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Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a (usually undesirable) disturbance caused in a radio receiver or other electrical circuit by electromagnetic radiation emitted from an external source.  The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.
EMI can be induced intentionally for radio jamming, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent.
Additional recommended knowledge
EMI or RFI may be broadly categorized into two types; narrowband and broadband.
Narrowband interference usually arises from intentional transmissions such as radio and TV stations, pager transmitters, cell phones, etc. Broadband interference usually comes from incidental radio frequency emitters. These include electric power transmission lines, electric motors, thermostats, bug zappers, etc. Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resembles that of synchrotron sources, stronger at low frequencies and diminishing at higher frequencies, though this noise is often modulated, or varied, by the creating device in some way. Included in this category are computers and other digital equipment as well as televisions. The rich harmonic content of these devices means that they can interfere over a very broad spectrum. Characteristic of broadband RFI is an inability to filter it effectively once it has entered the receiver chain.  
Power line noise
Virtually all power-line noise, originating from utility company equipment, is caused by a spark or arcing across some power-line related hardware. A breakdown and ionization of air occurs, and current flows between two conductors in a gap. The gap may be caused by broken or loose hardware such as a cracked insulator. Typical culprits include insufficient and inadequate hardware spacing such as a gap between a ground wire and a staple. Once an ionized path is established in the gap, current flows at all parts of the cycle where the voltage is higher than the breakdown voltage of the gap. This typically occurs only at the positive and negative voltage peaks -- the times of highest instantaneous voltage throughout the cycle.
As an example for a 60Hz system (i.e.power-lines carrying 60 Hz AC, such as in the US), the voltage on them passes through two peaks each cycle (one positive and one negative) and pass through zero twice each cycle. This gives 120 peaks and 120 zero crossings in each second (50Hz: 100 peaks and crossings correspondingly). Power-line noise follows this pattern, generally occurring in bursts at a rate of 120 bursts per second. This gives power-line noise a characteristic sound that is often described as a harsh and raspy hum or buzz. Because the peaks occur twice per cycle, true power-line noise has a strong 120-Hz modulation on the signal (50Hz system: 100Hz).
On integrated circuits, the most important means of reducing EMI are: the 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 depends 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 relation 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-mode 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. One cure for this is to use a braid-breaker or choke to reduce the common-mode signal.
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 EMI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or conductive-coated plastic, cases.
Switching power supplies can be a source of EMI, but have become less of a problem as design techniques have improved.
Most countries have legal requirements that mandates electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and should not emit EMI which could interfere with other equipment (such as radios).
Susceptibilities of different radio technologies
Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulation, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas used with broadcast systems. Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi, error-correction techniques can be used. Spread-spectrum and frequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly directional receiver, such as a parabolic antenna or a diversity receiver, can be used to select one signal in space to the exclusion of others.
The most extreme example of digital spread-spectrum signalling to date is ultra-wideband (UWB), which proposes the use of large sections of the radio spectrum at low amplitudes to transmit high-bandwidth digital data. UWB, if used exclusively, would enable very efficient use of the spectrum, but users of non-UWB technology are not yet prepared to share the spectrum with the new system because of the interference it would cause to their receivers. The regulatory implications of UWB are discussed in the Ultra-wideband article.
Interference to consumer devices
Complex electronic circuitry is found in all sorts of devices used in the home. This results in a vast interference potential that didn't exist in earlier, simpler decades. In the US, Public Law 97-259, enacted in 1982, gave the FCC the authority to regulate the susceptibility of consumer electronic equipment sold in the United States. The FCC, working with equipment manufacturers, decided to allow them to develop standards for EMI immunity and implement their own voluntary compliance programs.
Broadcast transmitters, two-way radio transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI. Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blankets, ultrasonic pest controls (bug zappers), heating pads, and touch controlled lamps.
The Special International Committee on Radio Interference (CISPR) sets standards for radiated and conducted electromagnetic interference.
Categories: Medical imaging | Radiography
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Electromagnetic_interference". A list of authors is available in Wikipedia.|