Radio Frequency Interference

(hardware, testing)(RFI) Electromagnetic radiation which isemitted by electrical circuits carrying rapidly changingsignals, as a by-product of their normal operation, and whichcauses unwanted signals (interference or noise) to be inducedin other circuits.

The most important means of reducing RFI are: use of bypass or"decoupling" capacitors on each active device (connectedacross the power supply, as close to the device as possible),risetime control of high speed signals using series resistorsand VCC filtering. Shielding is usually a last resort afterother techniques have failed because of the added expense ofRF gaskets and the like.

The efficiency of the radiation is dependant on the heightabove the ground or power plane (at RF one is as good as theother) and the length of the conductor in relationship to thewavelength of the signal component (fundamental, harmonic ortransient (overshoot, undershoot or ringing)). At lowerfrequencies, such as 133 MHz, radiation is almost exclusivelyvia I/O cables; RF noise gets onto the power planes and iscoupled to the line drivers via the VCC and ground pins. TheRf is then coupled to the cable through the line driver ascommon node noise. Since the noise is common mode, shieldinghas very little effect, even with differential pairs. The RFenergy is capacitively coupled from the signal pair to theshield and the shield itself does the radiating.

At higher frequencies, usually above 500 Mhz, traces getelectrically longer and higher above the plane. Twotechniques are used at these frequencies: wave shaping withseries resistors and embedding the traces between the twoplanes. 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 coatedplastic, cases.

Switching power supplies can be a source of RFI, but havebecome less of a problem as design techniques have improved.

Most countries have legal requirements that electronic andelectrical hardware must still work correctly when subjectedto certain amounts of RFI, and should not emit RFI which couldinterfere with other equipment (such as radios).

See also Electrostatic Discharge, Electromagnetic Compatibility.

单词	radio frequency interference
释义	DictionarySeeelectromagnetic interference Radio Frequency Interference Radio Frequency Interference (hardware, testing)(RFI) Electromagnetic radiation which isemitted by electrical circuits carrying rapidly changingsignals, as a by-product of their normal operation, and whichcauses unwanted signals (interference or noise) to be inducedin other circuits. The most important means of reducing RFI are: use of bypass or"decoupling" capacitors on each active device (connectedacross the power supply, as close to the device as possible),risetime control of high speed signals using series resistorsand VCC filtering. Shielding is usually a last resort afterother techniques have failed because of the added expense ofRF gaskets and the like. The efficiency of the radiation is dependant on the heightabove the ground or power plane (at RF one is as good as theother) and the length of the conductor in relationship to thewavelength of the signal component (fundamental, harmonic ortransient (overshoot, undershoot or ringing)). At lowerfrequencies, such as 133 MHz, radiation is almost exclusivelyvia I/O cables; RF noise gets onto the power planes and iscoupled to the line drivers via the VCC and ground pins. TheRf is then coupled to the cable through the line driver ascommon node noise. Since the noise is common mode, shieldinghas very little effect, even with differential pairs. The RFenergy is capacitively coupled from the signal pair to theshield and the shield itself does the radiating. At higher frequencies, usually above 500 Mhz, traces getelectrically longer and higher above the plane. Twotechniques are used at these frequencies: wave shaping withseries resistors and embedding the traces between the twoplanes. 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 coatedplastic, cases. Switching power supplies can be a source of RFI, but havebecome less of a problem as design techniques have improved. Most countries have legal requirements that electronic andelectrical hardware must still work correctly when subjectedto certain amounts of RFI, and should not emit RFI which couldinterfere with other equipment (such as radios). See also Electrostatic Discharge, Electromagnetic Compatibility. AcronymsSeeRFI
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