Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation Electromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified into several types emitted from an external source. ^[1] The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources, and switches, the Sun The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass. The mean distance of the Sun from the Earth is approximately 149,598,000 kilometres (92,956,000 mi), and its light travels this distance in 8 minutes and 19 seconds. But it varies or the Northern Lights Auroras, sometimes called the northern and southern lights or aurorae (singular: aurora), are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras. In northern latitudes, the effect is known as the aurora borealis, named.

EMI can be intentionally used for radio jamming Radio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without checking that it is in use first, or without being able to hear distant stations on the same frequency. Another form of unintentional jamming, as in some forms of electronic warfare The purpose of EW is to deny the opponent an advantage in the EMS and ensure friendly unimpeded access to the EM spectrum portion of the information environment. EW can be applied from air, sea, land, and space by manned and unmanned systems, or can occur unintentionally, as a result of spurious emissions for example through intermodulation products, and the like. It frequently affects the reception of AM radio AM radio began with the first, experimental broadcast on Christmas Eve of 1906 by Canadian experimenter Reginald Fessenden, and was used for small-scale voice and music broadcasts up until World War I. San Francisco, California radio station KCBS claims to be the direct descendant of KQW, founded by radio experimenter Charles "Doc" in urban areas. It can also affect cell phone A mobile phone or mobile is a long-range, electronic device used for mobile voice or data communication over a network of specialized base stations known as cell sites. In addition to the standard voice function of a mobile phone, telephone, current mobile phones may support many additional services, and accessories, such as SMS for text messaging,, FM radio FM broadcasting is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity sound over broadcast radio and television Commercially available since the late 1930s, the television set has become a common communications receiver in homes, businesses and institutions, particularly as a source of entertainment and news. Since the 1970s the availability of video cassettes, laserdiscs, DVDs and now Blu-ray discs, have resulted in the television set frequently being used reception, although to a lesser extent.

EMI/RFI types

Radiated EMI or RFI may be broadly categorized into two types; narrowband Narrowband refers to a situation in radio communications where the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum and broadband Broadband in telecommunications refers to a signaling method that includes or handles a relatively wide range of frequencies, which may be divided into channels or frequency bins. Broadband is always a relative term, understood according to its context. The wider the bandwidth, the greater the information-carrying capacity. In radio, for example,.

Narrowband interference usually arises from intentional transmissions such as radio and TV stations Broadcasting is the distribution of audio and/or video signals which transmit programs to an audience. The audience may be the general public or a relatively large sub-audience, such as children or young adults, pager transmitters, cell phones A mobile phone or mobile is a long-range, electronic device used for mobile voice or data communication over a network of specialized base stations known as cell sites. In addition to the standard voice function of a mobile phone, telephone, current mobile phones may support many additional services, and accessories, such as SMS for text messaging,, etc. Broadband interference usually comes from incidental radio frequency Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency emitters. These include electric power transmission Electric power transmission is the bulk transfer of electrical power , a process in the delivery of electricity to consumers. A power transmission network typically connects power plants to multiple substations near a populated area. The wiring from substations to customers is referred to as Electricity distribution, following the historic lines, electric motors An electric motor is a device using electrical energy to produce mechanical energy, nearly always by the interaction of magnetic fields and current-carrying conductors. The reverse process, that of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo. Traction motors used on vehicles often perform both, thermostats A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off or regulating the flow of a heat transfer fluid as needed to maintain the correct temperature, bug zappers, etc. Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resembles that of synchrotron Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of ultrarelativistic charged particles through magnetic fields. This may be achieved artificially in synchrotrons or storage rings, or naturally by fast electrons moving through magnetic fields in space. The radiation produced may sources, stronger at low frequencies and diminishing at higher frequencies, though this noise is often modulated Modulation is the process of varying one waveform in relation to another waveform. In telecommunications, modulation is used to convey a message, or a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Often a high-frequency sinusoid waveform is used as carrier signal to convey a lower frequency, or varied, by the creating device in some way. Included in this category are computers A computer is a machine that manipulates data according to a set of instructions and other digital A digital system is a data technology that uses discrete values represented by high and low states known as bits. By contrast, non-digital (or analog) systems use a continuous range of values to represent information. Although digital representations are discrete, the information represented can be either discrete, such as numbers, letters or equipment as well as televisions Commercially available since the late 1930s, the television set has become a common communications receiver in homes, businesses and institutions, particularly as a source of entertainment and news. Since the 1970s the availability of video cassettes, laserdiscs, DVDs and now Blu-ray discs, have resulted in the television set frequently being used. The rich harmonic In acoustics and telecommunication, a harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. For example, if the fundamental frequency is f, the harmonics have frequencies f, 2f, 3f, 4f, etc. The harmonics have the property that they are all periodic at the fundamental frequency, content of these devices means that they can interfere over a very broad spectrum. Characteristic of broadband RFI is an inability to filter it effectively once it has entered the receiver In consumer electronics, the terms radio and radio receiver are often used specifically for receivers designed for the sound signals transmitted by radio broadcasting services – historically the first mass-market radio application chain. ^[2][3][4]

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.

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 ^[5].

