Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!sumax!ole!ray
From: ray@ole.UUCP (Ray Berry)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 386 PCs and FCC Class A and Class B certification.
Summary: no easy solution
Keywords: business use ONLY, RF interference, low cost 386PCs
Message-ID: <665@ole.UUCP>
Date: 17 May 89 17:09:10 GMT
References: <6930@bunny.GTE.COM>
Reply-To: ray@ole.UUCP (Ray Berry)
Distribution: usa
Organization: Seattle Silicon Corporation, Bellevue, WA.
Lines: 36


    Conductive paint is helpful in shielding non-metallic enclosures, but
would be of no value in reducing the EMI from a pc in a steel cabinet. 

    Electromagnetic energy will escape from an enclosure in two ways - 
on the cables entering/leaving the box, and through any openings in the
enclosure, such as drive bays and even the seal between the slide-on
cabinet top and the bottom of the chassis.  There is an aperture effect
such that the minimum frequency energy which will escape is a function
of the size of an opening in the enclosure (e.g., the size of the holes
in a perforated panel).  

   To shield a machine well, you must provide an "rf-tight" box.  It is
not enough to screw the top on at 2-3 tops on the chassis- good electrical
contact must be maintained at every joint with gaps not exceeding an inch
or so.  Any conductors entering and leaving the box must be filtered, or
well shielded.  (Obviously you can't put a 1 Mhz low pass filter on your
video output :-).. 

   The incremental mfg cost of producing such a computer is probably not
that great (<$100) but few companies have the resources to do the requisite
engineering and testing.  Plus, there really isn't any public demand for
it.  There are a few companies who make "Tempest" rated pc's (VERY well
shielded, for the NSA types)- but those machines are priced so that only
the "TAXPAYERS" can afford them...  

   The only "quiet" pc I have seen offered to the public is a unit manu-
factured by Hal Communucations that is intended for use in ham radio in-
stallations.  
trivial, but 


-- 

Ray Berry  KB7HT uucp: ...uw-beaver!sumax!quick!ole!ray     CS 73407,3152
Seattle Silicon Corp. 3075 112th Ave NE. Bellevue WA 98004 (206) 828 4422