Electromagnetic interference

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.

Analog TV EMI
Electromagnetic interference in analog TV signal


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 USA 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.


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.

ITU definition

Interference with the meaning of electromagnetic interference, also radio-frequency interference (short: EMI | 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:

  • Permissible interference
  • Acceptable interference
  • Harmful interference

Conducted interference

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.

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 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).

Interference to consumer devices

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 can be caused by 802.11b and 802.11g 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, 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. 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).

EMI in integrated circuits

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 tape can be used. Most digital equipment is designed with metal or conductive-coated plastic cases.

RF immunity and testing

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).[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]

RFI in radio astronomy

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 90-110 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.

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]

RFI on environmental monitoring

Transmissions on adjacent bands to those used by passive remote sensing, such as weather satellites, have caused interference, sometimes significant.[22] 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.[23][24][25] 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.[26]

See also


  1. ^ Based on the "interference" entry of The Concise Oxford English Dictionary, 11th edition, online
  2. ^ Sue, M.K. "Radio frequency interference at the geostationary orbit". NASA. Jet Propulsion Laboratory. Retrieved 6 October 2011.
  3. ^ "Council Directive 89/336/EEC of 3 May 1989 on the approximation of the laws of the Member States relating to electromagnetic compatibility". EUR-Lex. 3 May 1989. Retrieved 21 January 2014.
  4. ^ "Radio Frequency Interference - And What to Do About It". Radio-Sky Journal. Radio-Sky Publishing. March 2001. Retrieved 21 January 2014.
  5. ^ Radio frequency interference / editors, Charles L. Hutchinson, Michael B. Kaczynski ; contributors, Doug DeMaw ... [et al.]. 4th ed. Newington, CT American Radio Relay League c1987.
  6. ^ Radio frequency interference handbook. Compiled and edited by Ralph E. Taylor. Washington Scientific and Technical Information Office, National Aeronautics and Space Administration; [was for sale by the National Technical Information Service, Springfield, Va.] 1971.
  7. ^ ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.166, definition: interference
  8. ^ Public Law 97-259
  9. ^ Paglin, Max D.; Hobson, James R.; Rosenbloom, Joel (1999), The Communications Act: A Legislative History of the Major Amendments, 1934-1996, Pike & Fischer - A BNA Company, p. 210, ISBN 0937275050
  10. ^ "Interference Handbook". Federal Communications Commission. Archived from the original on 16 October 2013. Retrieved 21 January 2014.
  11. ^ "Lab Note #103 Snubbers - Are They Arc Suppressors?". Arc Suppression Technologies. April 2011. Retrieved February 5, 2012.
  12. ^ "Lab Note #105 EMI Reduction - Unsuppressed vs. Suppressed". Arc Suppression Technologies. April 2011. Retrieved February 5, 2012.
  13. ^ Smith, Tony (7 November 2012). "WTF is... RF-MEMS?". TheRegister.co.uk. Retrieved 21 January 2014.
  14. ^ "Integrated Circuit EMC". Clemson University Vehicular Electronics Laboratory. Retrieved 21 January 2014.
  15. ^ ""Don't "despike" your signal lines, add a resistor instead."". Massmind.org. Retrieved 21 January 2014.
  16. ^ Fiori, Franco (November 2000). "Integrated Circuit Susceptibility to Conducted RF Interference". Compliance Engineering. Ce-mag.com. Archived from the original on 2 March 2012. Retrieved 21 January 2014.
  17. ^ a b Mehta, Arpit (October 2005). "A general measurement technique for determining RF immunity" (PDF). RF Design. Retrieved 21 January 2014.
  18. ^ "APPLICATION NOTE 3660: PCB Layout Techniques to Achieve RF Immunity for Audio Amplifiers". Maxim Integrated. 2006-07-04. Retrieved 21 January 2014.
  19. ^ LMV831-LMV834 Archived 2009-01-07 at the Wayback Machine
  20. ^ MAX9724
  21. ^ Characteristics of radio quiet zones (Report ITU-R RA.2259) (PDF). International Telecommunication Union. September 2012. Retrieved 22 April 2017.
  22. ^ Lubar, David G. (9 January 2019). "A Myriad of Proposed Radio Spectrum Changes—-Collectively Can They Impact Operational Meteorology?". 15th Annual Symposium on New Generation Operational Environmental Satellite Systems. Phoenix, AZ: American Meteorological Society.
  23. ^ Misra, Sidharth (10 January 2019). "The Wizard Behind the Curtain?—The Important, Diverse, and Often Hidden Role of Spectrum Allocation for Current and Future Environmental Satellites and Water, Weather, and Climate". 15th Annual Symposium on New Generation Operational Environmental Satellite Systems. Phoenix, AZ: American Meteorological Society.
  24. ^ Witze, Alexandra (26 April 2019). "Global 5G wireless networks threaten weather forecasts: Next-generation mobile technology could interfere with crucial satellite-based Earth observations". Nature News.
  25. ^ Brackett, Ron (1 May 2019). "5G Wireless Networks Could Interfere with Weather Forecasts, Meteorologists Warn". The Weather Channel.
  26. ^ Samenow, Jason (8 March 2019). "Critical weather data threatened by FCC 'spectrum' proposal, Commerce Dept. and NASA say". The Washington Post. Retrieved 2019-05-05.

