radio frequency interference

1 definition found

radio frequency interference - Free On-line Dictionary of Computing (26 May 2007) :

  Radio Frequency Interference
  RFI
  
     <hardware, testing> (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)