297 lines
17 KiB
Plaintext
297 lines
17 KiB
Plaintext
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_____________ _/_/ | | \ \ _/_/ _____________
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| ___________ _/_/ | | \ \ _/_/ ___________ |
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| | _/_/_____ | | > > _/_/_____ | |
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| | /________/ | | / / /________/ | |
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| | c o m m u n i c a t i o n s | |
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| |________________________________________________________________| |
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|____________________________________________________________________|
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...presents... TEMPEST in a Teapot
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by Grady Ward
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06/01/1996-#320
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__///////\ -cDc- CULT OF THE DEAD COW -cDc- /\\\\\\\__
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\\\\\\\/ Everything You Need Since 1986 \///////
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___ _ _ ___ _ _ ___ _ _ ___ _ _ ___
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|___heal_the_sick___raise_the_dead___cleanse_the_lepers___cast_out_demons___|
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TEMPEST is the code name for technology related to limiting unwanted
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electromagnetic emissions from data processing and related equipment. Its goal
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is to limit an opponent's capability to collect information about the internal
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data flow of computer equipment. Most information concerning TEMPEST
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specifications is classified by the United States Government and is not
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available for use by its citizens.
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The reason why TEMPEST technology is particularly important for computers
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and other data processing equipment is the kinds of signals components in a
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computer use to talk to each other ("square waves") and their clock speeds
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(measured in megahertz) produce a particularly rich set of unintentional
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signals in a wide portion of the electromagnetic spectrum. Because the
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spurious emissions occupy so wide a portion of that spectrum, technologies used
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to block one portion of the spectrum (as pulling the shades closed on a window
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to stop the visible light portion) are not necessarily effective in another
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portion.
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Unintentional emissions from a computer system can be captured and
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processed to reveal information about the target systems from simple levels of
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activity to even remotely copying keystrokes or capturing monitor information.
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It is speculated that poorly protected systems can be effectively monitored up
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to the order of one kilometer from the target equipment.
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This note will examine some practical aspects of reducing the
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susceptibility of your personal computer equipment to remote monitoring using
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easily-installed, widely available after-market components.
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_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
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One way of looking at TEMPEST from the lay person's point-of-view is that
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it is virtually identical to the problem of preventing electromagnetic
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interference ("EMI") by your computer system to others' radios, televisions, or
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other consumer electronics. That is, preventing the emission of wide-band
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radio "hash" from your computers, cabling, and peripherals both prevents
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interference to you and your neighbors television set and limits the useful
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signal available to a person surreptitiously monitoring.
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Viewing the problem in this light, there are quite a few useful documents
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available from the government and elsewhere attacking this problem and
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providing a wealth of practical solutions and resources. Very useful for the
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lay person are:
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Radio Frequency Interference: How to Find It and Fix It.
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Ed Hare, KA1CV, and Robert Schetgen, KU7G, editors
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The American Radio Relay League
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Newington, CT
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ISBN 0-87259-375-4
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1991
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Federal Communications Commission Interference Handbook
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FCC Consumers Assistance Branch
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Gettysburg, PA 17326
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717-337-1212
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1991
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and
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MIL-STD-188-124B in preparation
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(includes information on military shielding of tactical communications systems)
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Superintendent of Documents
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US Government Printing Office
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Washington, DC 20402
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202-783-3238
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Information on shielding a particular piece of consumer electronic
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equipment may be available from the:
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Electronic Industries Association (EIA)
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2001 Pennsylvania Ave NW
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Washington, DC 20006
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Preventing unintended electromagnetic emissions is a relative term. It is
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not feasible to reduce to zero all unintended emissions. My personal goal, for
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example, might be to reduce the amount and quality of spurious emission until
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the monitoring van a kilometer away would have to be in my front yard before it
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could effectively eavesdrop on my computer. Apartment dwellers with unknown
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neighbors only inches away (through a wall) might want to even more carefully
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adopt as many of the following suggestions as possible since signal available
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for detection decreases at approximately the inverse square of the distance
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from the monitoring equipment to your computer.
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Start with computer equipment that meets modern standards for emission.
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In the United States, the "quietest" standard for computers and peripherals is
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known as the "class B" level. Class A level is a less stringent standard for
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computers to be use in a business environment.
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You want to verify that all computers and peripherals you use meet the
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class B standard which permits only one-tenth the power of spurious emissions
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than the class A standard. If you already own computer equipment with an FCC
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ID, you can find out which standard applies. Contact the FCC Consumers
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Assistance Branch at 717-337-1212 for details on accessing their database.
