CHAPTER ONE
Introduction to Electromagnetic Compatibility (EMC)

Since the early days of radio and telegraph communications, it has been known that a spark gap generates electromagnetic waves rich in spectral content (frequency components) and that these waves can cause interference or noise in various electronic and electrical devices such as radio receivers and telephone communications. Numerous other sources of electromagnetic emissions such as lightning, relays, dc electric motors, and fluorescent lights also generate electromagnetic waves that are rich in spectral content and can cause interference in those devices. There are also sources of electromagnetic emissions that contain only a narrow band of frequencies. High-voltage power transmission lines generate electromagnetic emissions at the power frequency [60?Hz; 50?Hz in Europe]. Radio transmitters transmit desired emissions by encoding information (voice, music, etc.) on a carrier frequency. Radio receivers intercept these electromagnetic waves, amplify them, and extract the information that is encoded in the wave. Radar transmitters may transmit pulses of a single-frequency carrier or may transmit a band of frequencies using a chirp modulation scheme. The spectral content of such radar pulse transmissions is distributed over a larger band of frequencies around the carrier than are radio transmissions. Another important and increasingly significant source of electromagnetic emissions is associated particularly with digital computers, and digital electronic devices in general. These digital devices utilize pulses to signify a binary number, 0 (off) or 1 (on). Numbers and other symbols are represented as sequences of these binary digits. The transition time of the pulse from off to on and vice versa is perhaps the most important factor in determining the spectral content of the pulse. Fast (short) transition times generate a wider range of frequencies than do slower (longer) transition times. The spectral content of digital devices generally occupies a wide range of frequencies and can be a major contributor to electromagnetic interference in electrical and electronic devices.

This text is concerned with the ability of these types of electromagnetic emissions to cause interference in electrical and electronic devices. The reader has no doubt experienced noise produced in an AM radio by nearby lightning discharges. The lightning discharge is rich in frequency components, some of which pass through the input filter of the radio, causing noise to be superimposed on the desired signal. Also, even though a radio may not be tuned to a particular transmitter frequency, the transmission may be received, causing the reception of an unintended signal. These are examples of interference produced in intentional receivers. Of equal importance is the interference produced in unintentional receivers. For example, a strong transmission from an FM radio station or TV station may be picked up by a digital computer, causing the computer to interpret it as data or a control signal resulting in incorrect function of the computer. Conversely, a digital computer may create emissions that couple into a TV, causing interference.

This text is also concerned with the design of electronic systems such that interference from or to that system will be minimized. The emphasis will be on digital electronic systems. An electronic system able to function compatibly with other electronic systems and not produce or be susceptible to interference is said to be electromagnetically compatible with its environment. The objective of this text is to learn how to design electronic systems for electromagnetic compatibility (EMC). A system is electromagnetically compatible with its environment if it satisfies three criteria:

1. It does not cause interference with other systems.
2. It is not susceptible to emissions from other systems.
3. It does not cause interference with itself.

Designing for EMC is not only important for the desired functional performance; the device must also meet legal requirements in virtually all countries of the world before it can be sold. Designing an electronic product to perform a new and exciting function is a waste of effort if it cannot be placed on the market!

EMC design techniques and methodology have become an integral part of the design of electrical and electronic devices and systems. Consequently, the material in this text has become a fundamental part of an electrical engineer's background. This will no doubt increase in importance as the trend toward increased clock speeds and data rates of digital systems continues.

This text is intended for a university course in electromagnetic compatibility in an undergraduate/graduate curriculum in electrical engineering. There are textbooks available that concern EMC, but these are designed primarily for the industrial professional. Consequently, we will draw on a number of sources for reference material. These will be given at the end of each chapter, and their reference will be denoted in the text by brackets (e.g., [xx]). Numerous trade journals, EMC conference proceedings, and the Institute of Electrical and Electronics Engineers (IEEE) Transactions on Electromagnetic Compatibility contain useful tutorial articles on various aspects of EMC that we will discuss, and these will similarly be referenced where appropriate. The most important aspect in successfully dealing with EMC design is to have a sound understanding of the basic principles of electrical engineering (circuit analysis, electronics, signals, electromagnetics, linear system theory, digital system design, etc.). We will therefore review these basics so that the fundamentals will be understood and can be used effectively and correctly by the reader in solving the EMC problem. A representative set of such basic texts is [1-3]. A representative but not exhaustive list of texts that cover the general aspects of EMC is represented by [4-13]. The text by Ott [4] will form our primary EMC text reference. Other texts and journal articles that cover aspects of EMC will be referenced in the appropriate chapters. Textbooks on the design of high-speed digital systems are represented by [14-16]. For a discussion of the evolution of this EMC course, see [17, 18].

1.1 ASPECTS OF EMC

As illustrated above, EMC is concerned with the generation, transmission, and reception of electromagnetic energy. These three aspects of the EMC problem form the basic framework of any EMC design. This is illustrated in Fig. 1.1. A source (also referred to as an emitter) produces the emission, and a transfer or coupling path conveys the emission energy to a receptor (receiver), where it is processed, resulting in either desired or undesired behavior. Coupling, referred to often in this text, is the desired or undesired transfer of energy from one medium to another. Examples of coupling include capacitive coupling, inductive coupling, or even something as simple as a copper wire connecting two devices. Interference occurs if the received energy causes the receptor to behave in an undesired manner. Transfer of electromagnetic energy occurs frequently via unintended coupling modes. However, the unintentional transfer of energy causes interference only if the received energy is of sufficient magnitude and/or spectral content to cause the receptor to behave in an undesired fashion. Unintentional transmission or reception of electromagnetic energy is not necessarily detrimental; undesired behavior of the receptor constitutes interference. So the processing of the received energy by the receptor is an important part of the question of whether interference will occur. Quite often it is difficult to determine, a priori, whether a signal that is incident on a receptor will cause interference in that receptor. For example, clutter on a radar scope may cause a novice radar operator to incorrectly interpret the desired data, whereas the clutter may not create problems for an operator who has considerable experience. In one case we have interference and in the other we do not, although one could argue that the receptor is the radar operator and not the radar receiver. This points out that it is often difficult to uniquely identify the three aspects of the problem shown in Fig. 1.1!

FIGURE 1.1 The basic decomposition of the EMC coupling problem.

It is also important to understand that a source or receptor may be classified as intended or unintended. In fact, a source or receptor may behave in both modes. Whether the source or the receptor is intended or unintended depends on the coupling path, as well as the type of source or receptor. As an example, a radio station transmitter whose transmission is picked up by a radio receiver that is tuned to that carrier frequency constitutes an intended emitter. On the other hand, if the same radio transmission is processed by another radio receiver that is not tuned to the carrier frequency of the transmitter, then the emission is unintended. (Actually the emission is still intended but the coupling path is not.) There are some emitters whose emissions can serve no useful purpose. An example is the (nonvisible) electromagnetic emission from a fluorescent light.

This suggests that there are three ways to prevent interference:

1. Suppress the emission at its source.
2. Make the coupling path as inefficient...