Electromagnetic interference (EMI), also calledradio-frequency interference (RFI) when in theradio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit byelectromagnetic induction,electrostatic coupling, or conduction.[2] The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data.[3]
Both human-made and natural sources generate changing electrical currents and voltages that can cause EMI:ignition systems,cellular network of mobile phones,lightning,solar flares, andauroras (northern/southern lights).[citation needed] EMI frequently affectsAM radios. It can also affectmobile phones,FM radios, andtelevisions, as well as observations forradio astronomy andatmospheric science.
EMI can be used intentionally forradio jamming, as inelectronic warfare.
Since the earliest days of radio communications, the negative effects of interference from both intentional and unintentional transmissions have been felt and the need to manage the radio frequency spectrum became apparent.[4]
In 1933, a meeting of theInternational Electrotechnical Commission (IEC) in Paris recommended the International Special Committee on Radio Interference (CISPR) be set up to deal with the emerging problem of EMI. CISPR subsequently produced technical publications covering measurement and test techniques and recommended emission and immunity limits. These have evolved over the decades and form the basis of much of the world'sEMC regulations today.[5]
In 1979, legal limits were imposed on electromagnetic emissions from all digital equipment by theFCC in the US in response to the increased number of digital systems that were interfering with wired and radio communications. Test methods and limits were based on CISPR publications, although similar limits were already enforced in parts of Europe.[6]
In the mid 1980s, the European Union member states adopted a number of "new approach" directives with the intention of standardizing technical requirements for products so that they do not become a barrier to trade within the EC. One of these was the EMC Directive (89/336/EC)[7] and it applies to all equipment placed on the market or taken into service. Its scope covers all apparatus "liable to cause electromagnetic disturbance or the performance of which is liable to be affected by such disturbance".[6]
This was the first time there was a legal requirement on immunity, as well as emissions on apparatus intended for the general population. Although there may be additional costs involved for some products to give them a known level of immunity, it increases their perceived quality as they are able to co-exist with apparatus in the active EM environment of modern times and with fewer problems.[6]
Many countries now have similar requirements for products to meet some level ofelectromagnetic compatibility (EMC) regulation.[6]
Electromagnetic interference divides into several categories according to the source and signal characteristics.
The origin of interference, often called "noise" in this context, can be human-made (artificial) or natural.
Continuous, or continuous wave (CW), interference arises where the source continuously emits at a given range of frequencies. This type is naturally divided into sub-categories according to frequency range, and as a whole is sometimes referred to as "DC to daylight". One common classification is into narrowband and broadband, according to the spread of the frequency range.
Anelectromagnetic pulse (EMP), sometimes called atransient disturbance, arises where the source emits a short-duration pulse of energy. The energy is usually broadband by nature, although it often excites a relatively narrow-banddamped sine wave response in the victim.
Sources divide broadly into isolated and repetitive events.
Sources of isolated EMP events include:
Sources of repetitive EMP events, sometimes as regularpulse trains, include:
Conducted electromagnetic interference is caused by the physical contact of the conductors as opposed to radiated EMI, which is caused by induction (without physical contact of the conductors). Electromagnetic disturbances in the EM field of a conductor will no longer be confined to the surface of the conductor and will radiate away from it. This persists in all conductors and mutual inductance between two radiatedelectromagnetic fields will result in EMI.[8]
Some of the technical terms which are employed can be used with differing meanings. Some phenomena may be referred to by various different terms. These terms are used here in a widely accepted way, which is consistent with other articles in the encyclopedia.
The basic arrangement ofnoise emitter or source,coupling path and victim,receptor or sink is shown in the figure below. Source and victim are usuallyelectronic hardware devices, though the source may be a natural phenomenon such as alightning strike,electrostatic discharge (ESD) or, inone famous case, theBig Bang at the origin of the Universe.
There are four basic coupling mechanisms:conductive,capacitive,magnetic or inductive, andradiative. Any coupling path can be broken down into one or more of these coupling mechanisms working together. For example the lower path in the diagram involves inductive, conductive and capacitive modes.
Conductive coupling occurs when the coupling path between the source and victim is formed by direct electrical contact with a conducting body, for example a transmission line, wire, cable,PCB trace or metal enclosure. Conducted noise is also characterised by the way it appears on different conductors:
Inductive coupling occurs where the source and victim are separated by a short distance (typically less than awavelength). Strictly, "Inductive coupling" can be of two kinds, electrical induction and magnetic induction. It is common to refer to electrical induction ascapacitive coupling, and to magnetic induction asinductive coupling.
Capacitive coupling occurs when a varyingelectrical field exists between two adjacent conductors typically less than a wavelength apart, inducing a change involtage on the receiving conductor.
Inductive coupling or magnetic coupling occurs when a varyingmagnetic field exists between two parallel conductors typically less than a wavelength apart, inducing a change involtage along the receiving conductor.
Radiative coupling or electromagnetic coupling occurs when source and victim are separated by a large distance, typically more than a wavelength. Source and victim act as radio antennas: the source emits or radiates anelectromagnetic wave which propagates across the space in between and is picked up or received by the victim.