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Once you own good equipment, follow the manufacturer's recommendations for
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preserving the shielding integrity of the system. Don't operate the system
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with the cover off and keep "slot covers" in the back of the computer in place.
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Use only shielded cable for all system interconnections. A shielded cable
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surrounds the core of control wires with a metal braid or foil to keep signals
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confined to that core. In the late seventies it was common to use unshielded
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cable such as "ribbon" cable to connect the computer with, say, a disk drive.
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Unshielded cable acts just like an antenna for signals generated by your
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computer and peripherals. Most computer manufacturers supply shielded cable
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for use with their computers in order to meet FCC standards. Cables bought
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from third-parties are an unknown and should be avoided (unless you are willing
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to take one apart to see for yourself!).
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Try to avoid a "rat's nest" of wire and cabling behind your equipment by
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keeping all cables as short as possible. You want to reduce the length of
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unintended antennas and more easily predict the likely paths of electric and
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magnetic coupling from cable to cable so that it can be better filtered.
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Block radiation from the power cord(s) into the house wiring. Most
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computers have an EMI filter built into their body where the AC line cord
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enters the power supply. This filter is generally insufficient to prevent
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substantial re-radiation of EMI voltages back into the power wiring of your
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house and neighborhood. To reduce the power retransmitted down the AC power
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cords of your equipment, plug them in to special EMI filters that are in turn
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plugged into the wall socket. AC and other filters mentioned in this note are
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available from a wide variety of sources including, for example, Radio Shack.
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Some filters reduce retransmission of EMI by a factor of at least 1000 in the
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high-frequency range. Although ideally, every computer component using an AC
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line cord ought to be filtered, it is especially important for the monitor and
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computer CPU to be filtered in this manner as the most useful information
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available to opponents usually comes from these sources.
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Block retransmitted information from entering your fax/modem or telephone
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line. Telephone line is generally very poorly shielded. EMI from your
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computer can be retransmitted directly into the phone line through your modem
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or can be unintentionally picked up by the magnetic portion of the EMI spectrum
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through magnetic induction from power supplies or the yoke of your cathode ray
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tube "CRT" monitor.
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To prevent direct retransmission, EMI filters are specifically designed
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for modular telephone jacks to mount at the telephone or modem, and for
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mounting directly at the service entrance to the house. Your phone company or
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telephone manufacturer may be able to supply you with free modular filters,
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although the design frequencies of these filters may not be high enough to be
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effective through much of the EMI spectrum of interest. Keep telephone lines
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away from power supplies of computers or peripherals and the rear of CRTs: the
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magnetic field often associated with those device can inductively transfer to
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unshielded lines just as if the telephone line were directly electrically
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connected to them. Since this kind of coupling decreases rapidly with
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distance, this kind of magnetic induction can be virtually eliminated by
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keeping as much distance (several feet or more) as possible between the power
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supply/monitor yoke and cabling.
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Use ferrite toroids and split beads to prevent EMI from escaping on the
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surface of your cables. Ferrites are magnetic materials that, for certain
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ranges of EMI frequencies, attenuate the EMI by causing it to spend itself in
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heat in the material rather than continuing down the cable. They can be
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applied without cutting the cable by snapping together a "split bead" form over
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a thick cable such as a power cord or by threading thinner cable such as
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telephone several times around the donut-shaped ferrite form. Every cable
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leaving your monitor, computer, mouse, keyboard, and other computer peripherals
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should have at least one ferrite core attenuator. Don't forget the telephone
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lines from your fax, modem, telephone or the unshielded DC power cord to your
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modem. Ferrites are applied as close to the EMI emitting device as possible so
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as to afford the least amount of cable that can act as an antenna for the EMI.
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Other remedies that are somewhat more difficult to correctly apply include
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providing a good EMI "ground" shield for your computer equipment and other more
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intrusive filters such as bypass capacitor filters.
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You probably ought not to think about adding bypass capacitors unless you
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are familiar with electronic circuits and digital design. While quite
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effective, added improperly to the motherboard or cabling of a computer they
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can "smooth out" the square wave digital waveform- perhaps to the extent that
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signals are interpreted erroneously causing mysterious "crashes" of your
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system. In other cases, bypass capacitors can cause unwanted parasitic
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oscillation on the transistorized output drivers of certain circuits which
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could damage or destroy those circuits in the computer or peripherals. Also,
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unlike ferrite toroids, adding capacitors requires actually physically splicing
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them in or soldering them into circuits. This opens up the possibility of
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electric shock, damage to other electronic components or voiding the warranty
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on the computer equipment.
