Radio Frequency Interference

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, ElectromagneticCompatibility.

(1998-01-26)