Interference with the meaning ofelectromagnetic interference, alsoradio-frequency interference (EMI orRFI) is – according toArticle 1.166 of theInternational Telecommunication Union's (ITU)Radio Regulations (RR)[9] – defined as "The effect of unwanted energy due to one or a combination ofemissions,radiations, orinductions upon reception in aradiocommunication system, manifested by any performance degradation, misinterpretation, or loss of information which could be extracted in the absence of such unwanted energy".
This is also a definition used by thefrequency administration to providefrequency assignments and assignment of frequency channels toradio stations or systems, as well as to analyzeelectromagnetic compatibility betweenradiocommunication services.
In accordance with ITU RR (article 1) variations of interference are classified as follows:[10]
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Conducted EMI is caused by the physical contact of the conductors as opposed to radiated EMI which is caused byinduction (without physical contact of the conductors).
For lower frequencies, EMI is caused by conduction and, for higher frequencies, by radiation.
EMI through the ground wire is also very common in an electrical facility.
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Interference tends to be more troublesome with older radio technologies such as analogueamplitude modulation, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional antennas used with broadcast systems. Newer radio systems incorporate several improvements that enhance theselectivity. In digital radio systems, such asWi-Fi,error-correction techniques can be used.Spread-spectrum andfrequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. A highlydirectional receiver, such as aparabolic antenna or adiversity receiver, can be used to select one signal in space to the exclusion of others.
The most extreme example of digitalspread-spectrum signalling to date is ultra-wideband (UWB), which proposes the use of large sections of theradio 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 theultra-wideband article).
In theUnited States, the 1982 Public Law 97-259 allowed theFederal Communications Commission (FCC) to regulate the susceptibility of consumer electronic equipment.[11][12]
Potential sources of RFI and EMI include:[13] various types oftransmitters, doorbell transformers,toaster ovens,electric blankets, ultrasonic pest control devices, electricbug zappers,heating pads, andtouch controlled lamps. MultipleCRT computer monitors or televisions sitting too close to one another can sometimes cause a "shimmy" effect in each other, due to the electromagnetic nature of their picture tubes, especially when one of theirde-gaussing coils is activated.
Electromagnetic interference at 2.4 GHz may be caused by802.11b,802.11g and802.11n wireless devices,Bluetooth devices,baby monitors andcordless telephones,video senders, andmicrowave ovens.
Switching loads (inductive,capacitive, andresistive), such as electric motors, transformers, heaters, lamps, ballast, power supplies, etc., all cause electromagnetic interference especially at currents above 2 A. The usual method used for suppressing EMI is by connecting asnubber network, a resistor in series with acapacitor, across a pair of contacts. While this may offer modest EMI reduction at very low currents, snubbers do not work at currents over 2 A withelectromechanical contacts.[14][15]
Another method for suppressing EMI is the use of ferrite core noise suppressors (orferrite beads), which are inexpensive and which clip on to the power lead of the offending device or the compromised device.
Switched-mode power supplies can be a source of EMI, but have become less of a problem as design techniques have improved, such as integratedpower factor correction.
Most countries have legal requirements that mandateelectromagnetic 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).
Radio frequency signal quality has declined throughout the 21st century by roughly one decibel per year as the spectrum becomes increasingly crowded.[additional citation(s) needed] This has inflicted aRed Queen's race on the mobile phone industry as companies have been forced to put up more cellular towers (at new frequencies) that then cause more interference thereby requiring more investment by the providers and frequent upgrades of mobile phones to match.[16]
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The International Special Committee for Radio Interference or CISPR (French acronym for "Comité International Spécial des Perturbations Radioélectriques"), which is a committee of the International Electrotechnical Commission (IEC) sets international standards for radiated and conducted electromagnetic interference. These are civilian standards for domestic, commercial, industrial and automotive sectors. These standards form the basis of other national or regional standards, most notably the European Norms (EN) written by CENELEC (European committee for electrotechnical standardisation). US organizations include the Institute of Electrical and Electronics Engineers (IEEE), the American National Standards Institute (ANSI), and the US Military (MILSTD).
Integrated circuits are often a source of EMI, but they must usually couple their energy to larger objects such as heatsinks, circuit board planes and cables to radiate significantly.[17]
Onintegrated circuits, important means of reducing EMI are: the use of bypass ordecoupling capacitors on each active device (connected across the power supply, as close to the device as possible),rise time control of high-speed signals using series resistors,[18] andIC power supply pin filtering. Shielding is usually a last resort after other techniques have failed, because of the added expense of shielding components such as conductive gaskets.