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A good EMI ground is difficult to achieve. Unlike an electrical safety
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ground, such as the third wire in a three-wire AC power system, the EMI ground
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must operate effectively over a much wider part of the EMI spectrum. This
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effectiveness is related to a quality known as electrical impedance. You want
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to reduce the impedance to as low a value as possible over the entire range of
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EMI frequencies.
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Unlike the AC safety ground, important factors in achieving low impedance
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include having as short a lead from the equipment to a good EMI earth ground as
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possible (must be just a few feet); the gauge of the connecting lead (the best
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EMI ground lead is not wire but woven grounding "strap" or wide copper flashing
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sheets; and the physical coupling of the EMI into the actual earth ground. An
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8 ft. copper-plated ground may be fine for AC safety ground, but may present
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appreciable impedance resistance to an EMI voltage. Much better would be to
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connect a network of six to eight copper pipes arranged in a six-foot diameter
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circle driven in a foot or two into the ground, electrically bonded together
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with heavy ground strap and connected to the equipment to be grounded via a
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short (at most, several feet), heavy (at least 3/4-1" wide) ground strap.
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If you can achieve a good EMI ground, then further shielding possibilities
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open up for you such as surrounding your monitor and computer equipment in a
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wire-screen Faraday cage. You want to use mesh rather than solid sheet because
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you must preserve the free flow of cooling air to your equipment. Buy aluminum
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(not nylon) screen netting at your local hardware store. This netting
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typically comes in rolls 36" wide by several feet long. Completely surround
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your equipment you want to reduce the EMI being careful to make good electrical
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bonds between the different panels of netting and your good earth ground. I
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use stainless steel nuts, bolts, and lock washers along with special
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non-oxidizing electrical paste (available from electrical contractors supply
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houses) to secure my ground strapping to my net "cages". A good Faraday cage
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will add several orders of magnitude of EMI attenuation to your system.
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It is easy to get a general feeling about the effectiveness of your EMI
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shielding work with an ordinary portable AM radio. Bring it very close to the
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body of your computer and its cables in turn. Ideally, you should not hear an
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increased level of static. If you do hear relatively more at one cable than at
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another, apply more ferrite split beads or obtain better shielded cable for
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this component. The practice of determining what kind of operating system code
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is executing by listening to a nearby AM radio is definitely obsolete for an
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well-shielded EMI-proof system!
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To get an idea of the power and scope of your magnetic field emissions, an
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ordinary compass is quite sensitive in detecting fields. Bring a compass
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within a few inches of the back of your monitor and see whether it is
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deflected. Notice that the amount of deflection decreases rapidly with
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distance. You want to keep cables away from magnetic sources about as far as
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required not to see an appreciable deflection on the compass.
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If you start with good, shielded equipment that has passed the FCC level B
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emission standard then you are off to a great start. You may even be able to
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do better with stock OEM equipment by specifying "low-emission" monitors that
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have recently come on the market in response to consumer fears of extremely low
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frequency ("ELF") and other electromagnetic radiation. Consistently use
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shielded cables, and apply filtering and ferrite toroids to all cabling
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entering or leaving your computer equipment. Finally, consider a good EMI
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ground and Faraday cages. Beyond this there are even more effective means of
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confining the electrical and magnetic components of your system through the use
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of copper foil adhesive tapes, conductive paint sprays, "mu metal" and other
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less common components. The details of these I leave for you to discover.
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.-. _ _ .-.
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/ \ .-. ((___)) .-. / \
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/ \ / \ .-. [ x x ] .-. / \ / \
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-/-------\-------/-----\-----/---\--\ /--/---\-----/-----\-------/-------\-
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/ \ / \ / `-(' ')-' \ / \ / \
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WORLDWIDE \ / `-' (U) `-' \ / WORLDWIDE
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`-' .ooM `-' _
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Oooo / ) __
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/)(\ ( \ Copyright (c)1996 Grady Ward and Cult of the Dead Cow / (/ \
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\__/ ) / All rights reserved. Award-winning CULT OF THE DEAD COW \ ) \)(/
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(_/ is published by cDc communications, P.O. Box 53011, oooO _
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oooO Lubbock, TX, 79453, US of A. Edited by Swamp Ratte'. __ ( \
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/ ) /)(\ / \ ) \
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\ ( \__/ Save yourself! Go outside! Do something! \)(/ ( /
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\_) "THE COW WALKS AMONGST US" Oooo
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