The degree to which an electronic system is able to function compatibly with other electronic systems and not be susceptible to or produce interference. Such interference is commonly termed electromagnetic interference (EMI). If the frequency of the interference lies within the radiofrequency part of the electromagnetic spectrum, it is termed radiofrequency interference (RFI).
A system may be electromagnetically incompatible with other systems in a number of ways. Conducted/radiated emissions are said to occur when the system unintentionally produces signals that are conducted/radiated away from itself. These emissions may cause interference to other systems. The system is said to suffer from conducted/radiated susceptibility if signals that are present in its environment are transferred in or onto the system through conduction/radiation and cause the system to fail. In general, conducted interference travels along cables, while radiated interference travels through the space between source and victim. Conducted interference tends to be more important at lower frequencies (below about 50 MHz), and the radiated interference at higher frequencies.
It is important to note that a system may and probably will receive unintentional signals that do not cause it to fail. In this case the system is electromagnetically compatible with its environment – it is only when it fails to function in its intended manner that it constitutes interference. If a system does not fail under the presence of an unintentional signal, it is said to possess immunity from electromagnetic interference.
Generally, the failure of a system takes place when an unintentional signal passes into the system and interacts with an active device in that system. These devices usually interact with each other by way of transmission lines carrying differential-mode currents, where the current is carried into the device along one line and returned along the other. Ideally, differential-mode currents are equal in magnitude and oppositely directed along each line of the transmission line. In practice, the transmission line will not be exactly balanced, neither geometrically nor electrically, giving rise to common-mode currents that flow with equal magnitude in the same direction along each of the lines.
The presence of common-mode currents along a transmission line can significantly increase the radiated energy from that line, despite the relatively small size of these currents compared to differential-mode currents, because the transmission line is a more efficient radiator of common-mode signals than differential-mode. They thus considerably enhance the radiated interference from a system. Because an efficient radiator is also an efficient receiver of energy, common-mode currents are also easily induced on transmission lines compared to differential-mode currents. The nonideal nature of the transmission lines then ensures that some of the energy in the common-mode currents is converted to differential-mode current and passes into the terminating devices where it may cause failure.
A means of reducing the effects of common-mode radiation or reception is to use common-mode chokes, typically made of ferrite beads, which are placed around cabling and prevent the flow of common-mode currents while allowing the normal operation of the transmission line.
Another common failure mode is when a system is subject to electrostatic discharge (ESD) in which a burst of charge is transferred to the system causing it to fail, typically irrecoverably due to damage of the semiconductor devices. A typical source of ESD is from human contact where the peak voltage during the burst may approach 10 kV. It is particularly important to guard against ESD when assembling or replacing electronic components. In this case a wrist strap is often worn; this consists of a conducting lead with one end earthed to the same earth as the equipment being handled and the other end wrapped around the wrist of the person working on the equipment to make a good electrical contact. This ensures that both the person and the equipment are at the same potential and prevents the relative build-up of charge.
A similar type of failure mode can occur due to electromagnetic pulse effects from nuclear detonations. Here a pulse of energy is radiated out and may cause large currents to be induced in a system with a high risk of failure and, as in the case of ESD, with a significant probability of irrecoverable damage to the devices in a system.
Most countries have developed EMC standards for electronic products that specify acceptable levels of emissions and immunity. These involve a series of tests on the product to determine the radiated/conducted emissions from the product and, by subjecting it to standard intensities of radiation/current, determine the immunity of the product. In the case of radiated standards, antennas such as the log-periodic, biconical and Bilog may be used to produce/receive the radiation; in the case of conducted standards, typically a line impedance stabilization network (LISN) is inserted in series in the a.c. power supply to monitor the currents on the power cable. The LISN provides a reference supply impedance across a range of frequencies.
http://www.cvel.clemson.edu/emc/ A comprehensive resource on electromagnetic compatibility, from Clemson University