External links

16th Space Control Squadron

The 16th Space Control Squadron is an active United States Air Force unit, stationed at Peterson Air Force Base, Colorado as part of the 21st Operations Group. The squadron protects critical satellite communication links to detect, characterize, geolocate and report sources of electromagnetic interference on US military and commercial satellites. The squadron also provides combat-ready crews to deploy and employ defensive space control capabilities for theater combatant commanders. The squadron is Air Force Space Command's first defensive counterspace unit.

From 1967 through 1994, the squadron, originally the 16th Surveillance Squadron, operated the Cobra Dane space detection system at Eareckson Air Force Base, Alaska.

Balanced audio

Balanced audio is a method of interconnecting audio equipment using balanced lines. This type of connection is very important in sound recording and production because it allows the use of long cables while reducing susceptibility to external noise caused by electromagnetic interference.

Balanced connections typically use shielded twisted-pair cable and three-conductor connectors. The connectors are usually XLR or TRS phone connectors. When used in this manner, each cable carries one channel, therefore stereo audio (for example) would require two of them.


The Comité International Spécial des Perturbations Radioélectriques (CISPR; English: International Special

Committee on Radio Interference) was founded in 1934 to set standards for controlling electromagnetic interference in electrical and electronic devices, and is a part of the International Electrotechnical Commission (IEC).

Common-mode rejection ratio

In electronics, The common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e., those that appear simultaneously and in-phase on both inputs. An ideal differential amplifier would have infinite CMRR, however this is not achievable in practice. A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI). An example is audio transmission over balanced line in sound reinforcement or recording.

Differential signaling

Differential signaling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires (typically twisted together) or traces on a circuit board. The receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground. The opposite technique is called single-ended signaling.

Differential pairs are usually found on printed circuit boards, in twisted-pair and ribbon cables, and in connectors.

Electromagnetic compatibility

Electromagnetic compatibility (EMC) is the branch of electrical engineering concerned with the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage in operational equipment. Also, it is the ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment. The goal of EMC is the correct operation of different equipment in a common electromagnetic environment.

EMC pursues three main classes of issue. Emission is the generation of electromagnetic energy, whether deliberate or accidental, by some source and its release into the environment. EMC studies the unwanted emissions and the countermeasures which may be taken in order to reduce unwanted emissions. The second class, susceptibility, is the tendency of electrical equipment, referred to as the victim, to malfunction or break down in the presence of unwanted emissions, which are known as Radio frequency interference (RFI). Immunity is the opposite of susceptibility, being the ability of equipment to function correctly in the presence of RFI, with the discipline of "hardening" equipment being known equally as susceptibility or immunity. A third class studied is coupling, which is the mechanism by which emitted interference reaches the victim.

