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 dependent 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)