Electromagnetic interference (EMI) is unwanted noise or interference in an electrical path or circuit caused by an outside source. It is also known as radio frequency interference. EMI can cause electronics to operate poorly, malfunction or stop working completely. Show EMI can be caused by natural or human-made sources. Using high quality electronics, electrical shielding, and modern error correction can reduce the impact of EMI. A common example of EMI is when a cellphone is placed near powered audio equipment or speakers, and it causes a noise or series of beeps to be heard. What causes electromagnetic interference?EMI occurs because of the close relationship between electricity and magnetism. All electrical flow produces a small magnetic field. Conversely, a moving magnetic field produces an electrical current. These principals allow electric motors and generators to work. Additionally, all electrical conductors can operate as radio antennas. High powered electrical and radio sources can produce unwanted effects in devices far away. As electronics become smaller, faster, more tightly packed and more sensitive, they become more susceptible to these effects, causing EMI. The sources of EMI can be broadly split into two categories: naturally occurring and human-made. There are some sources in the natural world that can produce powerful enough electric fields to affect electronic devices. Lightning can produce strong electrostatic discharges and magnetic pulses. Solar storms and solar flares emit highly charged particles that can cause problems with satellite and terrestrial communications. Cosmic radiation has been known to cause bitflips in electronics. Human-made EMI can come from many places. High-power radio and electrical sources can cause unwanted EMI. Malfunctioning or improperly designed consumer devices can cause EMI in other devices. Using an electromagnetic pulse to intentionally induce EMI faults in victim devices is also a possible offensive action. Types of electromagnetic interferenceIn EMI there is a source, a path and a receptor (or victim). There are several types of paths, or methods of transmission of EMI from the source to the receptor. Radiated EMI happens when a high-power transmitter or electrical device produces a radio frequency that is picked up and causes unwanted effects in another device. If there is EMI and the source and receptor are far apart, then it is likely radiated EMI. Some examples of this would be a broken kitchen microwave causing a computer to reboot or old wireless telephones causing Wi-Fi to drop. Radiated EMI can be subdivided into narrowband and broadband interference.
Conducted EMI happens when there is a physical electrical path from the source to the receptor. This is often along power transmission lines. Some sources may be a large motor or power supply. An example of conducted EMI is the turning on of a treadmill or clothes dryer causing a computer on the same electrical mains circuit to reboot. Coupled EMI happens when the source and receptor are close together, but not electrically connected. Coupled EMI can be transmitted through induction or capacitance.
How to prevent EMIThe best way to prevent EMI is to use good quality electronics from reputable suppliers. In the United States, the FCC requires that all devices be tested against emission standards to prevent them from causing excessive EMI in other devices; similar rules exist in other countries. Poorly made, cheap or counterfeit electronics may not be properly tested or may not be EM shielded, making them more likely to cause EMI in other devices and more susceptible to EMI themselves. Modern error correction and filters can also reduce the effect of nearby EMI sources. Accounting for EMI is important in high-speed networking. For wired networks, power lines should be separated from data lines. Shielded and twisted pair cables offer higher signal integrity. Fiber optic cable can also be substituted for copper cables as it is not susceptible to EMI. Wireless network planning may also need to account for the number and power of transmission sites, nearby radio sources and high-power transmission lines. Using shielded and unshielded twist pair cabling help protect wired networks electromagnetic interference.Medical devices may have legal requirements for EMI shielding and testing. It is also common for hospitals to require that cellphones be switched off to prevent them from causing EMI in sensitive equipment. Consideration of EMI is important when designing electronics and circuit boards. This is especially true in modern high-speed devices. Board designers must account for component location and routing. They may also include metal shielding cans or conductive tape to block EMI from affecting sensitive components. In highly sensitive situations, a Faraday cage may be used to shield a device or room from outside EMI. Radio telescopes are often constructed in isolated areas away from population centers to reduce EMI. See wireless network configuration basics: 5 steps to follow, an overview of network management tasks and best practices, different types of configuration tools for network change management and how to troubleshoot wireless network connection problems in 10 steps. Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction.[1] 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.[2] Both man-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras (northern/southern lights). EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy and atmospheric science.