Interference mitigation and hence electromagnetic compatibility may be achieved by addressing any or all of these issues, i.e., quieting the sources of interference, inhibiting coupling paths and/or hardening the potential victims. In practice, many of the engineering techniques used, such as grounding and shielding, apply to all three issues.

Electromagnetic interference control

In telecommunication, electromagnetic interference control (EMI) is the control of radiated and conducted energy such that emissions that are unnecessary for system, subsystem, or equipment operation are reduced, minimized, or eliminated.

Note: Electromagnetic radiated and conducted emissions are controlled regardless of their origin within the system, subsystem, or equipment. Successful EMI control with effective susceptibility control leads to electromagnetic compatibility.

Environmental testing

Environmental testing is the measurement of the performance of equipment under specified environmental conditions, such as:

extremely high and low temperatures

large, swift variations in temperature

blown and settling sand and dust

salt spray and salt fog

very high or low humidity

wet environments, waterproofness, icing

presence of corrosive material

fungus, fluids susceptibility

vibrations (airborne and structural), gun fire


solar radiation

high and low pressures (especially for aeronautical and space equipment)

operating at angles (especially for marine, aeronautical and space equipment)

electromagnetic interference (EMI), ESD, Lightning

acoustic measurements

power input variationsSuch tests are most commonly performed on equipment used in military, maritime, aeronautical and space applications. See Environmental test chambers for more information about environmental testing equipment.

Environmental test standards include

MIL-STD-810, "Test Method Standard for Environmental Engineering Considerations and Laboratory Tests", presently (2010) version G, issued in 2009

MIL-HDBK-2036, "Preparation of Electronic Equipment Specifications", issued 1999

IEC 60068, "Environmental Testing", with many parts.

IEC 60945, "Maritime navigation and radiocommunication equipment and systems – General requirements – Methods of testing and required test results", issued 2002 and due for review in 2007

RTCA DO-160, "Environmental Conditions and Test Procedures for Airborne Equipment", first published in 1975

MIL-STD-461, "Department of Defense Interface Standard: Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment (11 DEC 2015)", presently version G.

Faraday cage

A Faraday cage or Faraday shield is an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of conductive material, or in the case of a Faraday cage, by a mesh of such materials. Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836.

A Faraday cage operates because an external electrical field causes the electric charges within the cage's conducting material to be distributed so that they cancel the field's effect in the cage's interior. This phenomenon is used to protect sensitive electronic equipment from external radio frequency interference (RFI). Faraday cages are also used to enclose devices that produce RFI, such as radio transmitters, to prevent their radio waves from interfering with other nearby equipment. They are also used to protect people and equipment against actual electric currents such as lightning strikes and electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.

Faraday cages cannot block stable or slowly varying magnetic fields, such as the Earth's magnetic field (a compass will still work inside). To a large degree, though, they shield the interior from external electromagnetic radiation if the conductor is thick enough and any holes are significantly smaller than the wavelength of the radiation. For example, certain computer forensic test procedures of electronic systems that require an environment free of electromagnetic interference can be carried out within a screened room. These rooms are spaces that are completely enclosed by one or more layers of a fine metal mesh or perforated sheet metal. The metal layers are grounded to dissipate any electric currents generated from external or internal electromagnetic fields, and thus they block a large amount of the electromagnetic interference. See also electromagnetic shielding. They provide less attenuation of outgoing transmissions than incoming: they can block EMP waves from natural phenomena very effectively, but a tracking device, especially in upper frequencies, may be able to penetrate from within the cage (e.g., some cell phones operate at various radio frequencies so while one cell phone may not work, another one will).

