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spark-generator

Electromagnetic compatibility

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.

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.

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 particluar 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.

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 victim, receptor or sink is shown in the figure below. Source and sink 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.

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 (also called common-impedance 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:

  • 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.

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 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 noise source and victim are separated by more than a wavelength. Source and victim act as radio antennas: the source emits an electromagnetic wave which propagates across the open space and is picked up by the victim.

EMC control

The control of electromagnetic intereference (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

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 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

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 airborne 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 in PCB Design, such as loops of circulating current or unbalanced transmission lines.

Emissions suppression

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.

Susceptibility hardening

Additional measures to reduce susceptibility include:

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

EMC testing

Testing is required to see if a particular device meets those legal requirements, and 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 are sometimes used, especially 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, 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 programme wmay 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 equipment under test (EUT).

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

Some electrostatic discharge testing is 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 an antenna so large that the EUT is placed inside it.

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 not usually relevant to overall emissions.

Typically a spectrum analyzer is used to measure the emission levels of the equipment under test (EUT) across a wide band of frequencies (frequency domain). For radiated interference this must be measured in all directions.

Some pulse emissions are more usefully characterised 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 and power lines. 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. Power stations were provided with output circuit breakers. Buildings and appliances would soon be provided with input fuses, and later 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 appear 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.

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. However 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.

With the increasing popularity of modern digital circuitry, an accompanying increase in their 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 draw up and safeguard 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.

Most 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 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.

References

EMC test equipment manufacturers (alphabetic)

See also

External links

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