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* '''[[Electromagnetic shielding|Shielded]] housings'''.
* '''[[Electromagnetic shielding|Shielded]] housings'''.
: In order to access the components, a housing is typically made in sections (such as a box and lid); an RF gasket is often used at the section joints to reduce the amount of interference that leaks through the joint. RF gaskets come in various types. One type is based on a waterproof flexible elastomeric base with chopped metal fibers dispersed into the interior or long metal fibers covering the surface or both. ("oriented wire gasket", "woven mesh gasket", etc.) Another type is RF fingerstock comprises springy metal "fingers".
: In order to access the components, a housing is typically made in sections (such as a box and lid); an RF gasket is often used at the section joints to reduce the amount of interference that leaks through the joint. RF gaskets come in various types. One type is based on a waterproof flexible elastomeric base with chopped metal fibers dispersed into the interior or long metal fibers covering the surface or both. ("oriented wire gasket", "woven mesh gasket", etc.) Another type is RF fingerstock comprises springy metal "fingers".
* '''Shielded cables''', where the conducting wires are surrouned by an outer conductive layer that is grounded at one or both ends.
* '''Shielded cables''', where the conducting wires are surrounded by an outer conductive layer that is grounded at one or both ends.
* Grounding or earthing schemes such as '''star earthing''' for audio equipment or '''ground planes''' for RF.
* Grounding or earthing schemes such as '''star earthing''' for audio equipment or '''ground planes''' for RF.


Revision as of 08:24, 28 January 2013

Anechoic RF chamber used for EMC testing (radiated emissions and immunity). The furniture has to be made of wood or plastic, and not metal
LPDA antenna measurement for outdoor

Electromagnetic compatibility (EMC) is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects (Electromagnetic interference, or EMI) that such energy may induce.

EMC aims to ensure that equipment items or systems will not interfere with or prevent each other's correct operation through spurious emission and absorption of EMI. EMC is sometimes referred to as EMI Control, and in practice EMC and EMI are frequently referred to as a combined term "EMC/EMI".

Introduction

While electromagnetic interference (EMI) is a phenomenon - the radiation emitted and its effects - electromagnetic compatibility (EMC) is an equipment characteristic or property - to not behave unacceptably in the EMI environment.

EMC ensures the correct operation, in the same electromagnetic environment, of different equipment items which use or respond to electromagnetic phenomena, and the avoidance of any interference effects. Another way of saying this is that EMC is the control of EMI so that unwanted effects are prevented.

EMC divides into a number of issues:

  • EMI is the radiation emitted and its effects on the victim.
  • Emission is the unwanted generation of electromagnetic energy by some emitter or source.
  • Susceptibility or Immunity is the ability of the receptor or victim equipment to operate correctly in the presence of electromagnetic disturbances. Susceptibility and immunity are opposites - an equipment which has high susceptibility has low immunity, and vice versa.
  • Coupling is the mechanisms by which EMI is able to travel from source to victim.

Besides understanding the phenomena in themselves, EMC also addresses the countermeasures, such as control regimes, design and measurement, which should be taken in order to prevent emissions from causing any adverse effect.

Types of interference

Main article: Electromagnetic interference

Electromagnetic interference divides into several categories according to the source and signal characteristics.

The origin of noise can be man made or natural.

Continuous interference

Continuous, or Continuous Wave (CW), interference arises where the source continuously emits at a given range of frequencies. This type is naturally divided into sub-categories according to frequency range, and as a whole is sometimes referred to as "DC to daylight".

  • Audio Frequency, from very low frequencies up to around 20 kHz. Frequencies up to 100 kHz may sometimes be classified as Audio. Sources include:
    • Mains hum from; power supply units, nearby power supply wiring, transmission lines and substations.
    • Audio processing equipment, such as audio power amplifiers and loudspeakers.
    • Demodulation of a high-frequency carrier wave such as an FM radio transmission.
  • Radio Frequency Interference (RFI), from typically 20 kHz to an upper limit which constantly increases as technology pushes it higher. Sources include:
    • Wireless and Radio Frequency Transmissions
    • Television and Radio Receivers
    • Industrial, scientific and medical equipment (ISM)
    • Digital processing circuitry such as microcontrollers
  • Broadband noise may be spread across parts of either or both frequency ranges, with no particular frequency accentuated. Sources include:

Pulse or transient interference

Electromagnetic Pulse, EMP, also sometimes called Transient disturbance, arises where the source emits a short-duration pulse of energy. The energy is usually broadband by nature, although it often excites a relatively narrow-band damped sine wave response in the victim.

