Part I--Underlying Concepts and Techniques: Introduction to Electromagnetic Compatibility

Electromagnetic compatibility (EMC) is the branch of science and engineering concerned with the design and operation of equipment in a manner that makes them immune to certain amounts of electromagnetic interference, while at the same time keeping equipment-generated interference within specified limits. The scope of EMC is thus very wide as it encompasses virtually all equipment powered by electrical supplies. Practically, all engineering systems incorporate power conditioning and information processing units and thus fall within the scope of EMC. The frequency range of interest extends from DC to light and in certain parts of this spectrum a strict international regulatory framework has been set up to ensure immunity of equipment to electromagnetic interference (EMI) and to control emission.

Interest in EMC is not new. Since the early days of radio, designers and listeners alike were alerted to noise, interference, and earthing problems. However, the rapid increase in the use of radio communications, digital systems, fast processors, and the introduction of new design practices have brought EMC to the forefront of advanced design. Three technological trends have provided the impetus for these changes. First, modern digital logic and signal processing are based on relatively low-threshold voltages (i.e., a few volts) compared to older technologies based on electronic valves (several hundred volts). The immunity of modern systems is there fore inherently lower. Second, in the process of seeking higher processing speeds, shorter pulse rise-times are used, contributing significant amounts of energy at high frequencies, which is capable of propagating by radiative mechanisms over long distances. Third, the modern physical design of equipment is based increasingly on the use of plastics in preference to metals. This significantly reduces the electromagnetic shielding inherent to an all-metal cabinet. Several other items could be added to this list, such as miniaturization and thus the trend for compact designs, which contribute to EMC problems. Close attention must thus be paid to EMC at all stages of design if equipment is to function properly and meet international EMC regulations.

EMC may be approached from two different directions. First, it may be argued that before anything is designed, a complete EMC study must be performed to predict the electromagnetic signature of equipment and its capacity to withstand externally generated interference. The difficulties in performing such a study are formidable, since many of the necessary predictive analytical and numerical tools are not currently available and the environment in which equipment is installed is not always fully specified. Second, it may be thought that EMC is essentially a fire-righting operation and that any problems that may arise in this area are best dealt with on an ad hoc basis. The dangers of this approach are obvious, since the complexity of modern designs and the nature of EMC preclude easy and inexpensive solutions as an afterthought. A balanced approach to EMC is to bring into this area every tool available to the designer. This includes numerical tools, in-house practical experience, and sound physical grasp of EMC and electromagnetic interactions. No single individual will be gifted in all these areas and it’s thus important to establish teams with the right blend of experience and approach to problem solving to form the focus for EMC design. Several years of emphasis in engineering education on digital design have reduced the grasp of radio frequency (RF) design issues among graduates to dangerously low levels and it’s therefore of considerable importance to strengthen awareness of EMC and the grasp of fundamental RF design techniques in academia and in industry.

EMC must be regarded as an issue that affects all aspects of design -- electrical, electronic, and mechanical. It cannot be adequately addressed in isolation. Typically, a complete design may consist of a number of subsystems that interact with each other through signal and power cables and through reactive (capacitive and/or inductive) or radiative mechanisms. The EMC behavior of a complete system cannot be easily predicted from the known behavior of subsystems, although the purpose of current EMC research is to develop the necessary methodologies and tools to achieve this. The difficulties are more pronounced in large extensive systems where a mixture of new and old technologies is used in the presence of severe external electromagnetic (EM) threats. Clearly, the designer will make every effort to solve EMC problems within a piece of equipment caused by interactions between various subsystems. Nevertheless, the majority of EMC problems seem to be internally generated. In an integrated system in which a number of different types of equipment, often provided by different suppliers, are connected together, EMC problems may develop and they can be difficult to tackle if each unit does not meet specified EMC limits and if insufficient thought has been given to system-level EM interactions.

A designer who has developed a system that operates successfully as far as internally generated electromagnetic interference (EMI) is concerned, has nevertheless to meet, in addition, certain EMC limits specified in national and international standards. Compliance with these limits can be demonstrated by making a number of measurements under specified conditions. In a typical case, these tests will cover emission of EMI from equipment and also the susceptibility or immunity of this equipment to externally generated interference. In an emission test the equipment is placed, depending on the particular standard, inside a screened room or on an open-field site and measurements are taken of the emitted electro magnetic fields at a specified distance using receivers of specified band width over a specified frequency range. The type and polarization of antennas used is also specified to produce, as far as possible, a repeatable measurement. This type of test is called a radiated emission test, to distinguish it from conducted emission tests where the EMI voltage on conductors is measured. In each case, the measured quantity must be below specified limits. A selection of such limits and a more detailed description of test procedures are given in Sections 13 and 14.

In immunity testing, the equipment is subject to a specified externally generated field or to interference currents injected on conductors and the requirement is that the equipment remains functional. These tests may cover a wide frequency range (typically at least up to 1 GHz) and may also involve pulse-like incident electromagnetic signals to check the response of equipment to transients and electrostatic discharges. For complex equipment with a large number of operating modes it can be difficult to demonstrate immunity.

The designer is obliged to ascertain the nature and significance of EM emissions from different parts of the equipment and the effects of externally generated interference on the functional integrity of an entire system.

He or she may, as far as possible, seek to minimize interference generation at its source; reduce or eliminate coupling paths by proper layout, shielding, filtering, and grounding practices; design hardware with an inherent immunity to EMI; and adopt defensive programming practices to develop software that has a high level of immunity to EMI. Very few of these options are cost free and many may have adverse effects on normal operating characteristics, size, appearance, and weight of equipment. It’s not an easy matter to seek optimum solutions to such design problems, which depend on so many interdependent parameters. The purpose of research into EMC is to develop the methodologies and tools that allow optimum EMC design procedures to be incorporated into the design process from the very beginning, so that, even at the conceptual stage of a design, EMC issues are taken into account and sensible choices are made with minimum costs.

A glance at the titles of the Sections that follow will convince the Learner of the complexity and importance of EMC and of its position at the forefront of teaching and research in advanced system design techniques.

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