 EMI can be used intentionally for radio jamming, as in electronic 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. In 1933, a meeting of the International 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's EMC regulations today. In 1979, legal limits were imposed on electromagnetic emissions from all digital equipment by the FCC 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)[3] 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 electromagnetic compatibility (EMC) regulation.[citation needed] Electromagnetic interference can be categorized as follows:
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 electromagnetic fields will result in EMI.[citation needed] Interference with the meaning of electromagnetic interference, also radio-frequency interference (EMI or RFI) is – according to Article 1.166 of the International Telecommunication Union's (ITU) Radio Regulations (RR)[7] – defined as "The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication 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 frequency administration to provide frequency assignments and assignment of frequency channels to radio stations or systems, as well as to analyze electromagnetic compatibility between radiocommunication services. In accordance with ITU RR (article 1) variations of interference are classified as follows:
Conducted EMI 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). 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.
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 antennas used with broadcast systems. Newer radio systems incorporate several improvements that enhance 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). In the United States, the 1982 Public Law 97-259 allowed the Federal Communications Commission (FCC) to regulate the susceptibility of consumer electronic equipment.[8][9] Potential sources of RFI and EMI include:[10] various types of transmitters, doorbell transformers, toaster ovens, electric blankets, ultrasonic pest control devices, electric bug zappers, heating pads, and touch controlled lamps. Multiple CRT 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 de-gaussing coils is activated. Electromagnetic interference at 2.4 GHz may be caused by 802.11b, 802.11g and 802.11n wireless devices, Bluetooth devices, baby monitors and cordless telephones, video senders, and microwave ovens. Switching loads (inductive, capacitive, and resistive), 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 a snubber network, a resistor in series with a capacitor, 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 electromechanical contacts.[11][12] Another method for suppressing EMI is the use of ferrite core noise suppressors (or ferrite 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 integrated power factor correction. Most countries have legal requirements that mandate 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). 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 a Red 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.[13]
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.[14] On integrated circuits, 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), rise time control of high-speed signals using series resistors,[15] and IC 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 the ground plane 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 frequency, harmonic or transient 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 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 or conductive tape can be used. Most digital equipment is designed with metal or conductive-coated plastic cases.[citation needed] 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).[16] 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.[17] This demodulation manifests itself as unwanted audible buzz in audio appliances such as microphone amplifier, speaker amplifier, car radio, telephones etc. Adding onboard EMI filters or special layout techniques can help in bypassing EMI or improving RF immunity.[18] Some ICs are designed (e.g., LMV831-LMV834,[19] MAX9724[20]) 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 anechoic chamber with a controlled RF environment where the test vectors produce a RF field similar to that produced in an actual environment.[17] Interference in radio 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. 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 21-cm HI line at 1420 MHz, are protected by regulation. This is called spectrum management. However, modern radio-astronomical observatories such as VLA, LOFAR, and ALMA 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 (amplifiers and analog-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. 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 citizens' 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 or digital audio transmitters.[citation needed] Another way to manage RFI is to establish a radio 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 is United States National Radio Quiet Zone (NRQZ), established in 1958.[21] Prior to the introduction of Wi-Fi, one of the biggest applications of 5 GHz band is the Terminal Doppler Weather Radar.[22][23] The decision to use 5 GHz spectrum for Wi-Fi was finalized in World Radiocommunication Conference in 2003; however, meteorological community was not involved in the process.[24][25] 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 S band.[23][26] Transmissions on adjacent bands to those used by passive remote sensing, such as weather satellites, have caused interference, sometimes significant.[27] There is concern that adoption of insufficiently regulated 5G could produce major interference issues. Significant interference can significantly impair numerical weather prediction performance and incur substantially negative economic and public safety impacts.[28][29][30] These concerns led US Secretary of Commerce Wilbur Ross and NASA Administrator Jim Bridenstine in February 2019 to urge the FCC to cancel proposed spectrum auctioning, which was rejected.[31]
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Are susceptible to interference from ac power lines.
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