Electromagnetic interference
Encyclopedia
Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation
emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. These effects can range from a simple degradation of data to a total loss of data. 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
.
EMI can be intentionally used for radio jamming
, as in some forms of electronic warfare
, or can occur unintentionally, as a result of spurious emission
s for example through intermodulation
products, and the like. 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.
and broadband
.
Narrowband interference usually arises from intentional transmissions such as radio and TV stations
, pager transmitters, cell phones, etc. Broadband interference usually comes from incidental radio frequency
emitters. These include electric power transmission
lines, electric motor
s, thermostat
s, bug zapper
s, etc. Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resemble 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. Included in this category are computer
s and other digital
equipment as well as television
s. The rich harmonic
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 chain.
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.
, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antenna
s 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.
, Public Law 97-259, enacted in 1982, gave the Federal Communications Commission
(FCC) 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.
Broadcast
transmitter
s, two-way radio
transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI. Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blanket
s, ultrasonic pest control devices, electric bug zapper
s, heating pad
s, 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 monitor
s and cordless telephone
s, video sender
s, and microwave oven
s.
Switch
ing inductive
loads, such as electric motor
s, 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 ohm
s in series with 100 nanofarad
s 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).
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) 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. And although there may be additional costs involved for some products to give them a known level of immunity, it increases their perceived quality as they are able to co-exist with apparatus in the active EM environment of modern times and with fewer problems.
Many countries now have similar requirements for products to meet some level of EMC regulation.
On integrated circuit
s, important means of reducing EMI are: the use of bypass or decoupling capacitor
s 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, and VCC
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
(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 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 pair
s. 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.
amplifier, speaker
amplifier, car radio, telephones etc. Adding on-board EMI filters or special layout techniques help in bypassing EMI or improving RF immunity.
Some ICs are designed (e.g., LMV831-LMV834, MAX9724) to have integrated RF filters and/or special design that prevent demodulation of high-frequency carrier. These ICs are also subjected to tests for measuring their capability to reject RF.
Designers often need to carry out special tests for testing the RF immunity of the parts to be used in the system. These tests are usually carried out inside a special anechoic chamber with a controlled RF environment where the test vectors produce an RF field similar to that produced in an actual environment.
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. These effects can range from a simple degradation of data to a total loss of data. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
or the Northern Lights
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
.
EMI can be intentionally used for radio jamming
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 first checking whether it is in use, or without being able to hear stations using the frequency...
, as in some forms of electronic warfare
Electronic warfare
Electronic warfare refers to any action involving the use of the electromagnetic spectrum or directed energy to control the spectrum, attack an enemy, or impede enemy assaults via the spectrum. The purpose of electronic warfare is to deny the opponent the advantage of, and ensure friendly...
, or can occur unintentionally, as a result of spurious emission
Spurious emission
A spurious emission is any radio frequency not deliberately created or transmitted, especially in a device which normally does create other frequencies...
s for example through intermodulation
Intermodulation
Intermodulation or intermodulation distortion is the amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities...
products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
reception
Television antenna
A television antenna, or TV aerial, is an antenna specifically designed for the reception of over the air broadcast television signals, which are transmitted at frequencies from about 41 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries...
, although to a lesser extent.
Types
Radiated EMI or RFI may be broadly categorized into two types; narrowbandNarrowband
In radio, narrowband describes a channel in which 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.The opposite of...
and broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
.
Narrowband interference usually arises from intentional transmissions such as radio and TV stations
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
, pager transmitters, cell phones, etc. Broadband interference usually comes from incidental radio frequency
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
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
lines, electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s, thermostat
Thermostat
A thermostat is the component of a control system which regulates 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...
s, bug zapper
Bug zapper
A bug zapper, or more formally an electrical discharge insect control system is a device that attracts and kills flying insects that are attracted by light. A light source attracts insects to an electrical grid, where they are electrocuted by touching two wires with a high voltage between them...
s, etc. Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resemble that of synchrotron
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
sources, stronger at low frequencies and diminishing at higher frequencies, though this noise is often modulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
, or varied, by the creating device in some way. Included in this category are computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s and other digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
equipment as well as television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
s. The rich harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
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 chain.
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.
Susceptibilities of different radio technologies
Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulationAmplitude 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...
, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
s used with broadcast systems.
Newer radio systems incorporate several improvements that enhance the selectivity
Selectivity
Selectivity may refer to:* Selectivity , in radio transmission* Binding selectivity, in pharmacology* Functional selectivity, in pharmacology* Socioemotional selectivity theory, in social psychology...
.
In digital radio systems, such as Wi-Fi
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
, 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
Directivity
In electromagnetics, directivity is a figure of merit for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator radiating the same total power.An antenna's directivity is a...
receiver, such as a parabolic antenna
Parabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish...
or a diversity receiver
Antenna diversity
Antenna diversity, also known as space diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and indoor environments, there is no clear line-of-sight between transmitter and...
, 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
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...
), 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
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...
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 StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Public Law 97-259, enacted in 1982, gave the Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) 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
Voluntary compliance
Voluntary compliance is one of possible ways of practicing corporate social responsibility.Voluntary compliance is seen as an alternative to the state-imposed regulations on company's behavior...
programs.
Broadcast
Broadcast
Broadcast or Broadcasting may refer to:* Broadcasting, the transmission of audio and video signals* Broadcast, an individual television program or radio program* Broadcast , an English electronic music band...
transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
s, two-way radio
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI. Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blanket
Electric blanket
In the US the electric blanket is a blanket with an integrated electrical heating device usually placed above the top bed sheet. In the UK and Commonwealth, electric blanket commonly refers to an electric mattress pad, which is placed below the bottom bed sheet. Electric blankets usually have a...
s, ultrasonic pest control devices, electric bug zapper
Bug zapper
A bug zapper, or more formally an electrical discharge insect control system is a device that attracts and kills flying insects that are attracted by light. A light source attracts insects to an electrical grid, where they are electrocuted by touching two wires with a high voltage between them...
s, heating pad
Heating pad
A heating pad is a pad used for warming of parts of the body in order to manage pain. Localized application of heat causes the blood vessels in that area to dilate, enhancing perfusion to the targeted tissue...
s, and touch controlled lamps. Multiple CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
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
Degaussing
Degaussing is the process of decreasing or eliminating an unwanted magnetic field. It is named after Carl Friedrich Gauss, an early researcher in the field of magnetism...
coils is activated.
Electromagnetic interference at 2.4 GHz
Electromagnetic interference at 2.4 GHz
Electromagnetic interference at 2.4 GHz can affect various devices.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.-Phone:...
can be caused by 802.11b and 802.11g wireless devices, Bluetooth
Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
devices, baby monitor
Baby monitor
A baby monitor, also known as a baby alarm, is a radio system used to remotely listen to sounds made by an infant. The transmitter unit, equipped with a microphone, is placed near to the child. It transmits the sounds the baby makes by radio waves to a receiver unit with a speaker carried by, or...
s and cordless telephone
Cordless telephone
A cordless telephone or portable telephone is a telephone with a wireless handset that communicates via radio waves with a base station connected to a fixed telephone line, usually within a limited range of its base station...
s, video sender
Video sender
A video sender is a device for transmitting domestic video signals wirelessly from one room to another, as for example sending the output of a satellite TV decoder located in the lounge, to a television set in the bedroom....
s, and microwave oven
Microwave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...
s.
Switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
ing inductive
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
loads, such as electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s, often cause interference, but it is easily suppressed by connecting a snubber
Snubber
A snubber is a device used to suppress voltage transients in electrical systems, pressure transients in fluid systems, or excess force or rapid movement in mechanical systems.-Electrical systems:...
network, a resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
in series with a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, across the switch. Exact values can be optimised for each case, but 100 ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s in series with 100 nanofarad
Farad
The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
s is usually satisfactory.
Switched-mode power supplies
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
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
Electromagnetic compatibility
Electromagnetic compatibility is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects that such energy may induce...
: 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).
History
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
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
(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
Electromagnetic compatibility
Electromagnetic compatibility is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects that such energy may induce...
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) 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. And although there may be additional costs involved for some products to give them a known level of immunity, it increases their perceived quality as they are able to co-exist with apparatus in the active EM environment of modern times and with fewer problems.
Many countries now have similar requirements for products to meet some level of EMC regulation.
