Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a (usually undesirable) disturbance that affects an electrical circuit due to electromagnetic radiation emitted from an external source. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. ^[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, the Sun or the Northern Lights. An electrical network is an interconnection of Electrical elements such as Resistors Inductors Capacitors Transmission lines Voltage The Sun (Sol is the Star at the center of the Solar System.

EMI can be induced intentionally for radio jamming, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like. Radio jamming is the (usually deliberate transmission of Radio signals that disrupt communications by decreasing the Signal to noise ratio Electronic warfare ( EW) is the use of the Electromagnetic spectrum to effectively deny the use of this medium by an adversary while optimizing its use by friendly A spurious emission is any Radio frequency not deliberately created or transmitted especially in a device which normally does create other frequencies Intermodulation or intermodulation distortion ( IMD) or intermod for short It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent. See also Frequency modulation, FM band FM broadcasting is a broadcast Technology invented by Edwin Howard Armstrong that Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic

Sound

info		help
EMI sound sample. A Wi-Fi signal interferes with a speaker system. Wi-Fi (ˈwaɪfaɪ is the trade name for the popular wireless technology used

EMI/RFI types

EMI or RFI may be broadly categorized into two types; narrowband and broadband. Narrowband refers to a situation in radio communications where the bandwidth of the message does not significantly exceed the channel's Coherence bandwidth. The term broadband can have different meanings in different contexts

Narrowband interference usually arises from intentional transmissions such as radio and TV stations, pager transmitters, cell phones, etc. For the band see Broadcast (band Broadcasting is the distribution of audio and/or Video signals which transmit Broadband interference usually comes from incidental radio frequency emitters. Frequency is a measure of the number of occurrences of a repeating event per unit Time. These include electric power transmission lines, electric motors, thermostats, bug zappers, etc. Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power An electric motor uses Electrical energy to produce Mechanical energy. A thermostat is a device for regulating the Temperature of a System so that the system's temperature is maintained near a desired setpoint A bug zapper is a device that attracts and kills Insects that are attracted by light Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resembles that of synchrotron sources, stronger at low frequencies and diminishing at higher frequencies, though this noise is often modulated, or varied, by the creating device in some way. This article concerns the physical phenomenon of synchrotron radiation In Telecommunications, modulation is the process of varying a periodic Waveform, i Included in this category are computers and other digital equipment as well as televisions. A computer is a Machine that manipulates data according to a list of instructions. A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic The rich harmonic content of these devices means that they can interfere over a very broad spectrum. In Acoustics and Telecommunication, the harmonic of a Wave is a component Frequency of the signal that is an Integer Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz Characteristic of broadband RFI is an inability to filter it effectively once it has entered the receiver chain. This article is about a radio receiver for other uses see Radio (disambiguation. ^[2][3]
[4]
Figure alongside shows various EMI sources and their classification based on electromagnetic spectrum.

Power line noise

Virtually all power-line noise originating from utility company equipment is caused by a spark or arcing across some power-line related hardware. A public utility (usually just utility) is an organization that maintains the Infrastructure for a public service (often also providing a service using A breakdown and ionization of air occurs, and current flows between two conductors in a gap. The gap may be caused by broken or loose hardware such as a cracked insulator. Typical culprits include inadequate hardware spacing such as a gap between a ground wire and a staple. Once an ionized path is established in the gap, current flows at all parts of the cycle where the voltage is higher than the breakdown voltage of the gap. Breakdown Voltage of an Insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. This typically occurs only at the positive and negative voltage peaks -- the times of highest instantaneous voltage throughout the cycle.

As an example for a 60Hz system (i. e. power-lines carrying 60 Hz AC, such as in the US), the voltage passes through two peaks each cycle (one positive and one negative) and pass through zero twice each cycle. This gives 120 peaks and 120 zero crossings in each second (50Hz: 100 peaks and crossings correspondingly). Power-line noise follows this pattern, generally occurring in bursts at a rate of 120 bursts per second. This gives power-line noise a characteristic sound that is often described as a harsh and raspy hum or buzz. Because the peaks occur twice per cycle, true power-line noise has a strong 120-Hz modulation on the signal (50Hz system: 100Hz). ^[5]

EMI in Integrated Circuits

Main article: Electromagnetic compatibility

Integrated circuits are often a source of EMI, but they are never the "antenna". Electromagnetic compatibility (EMC is the branch of electrical sciences which studies the unintentional generation propagation and reception of electromagnetic energy with reference to They must couple their energy to larger objects such as heatsinks, circuit board planes and cables to radiate significantly ^[6].

