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(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.