Standards
The International Special Committee for Radio Interference or CISPR (French acronym for "Comité International Spécial des Pertubations 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 regional and national standards most notably the European Norms (EN)written by CENELEC (European committee for electrotechnical standardisation).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.On integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s, important means of reducing EMI are: the use of bypass or decoupling capacitor
Decoupling capacitor
A decoupling capacitor is a capacitor used to decouple one part of an electrical network from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit....
s on each active device (connected across the power supply, as close to the device as possible), rise time
Rise time
In electronics, when describing a voltage or current step function, rise time refers to the time required for a signal to change from a specified low value to a specified high value...
control of high-speed signals using series resistors, and VCC
IC power supply pin
Almost all integrated circuits have at least two pins that connect to the power rails of the circuit in which they are installed. These are known as the IC's power supply pins...
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
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
or power plane
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.- Uses :...
(at RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
, one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental frequency
Fundamental frequency
The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the lowest frequency of a periodic waveform. In terms of a superposition of sinusoids The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the...
, harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
or transient
Transient (oscillation)
A transient event is a short-lived burst of energy in a system caused by a sudden change of state.The source of the transient energy may be an internal event or a nearby event...
(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 ground pins
IC power supply pin
Almost all integrated circuits have at least two pins that connect to the power rails of the circuit in which they are installed. These are known as the IC's power supply pins...
. The RF is then coupled to the cable through the line driver as common-mode noise
Common-mode interference
In telecommunication, the term common-mode interference has the following meanings:#Interference that appears on both signal leads , or the terminals of a measuring circuit, and ground....
. Since the noise is common-mode, shielding has very little effect, even with differential pair
Differential pair
A differential pair is a pair of conductors used for differential signaling. Differential pairs are usually found on a printed circuit board, in cables , and in connectors...
s. The RF energy is capacitively coupled
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker
Braid-breaker
A braid-breaker is a filter that prevents television interference . In many cases of TVI, caused by a high field strength of a nearby high frequency transmitter, the aerial down lead plugged into the back of the TV acts as a longwire antenna or as a simple vertical element...
or choke
Choke (electronics)
A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...
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
Integrated circuits tend to demodulate high-frequency carrier signals commonly found in regular environment due to presence of cell phones. These ICs demodulate the high frequency cell phone carrier (e.g., GSM850 and GSM1900, GSM900 and GSM1800) and produce low-frequency (e.g., 217 Hz) demodulated signals. This demodulation manifests itself into unwanted audible buzz in audio appliances such as microphoneMicrophone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
amplifier, speaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
amplifier, car radio, telephones etc. Adding on-board EMI filters or special layout techniques help in bypassing EMI or improving RF immunity.
Some ICs are designed (e.g., LMV831-LMV834, MAX9724) to have integrated RF filters and/or special design that prevent demodulation of high-frequency carrier. These ICs are also subjected to tests for measuring their capability to reject RF.
Designers often need to carry out special tests for testing the RF immunity of the parts to be used in the system. These tests are usually carried out inside a special anechoic chamber with a controlled RF environment where the test vectors produce an RF field similar to that produced in an actual environment.
Electromagnetic interference manufacturers
- MILMEGAMILMEGAMILMEGA is a company specializing in designing and manufacturing solid state, high power amplifiers for Electromagnetic compatibility testing...
- ON SemiconductorON SemiconductorON Semiconductor , is a semiconductors supplier company. Products include power and signal management, logic, discrete and custom devices for automotive, communications, computing, consumer, industrial, LED lighting, medical, military/aerospace and power applications...
- MTK Electronics, Inc.
See also
- Electromagnetic RadiationElectromagnetic radiationElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
- Faraday CageFaraday cageA Faraday cage or Faraday shield is an enclosure formed by conducting material or by a mesh of such material. Such an enclosure blocks out external static and non-static electric fields...
- Radio receiver
- Signal noise
- Twisted PairTwisted pairTwisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
- InterferenceInterference (communication)In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
External links
- ARRL, RFI
- Interference Handbook
- EMC Design Fundamentals
- EMSCAN Corporation
- Introduction to EMC
- Clemson's EMC Page (EMI Tools and Information)
- Interference Technology Magazine
- Radiometrics Midwest Corporation, EMI/EMC Testing & Engineering Services
- Compatible Electronics / EMI Testing, certification, & training
- EMC Tutorials