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^[7], and V_CC filtering. Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors In Electronics, when describing a Voltage or current Step function, rise time (also risetime) refers to the time required for a signal Almost all Integrated circuits (ICs have at least two pins which connect to the power rails of the circuit they are installed in Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like. Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz

The efficiency of the radiation depends on the height above the ground 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, harmonic or transient (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 V_CC and ground pins. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. 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. Differential pair is a pair of conductors with special characteristics used for Differential signaling. 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. A braid-breaker is a filter that prevents Television interference (TVI A choke is an Inductor designed to have a high reactance to a particular frequency when used in a signal-carrying circuit

At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. 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, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas used with broadcast systems. Amplitude modulation ( AM) is a technique used in electronic communication most commonly for transmitting information via a Radio Carrier wave A dipole antenna, developed by Heinrich Rudolph Hertz around 1886, is an antenna with a center- fed Driven element for transmitting Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi, error-correction techniques can be used. Wi-Fi (ˈwaɪfaɪ is the trade name for the popular wireless technology used In Mathematics, Computer science, Telecommunication, and Information theory, error detection and correction has great practical importance in Spread-spectrum and frequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. Spread-spectrum techniques are methods by which Energy generated in a particular bandwidth is deliberately spread in the Frequency domain, resulting Frequency-hopping spread spectrum ( FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using 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. In electromagnetics, directivity is a figure of merit for an antenna. The parabolic antenna is a high-gain reflector antenna used for radio television and data communications and also for radiolocation ( RADAR) on the UHF and Antenna diversity, also known as space diversity, is one in a superset of wireless Diversity schemes that utilizes two or more antennas to improve the quality and reliability

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. Spread-spectrum techniques are methods by which Energy generated in a particular bandwidth is deliberately spread in the Frequency domain, resulting Ultra-wideband (aka UWB, ultra-wide band, ultraband, etc is a radio technology that can be used at very low energy levels for short-range high-bandwidth 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. Ultra-wideband (aka UWB, ultra-wide band, ultraband, etc is a radio technology that can be used at very low energy levels for short-range high-bandwidth

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 US, Public Law 97-259, enacted in 1982, gave the FCC the authority to regulate the susceptibility of consumer electronic equipment sold in the United States. The United States of America —commonly referred to as the The FCC, working with equipment manufacturers, decided to allow them to develop standards for EMI immunity and implement their own voluntary compliance programs. ^[8]

Broadcast transmitters, two-way radio transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI. For the band see Broadcast (band Broadcasting is the distribution of audio and/or Video signals which transmit For biologic transmitters see Transmitter substance. A transmitter is an electronic device which usually with the aid of an antenna two-way radio is a radio that can both transmit and receive (a Transceiver) unlike a broadcast receiver which only receives content ^[9] 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. A toaster is a small electric Kitchen appliance designed to Toast slices of Bread. An electric blanket is a Blanket with an integrated Electrical Heating device usually placed above the top bed sheet A bug zapper is a device that attracts and kills Insects that are attracted by light 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. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or

Switched-mode power supply can be a source of EMI, but have become less of a problem as design techniques have improved, such as integrated power factor correction. A switched-mode power supply, switching-mode power supply or SMPS, is an electronic Power supply unit (PSU that incorporates a switching regulator

Most countries have legal requirements that mandates 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). Electromagnetic compatibility (EMC is the branch of electrical sciences which studies the unintentional generation propagation and reception of electromagnetic energy with reference to

Standards

The Special International Committee on Radio Interference (CISPR) sets standards for radiated and conducted electromagnetic interference. The Special International Committee on Radio Interference (abbreviated CISPR from the French name of the organization Comité international spécial des perturbations

See also

• Electromagnetic Radiation
• Radio receiver
• Twisted Pair
• Faraday Cage

References

External links

• ARRL, RFI
• Clemson Vehicular Electronics Laboratory: EMC Page
• Interference Handbook