Sources divide broadly into isolated and repetitive events.

  • Sources of isolated EMP events include:
    • Switching action of electrical circuitry, including inductive loads such as relays, solenoids, or electric motors.
    • Electrostatic Discharge (ESD), as a result of two charged objects coming into close proximity or even contact.
    • Lightning Electromagnetic Pulse (LEMP), although typically a short series of pulses.
    • Nuclear Electromagnetic Pulse (NEMP), as a result of a nuclear explosion.
    • Non-Nuclear Electromagnetic Pulse (NNEMP) weapons.
    • Power Line Surges/Pulses
  • Sources of repetitive EMP events, sometimes as regular pulse trains, include:
    • Electric Motors
    • Gasoline engine ignition systems
    • Continual switching actions of digital electronic circuitry.

Coupling mechanisms

Some of the technical words employed can be used with differing meanings. These terms are used here in a widely accepted way, which is consistent with other articles in the encyclopedia.

The basic arrangement of noise source, coupling path and victim, receptor or sink is shown in the figure below. Source and victim are usually electronic hardware devices, though the source may be a natural phenomenon such as a lightning strike, electrostatic discharge (ESD) or, in one famous case, the Big Bang at the origin of the Universe.

The four electromagnetic interference (EMI) coupling modes.

There are four basic coupling mechanisms: conductive, capacitive, magnetic or inductive, and radiative. Any coupling path can be broken down into one or more of these coupling mechanisms working together. For example the lower path in the diagram involves inductive, conductive and capacitive modes.

Conductive coupling

Conductive coupling occurs when the coupling path between the source and the receptor is formed by direct contact with a conducting body, for example a transmission line, wire, cable, PCB trace or metal enclosure.

Conducted noise is also characterised by the way it appears on different conductors:

  • Common-mode or common-impedance[1] coupling: noise appears in phase (in the same direction) on two conductors .
  • Differential-mode coupling: noise appears out of phase (in opposite directions) on two conductors .

Inductive coupling

Inductive coupling occurs where the source and receiver are separated by a short distance (typically less than a wavelength). Strictly, "Inductive coupling" can be of two kinds, electrical induction and magnetic induction. It is common to refer to electrical induction as capacitive coupling, and to magnetic induction as inductive coupling.

Capacitive coupling

Capacitive coupling occurs when a varying electrical field exists between two adjacent conductors typically less than a wavelength apart, inducing a change in voltage across the gap.

Magnetic coupling

Inductive coupling or magnetic coupling (MC) occurs when a varying magnetic field exists between two parallel conductors typically less than a wavelength apart, inducing a change in voltage along the receiving conductor.

Radiative coupling

Radiative coupling or electromagnetic coupling occurs when source and victim are separated by a large distance, typically more than a wavelength. Source and victim act as radio antennas: the source emits or radiates an electromagnetic wave which propagates across the open space in between and is picked up or received by the victim.

EMC control

The damaging effects of electromagnetic interference pose unacceptable risks in many areas of technology, and it is necessary to control such interference and reduce the risks to acceptable levels.

The control of electromagnetic interference (EMI) and assurance of EMC comprises a series of related disciplines:

  • Characterising the threat.
  • Setting standards for emission and susceptibility levels.
  • Design for standards compliance.
  • Testing for standards compliance.

For a complex or novel piece of equipment, this may require the production of a dedicated EMC control plan summarizing the application of the above and specifying additional documents required.

Characterising the threat

Characterisation of the problem requires understanding of:

  • The interference source and signal.
  • The coupling path to the victim.
  • The nature of the victim both electrically and in terms of the significance of malfunction.

The risk posed by the threat is usually statistical in nature, so much of the work in threat characterisation and standards setting is based on reducing the probability of disruptive EMI to an acceptable level, rather than its assured elimination.

Laws and regulators

Regulatory and standards bodies

Main article: List of EMC directives

Several international organizations work to promote international co-operation on standardization (harmonization), including publishing various EMC standards. Where possible, a standard developed by one organization may be adopted with little or no change by others. This helps for example to harmonize national standards across Europe. Standards organizations include:

Among the more well known national organizations are:

Laws

Compliance with national or international standards is usually required by laws passed by individual nations. Different nations can require compliance with different standards.

By European law, manufacturers of electronic devices are advised to run EMC tests in order to comply with compulsory CE-labeling. Undisturbed usage of electric devices for all customers should be ensured and the electromagnetic field strength should be kept on a minimum level. EU directive 2004/108/EC (previously 89/336/EEC) on EMC announces the rules for the distribution of electric devices within the European Union. A good overview of EME limits and EMI demands is given in List of EMC directives.

