Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage to operational equipment. The primary goal of EMC is the correct operation of equipment in a common electromagnetic environment.
An EMC engineer, an associated discipline within the electrical engineering, pursues mitigating EMI, resulting in improved electromagnetic compatibility, in products by focusing on three main EMC hazard classes:
Emission is the generation of electromagnetic energy, whether deliberate or accidental, from a source and its release into the environment. This hazard class can be mitigated by quieting the sources of interference. Susceptibility is the tendency of electrical equipment, referred to as the victim, to malfunction or break down in the presence of emissions, also collectively known as radio-frequency interference (RFI). Immunity is the opposite of susceptibility, being the ability of equipment to function correctly in the presence of RFI, with the discipline of "hardening" equipment focusing on reducing susceptibility or immunity. Coupling is the mechanism by which emitted interference reaches the victim. This hazard can be mitigated by inhibiting coupling paths.
EMI in practice is typically mitigated by using broad based approaches that may impact all three hazard classes, such as grounding and shielding.
After the unit is built, various EMS test standards may need to be met before entering the unit for sale into a country's market.
History of Electromagnetic Environment and EMC Susceptibility
Origins
The earliest EMC susceptibility issue was lightning strike (lightning electromagnetic pulse, or LEMP) on ships and buildings. Lightning rods or lightning conductors began to appear in the mid-18th century to carry the energy to ground and reduce damage.
With the advent of widespread electricity generation and power transmission lines from the late 19th century on, lighting strikes to these power lines caused fire and shock hazards to the transmission lines, along with possibly creating short-circuit failure on connected equipment. In the late 19th century, power stations, buildings circuits and even individual appliances were provided with fuses that reduced the potential for damage. Later in the mid-20th century miniature circuit breakers (MCB) would come into popular use to replace fuses for EMC purposes.
Early twentieth century
Radio interference and the need for mitigation arose with the first spark-gap experiment of Marconi in the late 1800s. As radio communications developed in the first half of the 20th century, interference between multiple broadcast radio signals began to occur and an international regulatory framework was set up by the international telegraph union to ensure interference-free communications.
Switching devices, distributors, relays, etc., became commonplace through the middle of the 20th century, typically in petrol powered cars and motorcycles but also in domestic appliances such as thermostats and refrigerators. This caused transient interference with domestic radio and (after World War II) TV reception, and in due course laws were passed requiring the suppression of such interference sources.
EMI that could cause electrostatic discharge (ESD) problems first arose with accidental electric spark discharges in hazardous environments such as coal mines and when refueling aircraft or motor cars. Safe working practices had to be developed.
Postwar period
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.
In 1947, 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 emission levels limited only by thermal heating safety standards. Later, the International Telecommunication Union adopted a Recommendation providing limits of radiation from ISM devices in order to protect radio communications. A variety of issues such as sideband and harmonic emissions, broadband sources, and the ever-increasing popularity of electrical switching devices and their victims, resulted in a steady development of standards and laws.
From the late 1970s, when the popularity of modern digital circuitry technology rapidly grew due to the information age and microelectronics, EMC increasingly became a source of concern due to ever-faster microprocessor switching frequencies over 1 MHz (increasing emissions) and below 5VDC circuit voltages (increasing susceptibility). 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. In 1979, the American FCC published a regulation that required the electromagnetic emissions of all "digital devices" to be below certain limits. 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, despite the development of on-chip hardening techniques, a new ESD regulatory regime had to be developed.
Modern era
From the 1980s on the explosive growth in mobile communications and broadcast media channels put huge pressure on the available airspace. Regulatory authorities began squeezing band allocations closer and closer together, relying on increasingly sophisticated EMC control methods, especially in the digital communications domain, to keep cross-channel interference to acceptable levels. Digital systems are inherently less susceptible than analogue systems, and also offer far easier ways (such as software) to implement highly sophisticated protection and error-correction measures.
In 1985, the USA released the ISM bands for low-power mobile digital communications, leading to the development of Wi-Fi and remotely-operated car door keys. 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 any bursts of interference.
Concepts
"Electromagnetic interference" (EMI) is defined as the "degradation in the performance of equipment or transmission channel or a system caused by an electromagnetic disturbance" ( 161–01–06) while "electromagnetic disturbance" is defined as "an electromagnetic phenomenon that can degrade the performance of a device, equipment or system, or adversely affect living or inert matter (IEV 161-01-05). The terms "electromagnetic disturbance" and "electromagnetic interference" designate respectively the cause and the effect,
Electromagnetic compatibility (EMC) is an equipment characteristic or property and is defined as " the ability of equipment or a system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment " (IEV 161–01–07).
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. Another way of saying this is that EMC is the control of EMI so that unwanted effects are prevented.
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.
Technical characteristics of interference
Types of interference EMC is often understood as the control of electromagnetic interference (EMI). Electromagnetic interference divides into several categories according to the source and signal characteristics.
The origin of interference, often called "noise" in this context, can be man-made (artificial) or natural.
Continuous, or continuous wave (CW), interference comprises 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". One common classification is into narrowband and broadband, according to the spread of the frequency range.
An electromagnetic pulse (EMP), sometimes called a transient disturbance, is a short-duration pulse of energy. This energy is usually broadband by nature, although it often excites a relatively narrow-band damped sine wave response in the victim. Pulse signals divide broadly into isolated and repetitive events.
Coupling mechanisms
When a source emits interference, it follows a route to the victim known as the coupling path. 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.
Conductive coupling occurs when the coupling path between the source and victim is formed by direct electrical contact with a conducting body.
Capacitive coupling occurs when a varying electrical field exists between two adjacent conductors, inducing a change in voltage on the receiving conductor.
Inductive coupling or magnetic coupling occurs when a varying magnetic field exists between two parallel conductors, inducing a change in voltage along the receiving conductor.
Radiative coupling or electromagnetic coupling occurs when source and victim are separated by a large distance. Source and victim act as radio antennas: the source emits or radiates an electromagnetic wave which propagates across the space in between and is picked up or received by the victim.
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.
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.
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.
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.
Design
Breaking a coupling path is equally effective at either the start or the end of the path, therefore many aspects of good EMC design practice apply equally to potential sources and to potential victims. A design which easily couples energy to the outside world will equally easily couple energy in and will be susceptible. A single improvement will often reduce both emissions and susceptibility.