Radio Frequency Interference definition
<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)
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