EMC design

A TV tuner card showing many small bypass capacitors, and 2 CCA shields -- a metal box shielding the coax inputs and another metal box shielding the S-Video connector
Main article: EMC problem (excessive field strength)

Electromagnetic noise is produced in the source due to rapid current and voltage changes, and spread via the coupling mechanisms described earlier.

Since breaking a coupling path is equally effective at either the start or the end of the path, many aspects of good EMC design practice apply equally to potential emitters and to potential victims. Further, a circuit which easily couples energy to the outside world will equally easily couple energy in and will be susceptible. A single design improvement often reduces both emissions and susceptibility.

Grounding and shielding

Grounding and shielding aim to reduce emissions or divert EMI away from the victim by providing an alternative, low-impedance path. Techniques include:

In order to access the components, a housing is typically made in sections (such as a box and lid); an RF gasket is often used at the section joints to reduce the amount of interference that leaks through the joint. RF gaskets come in various types. One type is based on a waterproof flexible elastomeric base with chopped metal fibers dispersed into the interior or long metal fibers covering the surface or both. ("oriented wire gasket", "woven mesh gasket", etc.) Another type is RF fingerstock comprises springy metal "fingers".
  • Shielded cables, where the conducting wires are surrounded by an outer conductive layer that is grounded at one or both ends.
  • Grounding or earthing schemes such as star earthing for audio equipment or ground planes for RF.

Other general measures

  • Decoupling or filtering at critical points such as cable entries and high-speed switches, using RF chokes and/or RC elements.
  • Transmission line techniques for cables and wiring, such as balanced differential signal and return paths, and impedance matching.
  • Avoidance of Antenna Structures, such as loops of circulating current, resonant mechanical structures, unbalanced cable impedances or poorly grounded shielding.

Emissions suppression

Spread spectrum method reduces EMC peaks. Frequency spectrum of the heating up period of a switching power supply which uses the spread spectrum method incl. waterfall diagram over a few minutes

Additional measures to reduce emissions include:

  • Avoid unnecessary switching operations. Necessary switching should be done as slowly as technically possible.
  • Noisy circuits (with a lot of switching activity) should be physically separated from the rest of the design.
  • High peaks can be avoided by using the spread spectrum method.
  • Harmonic Wave Filters.
  • Design for operation at lower signal levels, reducing the energy available for emission.

Susceptibility hardening

Additional measures to reduce susceptibility include:

  • Fuses, trip switches and circuit breakers.
  • Transient absorbers.
  • Design for operation at higher signal levels, reducing the relative noise level in comparison.

EMC testing

Testing is required to confirm that a particular device meets the required standards. It divides broadly into emissions testing and susceptibility testing.

RF testing of a physical prototype is most often carried out in a radio-frequency anechoic chamber.

Open-air test sites, or OATS, are the reference sites in most standards. They are especially useful for emissions testing of large equipment systems.

Sometimes computational electromagnetics simulations are used to test virtual models.

Like all compliance testing, it is important that the test equipment, including the test chamber or site and any software used, be properly calibrated and maintained.

Typically, a given run of tests for a particular piece of equipment will require an EMC test plan and follow-up Test report. The full test program may require the production of several such documents.

Susceptibility testing

Radiated field susceptibility testing typically involves a high-powered source of RF or EM pulse energy and a radiating antenna to direct the energy at the potential victim or device under test (DUT).

Conducted voltage and current susceptibility testing typically involves a high-powered signal or pulse generator, and a current clamp or other type of transformer to inject the test signal.

Transient immunity is used to test the immunity of the DUT against powerline disturbances including surges, lightning strikes and switching noise.[2] In motor vehicles, similar tests are performed on battery[3] and signal lines.[4]

Electrostatic discharge testing is typically performed with a piezo spark generator called an "ESD pistol". Higher energy pulses, such as lightning or nuclear EMP simulations, can require a large current clamp or a large antenna which completely surrounds the DUT. Some antennas are so large that they are located outdoors, and care must be taken not to cause an EMP hazard to the surrounding environment.

Emissions testing

Emissions are typically measured for radiated field strength and where appropriate for conducted emissions along cables and wiring. Inductive (magnetic) and capacitive (electric) field strengths are near-field effects, and are only important if the device under test (DUT) is designed for location close to other electrical equipment.