A common misconception is that a Faraday cage provides full blockage or attenuation; this is not true. The reception or transmission of radio waves, a form of electromagnetic radiation, to or from an antenna within a Faraday cage is heavily attenuated or blocked by the cage; however, a Faraday cage has varied attenuation depending on wave form, frequency or distance from receiver/transmitter, and receiver/transmitter power. Near-field high-powered frequency transmissions like HF RFID are more likely to penetrate. Solid cages generally attenuate fields over a broader range of frequencies than mesh cages.

Globally asynchronous locally synchronous

Globally asynchronous locally synchronous (GALS) is an architecture for designing electronic circuits which addresses the problem of safe and reliable data transfer between independent clock domains. GALS is a Model of Computation (MoC) that emerged in the 1980s. It allows to design computer systems consisting of several synchronous islands (using synchronous programming for each such island) interacting with other islands using asynchronous communication, e.g. with FIFOs.

A GALS circuit consists of a set of locally synchronous modules communicating with each other via asynchronous wrappers. Each synchronous subsystem ("clock domain") can run on its own independent clock (frequency). Advantages include much lower electromagnetic interference (EMI). The CMOS circuit (logic gates) requires relatively large supply current when changing state from 0 to 1. These changes are aggregated for synchronous circuit as most changes are initialised by an active clock edge. Therefore, large spikes on supply current occur at active clock edges. These spikes can cause large electromagnetic interference, and may lead to circuit malfunction. In order to limit these spikes large number of decoupling capacitors are used. Another solution is to use a GALS design style, i.e. design (locally) is synchronous (thus easier to be designed than asynchronous circuit) but globally asynchronous, i.e. there are different (e.g. phase shifted, rising and falling active edge) clock signal regimes thus supply current spikes do not aggregate at the same time. Consequently, GALS design style is often used in system-on-a-chip (SoC). It is especially used in Network-on-Chip (NoC) architectures for SoCs.


A humbucking pickup, humbucker, or double coil, is a type of electric guitar pickup that uses two coils to "buck the hum" (or cancel out the interference) picked up by coil pickups caused by electromagnetic interference, particularly mains hum. Most pickups use magnets to produce a magnetic field around the strings, and induce an electrical current in the surrounding coils as the strings vibrate (a notable exception is the piezoelectric pickup). Humbuckers work by pairing a coil with the north poles of its magnets oriented "up", (toward the strings) with another coil right next to it, which has the south pole of its magnets oriented up. By connecting the coils together out of phase, the interference is significantly reduced via phase cancellation: the string signals from both coils add up instead of canceling, because the magnets are placed in opposite polarity. The coils can be connected in series or in parallel in order to achieve this hum-cancellation effect, although it's much more common for the coils of a humbucker pickup to be connected in series (see "series/parallel" below). In addition to electric guitar pickups, humbucking coils are sometimes used in dynamic microphones to cancel electromagnetic hum.

Hum is caused by the alternating magnetic fields created by transformers and power supplies inside electrical equipment using alternating current. While playing a guitar without humbuckers, a musician would hear a hum through the pickups during quiet sections of music. Sources of studio and stage hum include high-power amps, processors, mixers, motors, power lines, and other equipment. Compared to single coil pickups, especially unshielded ones, humbuckers dramatically reduce hum, and (especially when the coils are connected in series) produce a louder signal with more mid-range presence.

Impulse noise (audio)

Impulse noise is a category of (acoustic) noise which includes unwanted, almost instantaneous (thus impulse-like) sharp sounds (like clicks and pops). Noises of the kind are usually caused by electromagnetic interference, scratches on the recording disks, gunfire, explosions and ill synchronization in digital recording and communication. High levels of such a noise (200+ decibels) may damage internal organs, while 180 decibels are enough to destroy or damage human ears.

