Source : Free On-Line Dictionary of Computing

Radio Frequency Interference
     
         (RFI) Electromagnetic radiation which is
        emitted by electrical circuits carrying rapidly changing
        signals, as a by-product of their normal operation, and which
        causes unwanted signals (interference or noise) to be induced
        in other circuits.
     
        The most important means of reducing RFI are: use of bypass or
        "decoupling" {capacitors} on each active device (connected
        across the power supply, as close to the device as possible),
        risetime control of high speed signals using series resistors
        and {VCC filtering}.  Shielding is usually a last resort after
        other techniques have failed because of the added expense of
        RF gaskets and the like.
     
        The efficiency of the radiation is dependant on the height
        above the ground or power plane (at RF one is as good as the
        other) and the length of the conductor in relationship to the
        wavelength of the signal component (fundamental, harmonic or
        transient (overshoot, undershoot or ringing)).  At lower
        frequencies, such as 133 MHz, radiation is almost exclusively
        via I/O cables; RF noise gets onto the power planes and is
        coupled to the line drivers via the VCC and ground pins.  The
        Rf is then coupled to the cable through the line driver as
        common node noise.  Since the noise is common mode, shielding
        has very little effect, even with differential pairs.  The RF
        energy is capacitively coupled from the signal pair to the
        shield and the shield itself does the radiating.
     
        At higher frequencies, usually above 500 Mhz, traces get
        electrically longer and higher above the plane.  Two
        techniques are used at these frequencies: wave shaping with
        series resistors and embedding the traces between the two
        planes.  If all these measures still leave too much RFI,
        sheilding such as RF gaskets and copper tape can be used.
        Most digital equipment is designed with metal, or coated
        plastic, cases.
     
        Switching power supplies can be a source of RFI, but have
        become less of a problem as design techniques have improved.
     
        Most countries have legal requirements that electronic and
        electrical hardware must still work correctly when subjected
        to certain amounts of RFI, and should not emit RFI which could
        interfere with other equipment (such as radios).
     
        See also {Electrostatic Discharge}, {Electromagnetic
        Compatibility}.
     
        (1998-01-26)