Typically a spectrum analyzer is used to measure the emission levels of the DUT across a wide band of frequencies (frequency domain). Specialized spectrum analyzers for EMC testing are available, called EMI Test Receivers or EMI Analyzers. These incorporate bandwidths and detectors as specified by international EMC standards. EMI Receivers along with specified transducers can often be used for both conducted and radiated emissions. Pre-selector filters may also be used to reduce the effect of strong out-of-band signals on the front-end of the receiver.

For conducted emissions, typical transducers include the LISN (Line Impedance Stabilisation Network) or AMN (Artificial Mains Network) and the RF current clamp.

For radiated emission measurement, antennas are used as transducers. Typical antennas specified include dipole, biconical, log-periodic, double ridged guide and conical log-spiral designs. Radiated emissions must be measured in all directions around the DUT.

Some pulse emissions are more usefully characterized using an oscilloscope to capture the pulse waveform in the time domain.

History

The earliest EMC issue was lightning strike (Lightning Electromagnetic Pulse, or LEMP) on buildings. Lightning rods or lightning conductors began to appear in the mid-18th century. With the advent of widespread electricity generation and power supply lines from the late 19th century on, problems also arose with equipment short-circuit failure affecting the power supply, and with local fire and shock hazard when the power line was struck by lightning. Power stations were provided with output circuit breakers. Buildings and appliances would soon be provided with input fuses, and later in the 20th century miniature circuit breakers (MCB) would come into use.

As radio communications developed in the first half of the 20th century, interference between broadcast radio signals began to occur and an international regulatory framework was set up to ensure interference-free communications.

As switching devices became commonplace, typically in petrol powered cars and motorcycles but also in domestic appliances such as thermostats and refrigerators, transient interference with domestic radio and (after World War II) TV reception became problematic, and in due course laws were passed requiring the suppression of such interference sources.

ESD problems first arose with accidental electric spark discharges in hazardous environments such as coal mines and when refuelling aircraft or motor cars. Safe working practices had to be developed.

After World War II the military became increasingly concerned with the effects of nuclear electromagnetic pulse (NEMP), lightning strike, and even high-powered radar beams, on vehicle and mobile equipment of all kinds, and especially aircraft electrical systems.

When high RF emission levels from other sources became a potential problem (such as with the advent of microwave ovens), certain frequency bands were designated for Industrial, Scientific and Medical (ISM) use, allowing unlimited emissions. A variety of issues such as sideband and harmonic emissions, broadband sources, and the increasing popularity of electrical switching devices and their victims, resulted in a steady development of standards and laws.

From the 1970s, the popularity of modern digital circuitry rapidly grew. As the technology developed, with faster switching speeds (increasing emissions) and lower circuit voltages (increasing susceptibility), EMC increasingly became a source of concern. Many more nations became aware of EMC as a growing problem and issued directives to the manufacturers of digital electronic equipment, which set out the essential manufacturer requirements before their equipment could be marketed or sold. Organizations in individual nations, across Europe and worldwide, were set up to maintain these directives and associated standards. This regulatory environment led to a sharp growth in the EMC industry supplying specialist devices and equipment, analysis and design software, and testing and certification services.

Low-voltage digital circuits, especially CMOS transistors, became more susceptible to ESD damage as they were miniaturised, and a new ESD regulatory regime had to be developed.

From the 1980s, the ever-increasing use of mobile communications and broadcast media channels has put huge pressure on the available airspace. Regulatory authorities are squeezing band allocations closer and closer together, relying on increasingly sophisticated EMC control methods, especially in the digital communications arena, to keep cross-channel interference to acceptable levels. Digital systems are inherently less susceptible than the old analog systems, and also offer far easier ways (such as software) to implement highly sophisticated protection measures.

Most recently, even the ISM bands are being used for low-power mobile digital communications. This approach relies on the intermittent nature of ISM interference and use of sophisticated error-correction methods to ensure lossless reception during the quiet gaps between bursts of interference.

EMC test equipment manufacturers (alphabetic)

See also

References

  1. ^ LearnEMC Web Site: Common-Impedance Coupling.
  2. ^ EMC Testing and Standards in Transient Immunity Testing, RF Immunity. Electronics-project-design.com. Retrieved on 2011-07-19.
  3. ^ ISO 7637-2:2004/Amd 1:2008. Iso.org (2011-03-01). Retrieved on 2011-07-19.
  4. ^ ISO 7637-3:2007 – Road vehicles – Electrical disturbances from conduction and coupling – Part 3: Electrical transient transmission by capacitive and inductive coupling via lines other than supply lines. Iso.org (2010-09-06). Retrieved on 2011-07-19.
The Wikibook Practical Electronics has a page on the topic of: EMC design guidelines

Web sites

General introductions

Specific topics

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