100BaseT EMI ?

Velocet math at velocet.ca
Wed Mar 27 11:01:12 PST 2002

On Wed, Mar 27, 2002 at 09:45:09AM -0800, Jim Lux's all...
> 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...)

There's 100 Base-Fx fibre cards out there IIRC - priced them out at one point,
looking for solutions to longer distance interconnects between buildings
without needing repeaters, worrying about ground loops, differing ground
levels and other electrically related problems. I had no worries of EMI, but
it fits the bill here as well.  Some people may be dumping 100baseFX gear as
they move to fibre GBE, or you can always source it new if budget is less of a

If you have a real concern with EMI from the entire cluster (and without
really good cases, CPUs/mainboards can leak alot more EMI at certain freqs
than ethernet), then you could put the whole thing inside a milspec
faraday cage if you really needed 0 emissions. :)


> 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:
> >Greetings,
> >
> >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.
> >
> >
> >Regards,
> >
> >Gary Huntress
> >Code 4113
> >Naval Undersea Warfare Center
> >Newport, RI  02841
> >1-800-669-6892 x28990
> >
> >_______________________________________________
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> 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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Ken Chase, math at velocet.ca  *  Velocet Communications Inc.  *  Toronto, CANADA 

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