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Electromagnetic compatibility

 
Sci-Tech Dictionary: electromagnetic compatibility
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(i¦lek·trō·mag′ned·ik kəm′pat·ə′bil·əd·ē)

(electronics) The capability of electronic equipment or systems to be operated in the intended electromagnetic environment at design levels of efficiency.

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Sci-Tech Encyclopedia: Electromagnetic compatibility
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The situation in which electrical and electronic devices and systems work as intended, both within themselves and in their electromagnetic environment.

Electromagnetic interference (EMI) is said to exist when unwanted voltages or currents are present so that they adversely affect the performance of a device or system. Such voltages or currents may reach the victim circuit or device by conduction or by nonionizing radiation. In all cases, electromagnetic interference arises because of a combination of three factors: a source, a transmission path, and a response, at least one of which is unplanned. Electromagnetic interference control refers to the process of making design changes or adjustments of signal or noise levels in order to achieve electromagnetic compatibility (EMC).

The purpose of shielding is to confine radiated energy to a specific region, or to prevent radiated energy from entering a specific region. The most effective shield is a solid metallic enclosure, made of a permeable metal (for example, iron or steel) if frequencies below 100 kHz are to be shielded, or of any metal if higher frequencies are to be shielded. However, the solid shield does not permit light, air, water, or other substances to be passed through it, so shields with holes, including screens, braids, and honeycomb arrangements, as well as conductive glass may be needed. The widespread use of plastic enclosures has made thin film shields vital in achieving the needed shielding effectiveness in the use of such enclosures. See also Electrical shielding; Magnetic materials.

An electrical filter offers relatively little opposition to the passage of certain frequencies or direct current (dc) while blocking the passage of other frequencies. Accordingly, filters play a significant role in reducing conducted interference to the extent that such interference has a spectral content different from that of the desired signals.

A filter may be either reflective or lossy. Reflective filters present an impedance mismatch to unwanted frequencies, thereby returning them to the input, whereas lossy filters absorb unwanted frequencies. A filter may be designed on a time-domain basis as well as on a frequency-domain basis. See also Electric filter; Impedance matching.

Digital systems, such as computers, tend to interfere with analog systems, such as voice and video communications, more readily than analog systems interfere with digital systems. Therefore, data streams to be transmitted over analog voice circuits are converted to a quasianalog tone form first. Computer clocks also may have to be shielded and their output circuits may have to be filtered to prevent interference to communication equipment. In addition, personal computers must be connected to television receivers so that the video output of the personal computer does not reach the television receiving antenna, which then would radiate such signals. See also Electrical noise.

Military Dictionary: electromagnetic compatibility
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(DOD) The ability of systems, equipment, and devices that utilize the electromagnetic spectrum to operate in their intended operational environments without suffering unacceptable degradation or causing unintentional degradation because of electromagnetic radiation or response. It involves the application of sound electromagnetic spectrum management; system, equipment, and device design configuration that ensures interference-free operation; and clear concepts and doctrines that maximize operational effectiveness. Also called EMC. See also electromagnetic spectrum; electronic warfare; spectrum management.

Wikipedia: Electromagnetic compatibility
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Anechoic RF chamber used for EMC testing (radiated emissions and immunity)

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. The goal of EMC is the correct operation, in the same electromagnetic environment, of different equipment which use electromagnetic phenomena, and the avoidance of any interference effects.

In order to achieve this, EMC pursues two different kinds of issues. Emission issues are related to the unwanted generation of electromagnetic energy by some source, and to the countermeasures which should be taken in order to reduce such generation and to avoid the escape of any remaining energies into the external environment. Susceptibility or immunity issues, in contrast, refer to the correct operation of electrical equipment, referred to as the victim, in the presence of unplanned electromagnetic disturbances.

Interference, or noise, mitigation and hence electromagnetic compatibility is achieved primarily by addressing both emission and susceptibility issues, i.e., quieting the sources of interference and hardening the potential victims. The coupling path between source and victim may also be separately addressed to increase its attenuation.

Contents

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 Interference arises where the source regularly emits 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.
  • Radio Frequency Interference, RFI, from 20 kHz to a 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
    • High Frequency Circuit Signals (For example microcontroller activity)
  • 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:

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 Wikipedia pages.

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.

Conduction modes

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

  • Common-mode or common-impedance[1]) coupling: noise appears on two conductors in the same direction.
  • Differential-mode coupling: noise appears on two conductors in the opposite direction to each other.

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 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 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 piece of equipment, this may require the production of a dedicated EMC control plan summarising 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. These 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/CE (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

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 divert EMI away from the victim by providing an alternative, low-impedance path. Techniques include:

  • Shielded Housings.
  • Shielded Lines.
  • Grounding or earthing schemes such as Star Earthing for audio equipment, or Ground planes for RF.

Other general measures

  • Decoupled Cable Entries (Line filter, Signal filter) using RF chokes, 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, unbalanced transmission lines 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 view 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.

Susceptibility hardening

Additional measures to reduce susceptibility include:

  • Fuses, trip switches and circuit breakers.
  • Transient absorbers.

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 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. An 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 are the LISN (Line Impedence Stabilisation Network) also sometimes called as the AMN (Artificial Mains Network) and the RF current probe.

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. 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.

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 mobile vehicles of all kinds, and especially aircraft electrical systems.

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

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 1970's, 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 growing 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 1980's, 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 design methods, especially in the digital communications arena, to keep cross-channel interference to acceptable levels. Digital systems are inherently less susceptible than the old analogue 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. ^ Clemson Vehicular Electronics Laboratory Web Site: Common-Impedance Coupling.
  2. ^ http://www.electronics-project-design.com/EMCTesting.html
  3. ^ http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=45931
  4. ^ http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=40581

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Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Military Dictionary. US Department of Defense Dictionary of Military and Associated Words, 2003.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Electromagnetic compatibility".  Read more