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Electromagnetic compatibility

In the field of electrical engineering, Electromagnetic compatibility (EMC) refers to the ability of an electrical device to work satisfactorily in its electromagnetic environment without adversely either influencing the surrounding devices, or being influenced by them.

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1 Methods
2 Coupling Mechanisms
3 Types of Interference
4 EMC Design
5 Laws and regulators
6 History
7 See also

Methods

Electromagnetic compatibility (EMC) is also used to describe the branch of electrical sciences which studies the related effects, such as Electromagnetic Interference (EMI). The goal of the EMC discipline is the correct operation, in a common 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, 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 in the presence of unplanned electromagnetic disturbances.

Interference, or noise, mitigation and hence electromagnetic compatibility is achieved by addressing both emission and susceptibility issues, i.e., quieting the sources of interference, making the coupling path between source and victim less efficient, and making the potential victim systems less vulnerable.

Coupling Mechanisms

Some of the technical words employed can be used with differing meanings. The 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 receptor (sink) is shown the figure below. Source and sink are usually electronic hardware devices, though the source may be something more exotic such as a lightning strike, electrostatic discharge (ESD) or, in one famous case, the Big Bang at the origin of the Universe.

Any such coupling path can be broken down into one or more coupling mechanisms working together. There are four basic mechanisms: conductive, capacitive, magnetic or inductive, and radiative coupling.

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, or PCB trace).

Inductive coupling is sometimes regarded as a form of conductive coupling.

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 commonplace 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, such as PCB traces, typically less than a wavelength apart.

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 voltage across the receiving conductor.

Radiative Coupling

Radiative coupling or electromagnetic coupling occurs when noise source and sink are separated by more than a wavelength. Source and sink act as radio antennnas: the source emits an electromagnetic wave which propagates across the open space and is received by the sink.

Types of Interference

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

The origin of noise can be man made or natural. Examples are:

Lightning Electromagnetic Pulse (LEMP)
Nuclear Electromagnetic Pulse (NEMP)
Power Line Surges/Pulses
Power Transmission and Substations
Electrostatic Discharge (ESD)
Electric Fast Transient/Bursts (EFT)
Electric Motors
High Frequency Circuit Signals (For example microcontroller activity)
Television and Radio Receivers
Wireless and Radio Frequency Transmissions
Solar Activity

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.
Radio Frequency Interference, RFI, from 20 kHz to a limit which constantly increases as technology pushes it higher.

Pulse or Transient Interference

Electromagnetic Pulse, EMP, also sometimes called Transient disturbance, arises from:

Switching action of electrical circuitry.
Electrostatic Discharge, ESD.
Lightning strike.
Nuclear Electromagnetic Pulse, NEMP as a result of a nuclear explosion.
Non-Nuclear Electromagnetic Pulse, NNEMP weapons.

Conduction Modes

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

Differential-Mode noise appears on two conductors in the opposite direction to each other.
Common-Mode noise appears on two conductors in the same direction.

EMC Design

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.

Typical design measures include:

Shielded Lines.
Decoupled Cable Entries (Line filter, Signal filter) using RF chokes, or RC elements.
Shielded Housings.
Avoidance of Antenna Structures in PCB Design, such as loops of circulating current or unbalanced transmission lines.

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.
Harmonic Wave Filters.

Additional measures to reduce susceptibility include:

Fuses, trip switches and circuit breakers.
Transient absorbers.

Laws and regulators

Among the more well known national organizations are:

The Federal Communications Commission for the United States; CEN (Comité Européen de Normalisation or European Committee for Standardization); CENELEC (Comité Européen de Normalisation Electrotechniques or European Committee for Electrotechnical Standardization); ETSI (European Telecommunications Standards Institute) for Europe; and BSI (British Standards Institution) for Britain.

There are also several international organizations who try "to promote international co-operation on all questions of standardization" (harmonization), including EMC standards.

The most important international organization is the International Electrotechnical Commission (IEC), which has several committees working full time on EMC issues.

These are Technical Committee 77 ("TC77") working on "electromagnetic compatibility between equipment including networks", and the CISPR (Comité international spécial des perturbations radioélectriques or International Special Committee on Radio Interference).

Co-ordination of the IEC's work on EMC between these committees is the responsibility of the ACEC, the advisory committee on EMC.

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 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.

History

In the past, outside of the military field, EMC was not of paramount importance to equipment manufacturers and there was a relaxed EMC regime. Power input fuses on electrical appliances was just about it for the commercial world. Where high emission levels were a potential problem (such as microwave ovens), equipment manufacturers could usually avoid emissions limits through the choice of certain unlimited Industrial, Scientific and Medical (ISM) frequency bands.

With the proliferation of the clocks that synchronize modern digital circuits, a concomitant increase in their switching speeds (increasing emissions), and lower circuit voltages (increasing susceptibility), EMC increasingly became a source of concern. Many 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 each nation and across Europe were set up to draw up and safeguard these directives.

More recently, 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 processing 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.