100BaseT EMI ?

Jim Lux 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, 
100 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.
>Gary Huntress
>Code 4113
>Naval Undersea Warfare Center
>Newport, RI  02841
>1-800-669-6892 x28990
>Beowulf mailing list, Beowulf at beowulf.org
>To change your subscription (digest mode or unsubscribe) visit 

Jim Lux
Spacecraft Telecommunications Equipment Section
Jet Propulsion Laboratory
4800 Oak Grove Road, Mail Stop 161-213
Pasadena CA 91109

818/354-2075, fax 818/393-6875

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