On integrated circuits In electronics, an integrated circuit is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) that has been manufactured in the surface of a thin substrate of semiconductor material. Integrated circuits are used in almost all electronic equipment in use today and have revolutionized the, important means of reducing EMI are: the use of bypass or "decoupling" capacitors A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the plates. The effect is greatest between wide, 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^[6], and V_CC 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 In electrical engineering, a ground plane is an electrically conductive surface or power plane In printed circuit board design, a power plane is the counterpart to the ground plane and behaves as an AC signal ground, whilst providing DC voltage for powering circuits mounted on the PCB (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 The fundamental tone, often referred to simply as the fundamental and abbreviated f0 or F0, is the lowest frequency in a harmonic series, harmonic In acoustics and telecommunication, a harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. For example, if the fundamental frequency is f, the harmonics have frequencies f, 2f, 3f, 4f, etc. The harmonics have the property that they are all periodic at the fundamental frequency, or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz The hertz is a unit of frequency. It is defined as the number of cycles per second. It is the basic unit of frequency in the International System of Units (SI), and is used worldwide in both general-purpose and scientific contexts. Hertz can be used to measure any periodic event; the most common uses for hertz are to describe radio and audio, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the V_CC and ground pins. The RF is then coupled to the cable through the line driver as common-mode noise. Since the noise is common-mode, shielding has very little effect, even with differential pairs. The RF energy is capacitively coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker or choke A choke is an inductor designed to have a high reactance to a particular frequency when used in a signal-carrying circuit to reduce the common-mode signal.

At higher frequencies, usually above 500 MHz The hertz is a unit of frequency. It is defined as the number of cycles per second. It is the basic unit of frequency in the International System of Units (SI), and is used worldwide in both general-purpose and scientific contexts. Hertz can be used to measure any periodic event; the most common uses for hertz are to describe radio and audio, 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.

Susceptibilities of different radio technologies

Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulation Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker,, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas A dipole antenna, developed by Heinrich Rudolph Hertz around 1886, is an antenna that can be made by a simple wire, with a center-fed driven element for transmitting or receiving radio frequency energy. These antennas are the simplest practical antennas from a theoretical point of view; the current amplitude on such an antenna decreases uniformly used with broadcast systems. Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi Wi-Fi is a trademark of the Wi-Fi Alliance for certified products based on the IEEE 802.11 standards. This certification warrants interoperability between different wireless devices, error-correction In mathematics, computer science, telecommunication, and information theory, error detection and correction has great practical importance in maintaining data integrity across noisy channels and less-than-reliable storage media techniques can be used. Spread-spectrum Spread-spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural and frequency-hopping Frequency-hopping spread spectrum is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is utilized as a multiple access method in the frequency-hopping code division multiple access (FH-CDMA) scheme techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly directional receiver, such as a parabolic antenna A parabolic antenna is a high-gain reflector antenna used for radio, television and data communications, and also for radiolocation , on the UHF and SHF parts of the electromagnetic spectrum. The relatively short wavelength of electromagnetic (radio) energy at these frequencies allows reasonably sized reflectors to exhibit the very desirable 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 Spread-spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural signalling to date is ultra-wideband (UWB Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging. Most recent applications target sensor data collection, precision locating and tracking applications.[), 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 Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging. Most recent applications target sensor data collection, precision locating and tracking applications.[ article.

Interference to consumer devices

Complex electronic circuitry is found in all sorts of devices used in the home. This results in a vast interference potential that didn't exist in earlier, simpler decades. In the United States The United States of America is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the, Public Law 97-259, enacted in 1982, gave the FCC The Federal Communications Commission is an independent agency of the United States government, created, directed, and empowered by Congressional statute (see 47 U.S.C. § 151 and 47 U.S.C. § 154), and with the majority of its commissioners appointed by the current President. The FCC works towards six strategic goals in the areas of broadband, the authority to regulate the susceptibility of consumer electronic equipment sold in the country. The FCC, working with equipment manufacturers, decided to allow them to develop standards for EMI immunity and implement their own voluntary compliance programs.^[7]

Broadcast Broadcasting is the distribution of audio and/or video signals which transmit programs to an audience. The audience may be the general public or a relatively large sub-audience, such as children or young adults transmitters A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications, two-way radio A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI.^[8] Other possible sources of interference include a wide variety of devices, such as 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 another, 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 inductive loads, such as electric motors, often cause interference, but it is easily suppressed by connecting a snubber network, a resistor in series with a capacitor, across the switch. Exact values can be optimised for each case, but 100 ohms in series with 100 nano farads is usually satisfactory.

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).

Standards

The International Special Committee for Radio Interference (CISPR) sets standards for radiated and conducted electromagnetic interference.

See also

• Electromagnetic Radiation
• Faraday Cage
• Radio receiver
• Signal noise
• Twisted Pair
• Interference

References

1. ^ Based on the "interference" entry of The Concise Oxford English Dictionary, 11th edition, online
2. ^ http://www.radiosky.com/journal0901.html RadioSky Journal
3. ^ Radio frequency interference / editors, Charles L. Hutchinson, Michael B. Kaczynski ; contributors, Doug DeMaw ... [et al.]. 4th ed. Newington, CT American Radio Relay League c1987.
4. ^ 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.
5. ^ Clemson Vehicular Electronics Laboratory Web Site: Integrated Circuit EMC page.
6. ^ "Don't "despike" your signal lines, add a resistor instead."
7. ^ http://www.arrl.org/tis/info/rfigen.html RadioFrequency Interference/ElectroMagnetic Interference, ARRL
8. ^ http://www.kyes.com/antenna/interference/tvibook.html INTERFERENCE HANDBOOK

External links

• ARRL, RFI
• Interference Handbook
• Introduction to EMC
• Clemson's EMC Page (EMI Tools and Information)

Categories: Electromagnetic compatibility

See Also:

• Electromagnetic Interference

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