, and have zero emissions, and
since they are entirely nonmetallic, can't provide a sneak path for signals
to creep out of the box. I don't know that optical hubs/switches are
available (at a reasonable price...).
As an alternative, designed specifically for low EMI, there is SpaceWire
(IEEE 1355.2) .. point to point connections(with wormhole routers) at up to
400 MBps. Double Shielded cables with careful attention to grounding is
part of the spec. It also uses LVDS signalling (greatly reducing radiated
emission). There are standard interface cards with 3 ports available for
PCI bus. They're a bit pricey today (because they use an ASIC designed for
the space industry), but there might be less expensive versions in the
pipeline (or already available).
At 10:24 AM 3/27/2002 -0500, Huntress Gary B NPRI wrote:
>I am drafting a proposal for a small cluster that will be used in a
>tactical military environment (Naval), thus I am bound by some
>environmental issues, particularly in the area of electromagnetic interference.
>Right now I don't want to address the whole cluster, but is anyone aware
>of EMI issues specifically related to 100 Base T ethernet? My gut feeling
>is that good quality shielded cables will be the first line of defense,
>however I'm concerned about radiated emission levels at the connectors.
>As always, any info is greatly appreciated.
>Naval Undersea Warfare Center
>Newport, RI 02841
>Beowulf mailing list, Beowulf at beowulf.org
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