Electromagnetic Compatibility (EMC) is defined as the ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing electromagnetic disturbance to anything in that environment.

Electromagnetic environment is the totality of the electromagnetic phenomena existing a given location.  In general, the electromagnetic environment is time dependent and its description may require a statistical approach.  Electromagnetic disturbance, however, is any electromagnetic phenomena which may degrade the performance of a device, equipment or system, or adversely effect living or inert matter.  Electromagnetic disturbance may be an electromagnetic noise, an unwanted signal or a change in the propagation medium itself.

In essence electromagnetic interference is a major concern for designers and users of electrical/electronics equipment.  Electrical and electronics equipment have to be designed such that they have adequate immunity from self generated electrical noise plus that anticipated within the installed environment.  In addition, electrical and electronic equipment have to be designed such that the electromagnetic emissions it emits is within regulated limits in the country (or countries) where it is to be sold or installed. A user of electrical and electronic equipment requires assurance that the equipment in question will operate as intended within the intended electromagnetic environment and that the user would not be cited or the equipment (or installation) shut down due to excessive emissions. Electromagnetic compatibility is regulated in most countries and violation of EMC requirements may result in large fines and incarceration.

The proliferation of higher frequency electronics and increased use of communication links coupled with the EMC regulations and cost contributes to the difficulty of assuring electromagnetic compatibility. The most difficult challenge is that of complying with the specified EMC requirements at minimum cost, this is particularly difficult in the Third World Countries. The minimum cost objective is essentially true for all facets of product design, however no  other performance characteristics is more difficult to accurately predict or control than electromagnetic emissions.  Essentially, complex electronics equipment are antenna fields with an infinite number of variables, many of which are unique to the installation.

Any charge in motion emits energy into space.  Maxwell’s equations show that any time-varying charge has an electromagnetic field has an electromagnetic field associated with it, which in turn, will radiated a portion of its energy into the surrounding environment.  The amount of radiated energy is a function of current flow and efficiency of the radiating element.  In electrical and electronics equipment, all electrical conductors, including shields and grounds, are electromagnetic radiators and if of sufficient physical size emit harmful interference.  Thus, my role, as an EMC Engineer, is to assure that the combination of energy sources, feed systems and radiating elements are adequately controlled to assure that the radiated energy remains below that permitted by law or specified for the equipment or system.

In practice, all electromagnetic interference problems have elements which are unique to the particular system and environment in question.  Standards are produced which define limits of emissions and levels of immunity.  For any given environment, the standards give a margin of compatibility between a system and the environment.  The standards also takes into account ageing of the system so that as the system deteriorates with time there will still be a margin of compatibility.

Electromagnetic Emissions and Immunity

The main EMC objective in product development is the reduction of emissions from electrical devices as contract with improving susceptibility to noise.  In practice, both emission and immunity are important, however emissions are normally more difficult to control. The law of reciprocity usually applies relative to emissions and immunity, and thus any design activity to control emission usually ends up improving the immunity of the device.  The cause effect of the level of electromagnetic emissions on a device is best explained by

For an electromagnetic interference problem to exist, there must be a noise source, a victim and a path for the energy to be transferred from the source to the victim.  In this context, the victim is any circuit or device where the electromagnetic interference problem occurs. Effective electromagnetic interference control usually involves all three elements.  Since it is not possible to eliminate any of the three basic elements, the design for EMC is essentially that of controlling these elements such that the amplitude of the emitted noise energy is within specified limits and thresholds are adequate to discriminate against the environmental noise energy. Altering the coupling path via shielding may also be required.

Electromagnetic noise may be transferred from a source to a victim circuit by either a conductor (conducted emission) or a dielectric (induced or radiated emission) .  In this context, transfers by induction implies predominate electric field or magnetic field coupling (near field) whereas radiation implies electromagnetic waves (far field) coupling.  Intra-system interference normally results from conducted or induced energy transfers, whereas inter-system interference normally results from radiated energy transfer. 

Intra-system generated electrical noise can  generally be classified into three types:

a.) Thermal Noise (or Johnson’s Noise):  This type of noise is due to the thermal agitation of electrons in conducting materials at temperature above 0 K.  Thermal noise per hertz is equal throughout the frequency spectrum  and depends only conductor temperature, K and Boltzmann’s constant.

The Thermal Noise Power, is given by:

b.) Shot Noise: This type of electrical noise arises from the statistical fluctuation of electron flow within a conductor. It normally occurs when there is a potential barrier (such as in PN junction diode’s). When electrons and holes cross the barrier, shot noise is produced . 

The current fluctuation is given by:

c.)  One over F Noise:  This type of noise results from the variation in the emission efficiency of the cathode or of an emitter - an effect which becomes more pronounced as the frequency of operation increases.  It is produced in many natural phenomena such as nuclear radiation, electron flow through a conductor etc.  In electrical engineering, it is referred to as flicker noise. It can be described as time series with random fluctuations. 

The power spectra as a function of  frequency is given by:

Inter-system generated electrical noise are generated by sources location outside the  boundary of the system in question.  Typical sources could be car ignition systems, fluorescent lighting, electrical motors or generators, microprocessors, thunderstorms, solar disturbances etc. It is these external sources of electromagnetic interference that are normally of greatest significance when dealing with EMC.

Coupling Mechanism

For electromagnetic disturbance to cause interference, they must be propagated in some way. There are various mechanism by which electromagnetic noise couples to a victim circuit, these mechanism are explored in Table 1 below.

Following from Table 1 above, it is possible to categorise the coupling mechanism into three main types:

a.) Conduction;

b.) Reactive coupling; and

c.) Radiation.

Electromagnetic energy may be conducted in either common mode or differential mode.  This conduction can be via power cables, earth conductors, signal cables, antenna feeders or other low impedance paths.  There is a greater risk from conducted interference at frequencies below 30 MHz; above this frequency, conducted interference suffers substantial attenuation and other propagation mechanisms become dominant.  Electromagnetic energy can also be propagated by reactive coupling, either inductive or capacitive.  The precise effects depends upon distance, orientation, size, earthing and other factors all of which will tend to be unique to the system. Reactive coupling may exist within a system or between systems.  In general, inductive coupling tends to be associated with high current/low impedance situations, whilst capacitive coupling tends to occur in high voltage/ high impedance situations. 

For frequencies above 30 MHz, radiation tends to be the dominant propagation mechanism.  There are two types of radiating emitters: intentional and unintentional. Intentional emitters, such as radio and radar, produce spurious emissions together with their intended signal.  These may be in the form of harmonics or intermodulation products of the intended signal and are directly associated with the primary function of the equipment. Broadband noise is also radiated by practical radio transmitters. Unintentional emitters, for example personal computers or automobile, generate emissions as a by product of the primary function of the equipment.

A key point to note from Table 1, is that in any given situation, electromagnetic interference propagation may well occur via a combination of two or three mechanisms, rather than being due to a single mechanism in isolation.

As mentioned above, EMC is about controlling the three elements of the interference model such that compatibility is assured. In the following pages, I shall explore the control of the coupling path in more detailed.

Shielding Theory