An impulse noise filter can be used to enhance the quality of noisy signals, in order to achieve robustness in pattern recognition and adaptive control systems. A classic filter used to remove impulse noise is the median filter, at the expense of signal degradation. Thus it's quite common, in order to get better performing impulse noise filters, to use model-based systems that know the properties of the noise and source signal (in time or frequency), in order to remove only impulse obliterated samples.

Inductive coupling

In electrical engineering, two conductors are said to be inductively coupled or magnetically coupled when they are configured such that a change in current through one wire induces a voltage across the ends of the other wire through electromagnetic induction. A changing current through the first wire creates a changing magnetic field around it by Ampere's circuital law. The changing magnetic field induces an electromotive force (EMF or voltage) in the second wire by Faraday's law of induction. The amount of inductive coupling between two conductors is measured by their mutual inductance.

The coupling between two wires can be increased by winding them into coils and placing them close together on a common axis, so the magnetic field of one coil passes through the other coil. Coupling can also be increased by a magnetic core of a ferromagnetic material like iron or ferrite in the coils, which increases the magnetic flux. The two coils may be physically contained in a single unit, as in the primary and secondary windings of a transformer, or may be separated. Coupling may be intentional or unintentional. Unintentional inductive coupling can cause signals from one circuit to be induced into a nearby circuit, this is called cross-talk, and is a form of electromagnetic interference.

An inductively coupled transponder consists of a solid state transceiver chip connected to a large coil that functions as an antenna. When brought within the oscillating magnetic field of a reader unit, the transceiver is powered up by energy inductively coupled into its antenna and transfers data back to the reader unit inductively.

Magnetic coupling between two magnets can also be used to mechanically transfer power without contact, as in the magnetic gear.

Interference (communication)

In electronic communications, especially in telecommunications, an interference is that which modifies a signal in a disruptive manner, as it travels along a channel between its source and receiver. The term is often used to refer to the addition of unwanted signals to a useful signal. Common examples are:

Electromagnetic interference (EMI)

Co-channel interference (CCI), also known as crosstalk

Adjacent-channel interference (ACI)

Intersymbol interference (ISI)

Inter-carrier interference (ICI), caused by doppler shift in OFDM modulation (multitone modulation).

Common-mode interference (CMI)

Conducted interferenceInterference is typically but not always distinguished from noise, for example white thermal noise.

Radio resource management aims at reducing and controlling the co-channel and adjacent-channel interference.

List of 2.4 GHz radio use

There are several uses of the 2.4 GHz band. Interference may occur between devices operating at 2.4 GHz. This article details the different users of the 2.4 GHz band, how they cause interference to other users and how they are prone to interference from other users.

Silverliner V

The Silverliner V is an electric railcar designed and built by Hyundai Rotem. It is used by Philadelphia's SEPTA Regional Rail and Denver, Colorado's Regional Transportation District. This is the fifth generation railcar in the Silverliner family of single level EMUs.

TWA Flight 800 conspiracy theories

TWA Flight 800 conspiracy theories suggest that the crash of Trans World Airlines Flight 800 (TWA 800) was due to causes other than those determined by the National Transportation Safety Board (NTSB). The NTSB found that the probable cause of the crash of TWA Flight 800 was an explosion of flammable fuel/air vapors in a fuel tank, most likely from a short-circuit. Conspiracy theories say the crash was due to a U.S. Navy missile test gone awry, a terrorist missile strike, or an on-board bomb. On June 19, 2013, a documentary alleging that the investigation into the crash was a cover-up made news headlines with statements from six members of the original investigation team, now retired, who also filed a petition to reopen the probe.

Transition-minimized differential signaling

Transition-minimized differential signaling (TMDS), a technology for transmitting high-speed serial data, is used by the DVI and HDMI video interfaces, as well as by other digital communication interfaces.

The transmitter incorporates an advanced coding algorithm which reduces electromagnetic interference over copper cables and enables robust clock recovery at the receiver to achieve high skew tolerance for driving longer cables as well as shorter low-cost cables.

Twisted pair

Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighboring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell.

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