The efficiency of the radiation depends on the height above theground plane orpower plane (atRF, one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental frequency,harmonic ortransient such as 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 GND pins. The RF is then coupled to the cable through the line driver ascommon-mode noise. Since the noise is common-mode, shielding has very little effect, even withdifferential pairs. The RF energy iscapacitively coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use abraid-breaker orchoke 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 or conductive tape can be used. Most digital equipment is designed with metal or conductive-coated plastic cases.[citation needed]
Any unshielded semiconductor (e.g. an integrated circuit) will tend to act as a detector for those radio signals commonly found in the domestic environment (e.g. mobile phones).[19] Such a detector can demodulate the high frequency mobile phone carrier (e.g., GSM850 and GSM1900, GSM900 and GSM1800) and produce low-frequency (e.g., 217 Hz) demodulated signals.[20] This demodulation manifests itself as unwanted audible buzz in audio appliances such asmicrophone amplifier,speaker amplifier, car radio, telephones etc. Adding onboard EMI filters or special layout techniques can help in bypassing EMI or improving RF immunity.[21]Some ICs are designed (e.g., LMV831-LMV834,[22] MAX9724[23]) to have integrated RF filters or a special design that helps reduce any demodulation of high-frequency carrier.
Designers often need to carry out special tests for RF immunity of parts to be used in a system. These tests are often done in ananechoic chamber with a controlled RF environment where the test vectors produce a RF field similar to that produced in an actual environment.[20]
Interference inradio astronomy, where it is commonly referred to as radio-frequency interference (RFI), is any source of transmission that is within the observed frequency band other than the celestial sources themselves. Because transmitters on and around the Earth can be many times stronger than the astronomical signal of interest, RFI is a major concern for performing radio astronomy.[24]
Some of the frequency bands that are very important for radio astronomy, such as the21-cm HI line at 1420 MHz, are protected by regulation.[25] However, modern radio-astronomical observatories such asVLA,LOFAR, andALMA have a very large bandwidth over which they can observe.[citation needed] Because of the limited spectral space at radio frequencies, these frequency bands cannot be completely allocated to radio astronomy; for example,redshifted images of the 21-cm line from thereionization epoch can overlap with theFM broadcast band (88–108 MHz), and therefore radio telescopes need to deal with RFI in this bandwidth.[24]
Techniques to deal with RFI range from filters in hardware to advanced algorithms in software. One way to deal with strong transmitters is to filter out the frequency of the source completely. This is for example the case for the LOFAR observatory, which filters out the FM radio stations between 90 and 110 MHz. It is important to remove such strong sources of interference as soon as possible, because they might "saturate" the highly sensitive receivers (amplifiers andanalogue-to-digital converters), which means that the received signal is stronger than the receiver can handle. However, filtering out a frequency band implies that these frequencies can never be observed with the instrument.[citation needed]
A common technique to deal with RFI within the observed frequency bandwidth, is to employ RFI detection in software. Such software can find samples in time, frequency or time-frequency space that are contaminated by an interfering source. These samples are subsequently ignored in further analysis of the observed data. This process is often referred to asdata flagging. Because most transmitters have a small bandwidth and are not continuously present such as lightning orcitizens' band (CB) radio devices, most of the data remains available for the astronomical analysis. However, data flagging can not solve issues with continuous broad-band transmitters, such as windmills,digital video ordigital audio transmitters.[citation needed]
Another way to manage RFI is to establish aradio quiet zone (RQZ). RQZ is a well-defined area surrounding receivers that has special regulations to reduce RFI in favor of radio astronomy observations within the zone. The regulations may include special management of spectrum and power flux or power flux-density limitations. The controls within the zone may cover elements other than radio transmitters or radio devices. These include aircraft controls and control of unintentional radiators such as industrial, scientific and medical devices, vehicles, and power lines. The first RQZ for radio astronomy isUnited States National Radio Quiet Zone (NRQZ), established in 1958.[26]
Prior to the introduction of Wi-Fi, one of the biggest applications of 5 GHz band was theTerminal Doppler Weather Radar.[27][28] The decision to use 5 GHz spectrum for Wi-Fi was finalized at theWorld Radiocommunication Conference in 2003; however, meteorological authorities were not involved in the process.[29][30] The subsequent lax implementation and misconfiguration of DFS had caused significant disruption in weather radar operations in a number of countries around the world. In Hungary, the weather radar system was declared non-operational for more than a month. Due to the severity of interference, South African weather services ended up abandoning C band operation, switching their radar network toS band.[28][31]
Transmissions on adjacent bands to those used by passiveremote sensing, such asweather satellites, have caused interference, sometimes significant.[32] There is concern that adoption of insufficiently regulated5G could produce major interference issues. Significant interference can impairnumerical weather prediction performance and incur negative economic and public safety impacts.[33][34][35] These concerns led US Secretary of CommerceWilbur Ross and NASA AdministratorJim Bridenstine in February 2019 to urge the FCC to cancel a proposedspectrum auction, which was rejected.[36]
The FCC ruling is re-opening the Terminal Doppler Weather Radar (TDWR) band (channels 120, 124, 128) with new test requirements for DFS protection.
Since 2006, interference to C-band radars from RLAN is increasingly experienced by most OPERA members. ... The South African weather services initially tried to implement specific software filtering to improve the situation but then decided in 2011 to move its meteorological radar network to S band.
Decision ERC/DEC/(99)23 adds 5250–5350MHz and 5470–5725MHz with more Tx power but with the added caveat that DFS was required to protect legacy users (Military Radar and Satellite uplinks)
More than 12 European countries experienced such interference cases (other cases have now been reported in number of countries in the world). Definitively harmful interference (in Hungary, the radar was declared as non-operational for more than 1 month)
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