HistorySince 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. In 1933, a meeting of the (IEC) in Paris recommended the International Special Committee on Radio Interference () 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's regulations today. In 1979, legal limits were imposed on electromagnetic emissions from all digital equipment by the 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. 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) 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". 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. Many countries now have similar requirements for products to meet some level of (EMC) regulation.
TypesElectromagnetic interference can be categorized as follows: * EMI or RFI, which typically emanates from intended transmissions such as or s * EMI or RFI, which is unintentional radiation from sources such as lines. 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 radiated will result in EMI.
ITU definitionInterference with the meaning of ''electromagnetic interference'', also ''radio-frequency interference'' (''EMI'' or ''RFI'') is according to ''Article 1.166'' of the s (ITU) (RR) defined as "The effect of unwanted energy due to one or a combination of , , or upon reception in a 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 the to provide s and assignment of frequency channels to s or systems, as well as to analyze between s. In accordance with ITU RR (article 1) variations of interference are classified as follows: * permissible interference * acceptable interference * harmful interference
Conducted interferenceConducted EMI is caused by the physical contact of the conductors as opposed to radiated EMI which is caused by (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.
Susceptibilities of different radio technologiesInterference tends to be more troublesome with older radio technologies such as analogue , 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 the . In digital radio systems, such as , techniques can be used. and techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly receiver, such as a or a , can be used to select one signal in space to the exclusion of others. The most extreme example of digital signalling to date is ultra-wideband (), which proposes the use of large sections of the 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 article).
Interference to consumer devicesIn the , the 1982 Public Law 97-259 allowed the (FCC) to regulate the susceptibility of consumer electronic equipment. Potential sources of RFI and EMI include: various types of s, doorbell transformers, s, s, ultrasonic pest control devices, electric s, s, and . Multiple 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 their coils is activated. can be caused by and wireless devices, devices, s and s, s, and s. ing loads (, , and ), such as electric motors, transformers, heaters, lamps, ballast, power supplies, etc., all cause electromagnetic interference especially at currents above 2 . The usual method used for suppressing EMI is by connecting a network, a resistor in series with a , 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 with contacts. Another method for suppressing EMI is the use of ferrite core noise suppressors (or s), which are inexpensive and which clip on to the power lead of the offending device or the compromised device. can be a source of EMI, but have become less of a problem as design techniques have improved, such as integrated . Most countries have legal requirements that mandate : 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. This has inflicted a 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.
StandardsThe 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).
EMI in integrated circuitsIntegrated 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. On s, important means of reducing EMI are: the use of bypass or s on each active device (connected across the power supply, as close to the device as possible), control of high-speed signals using series resistors, and 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 the or (at , one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (, or such as overshoot, undershoot or ringing). At lower frequencies, such as 133 , 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 as . Since the noise is common-mode, shielding has very little effect, even with s. The RF energy is from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a or 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.
RF immunity and testingAny 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). 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. This demodulation manifests itself as unwanted audible buzz in audio appliances such as amplifier, amplifier, car radio, telephones etc. Adding onboard EMI filters or special layout techniques can help in bypassing EMI or improving RF immunity. Some ICs are designed (e.g., LMV831-LMV834, MAX9724) 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 an with a controlled RF environment where the test vectors produce a RF field similar to that produced in an actual environment.
RFI in radio astronomyInterference in , 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. Natural sources of interference, such as lightning and the Sun, are also often referred to as RFI. Some of the frequency bands that are very important for radio astronomy, such as the at 1420 MHz, are protected by regulation. This is called . However, modern radio-astronomical observatories such as , , and have a very large bandwidth over which they can observe. Because of the limited spectral space at radio frequencies, these frequency bands cannot be completely allocated to radio astronomy. Therefore, observatories need to deal with RFI in their observations. 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 (s and s), 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. 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 as ''data flagging''. Because most transmitters have a small bandwidth and are not continuously present such as lightning or (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, or transmitters. Another way to manage RFI is to establish a (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 is (NRQZ), established in 1958.
RFI on environmental monitoringPrior to the introduction of Wi-Fi, one of the biggest applications of 5 GHz band is the . The decision to use 5 GHz spectrum for Wi-Fi was finalized in in 2003; however, meteorological community was not involved in the process. 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 to . Transmissions on adjacent bands to those used by passive , such as s, have caused interference, sometimes significant. There is concern that adoption of insufficiently regulated could produce major interference issues. Significant interference can significantly impair performance and incur substantially negative economic and public safety impacts. These concerns led US Secretary of Commerce and NASA Administrator in February 2019 to urge the FCC to cancel proposed ing, which was rejected.
See also* * * * * * * * *