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PART I
EMC Theory
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1
Electromagnetic Compatibility
1.1 INTRODUCTION
The widespread use of electronic circuits for communication, computation, automation, and other purposes makes it necessary for diverse circuits to operate in close proximity to each other. All too often, these circuits affect each other adversely. Electromagnetic interference (EMI) has become a major problem for circuit designers, and it is likely to become even more severe in the future. The large number of electronic devices in common use is partly responsible for this trend. In addition, the use of integrated circuits and large-scale integration has reduced the size of electronic equipment. As circuitry has become smaller and more sophisticated, more circuits are being crowded into less space, which increases the probability of interference. In addition, clock frequencies have increased dramatically over the yearsâin many cases to over a gigahertz. It is not uncommon today for personal computers used in the home to have clock speeds in excess of 1 GHz.
Todayâs equipment designers need to do more than just make their systems operate under ideal conditions in the laboratory. Besides that obvious task, products must be designed to work in the âreal world,â with other equipment nearby, and to comply with government electromagnetic compatibility (EMC) regulations. This means that the equipment should not be affected by external electromagnetic sources and should not itself be a source of electromagnetic noise that can pollute the environment. Electromagnetic compatibility should be a major design objective.
1.2 NOISE AND INTERFERENCE
Noise is any electrical signal present in a circuit other than the desired signal. This definition excludes the distortion products produced in a circuit due to nonlinearities. Although these distortion products may be undesirable, they are not considered noise unless they are coupled into another part of the circuit. It follows that a desired signal in one part of a circuit can be considered to be noise when coupled to some other part of the circuit.
Noise sources can be grouped into the following three categories: (1) intrinsic noise sources that arise from random fluctuations within physical systems, such as thermal and shot noise; (2) man-made noise sources, such as motors, switches, computers, digital electronics, and radio transmitters; and (3) noise caused by natural disturbances, such as lightning and sunspots.
Interference is the undesirable effect of noise. If a noise voltage causes improper operation of a circuit, it is interference. Noise cannot be eliminated, but interference can. Noise can only be reduced in magnitude, until it no longer causes interference.
1.3 DESIGNING FOR ELECTROMAGNETIC COMPATIBILITY
Electromagnetic compatibility (EMC) is the ability of an electronic system to (1) function properly in its intended electromagnetic environment and (2) not be a source of pollution to that electromagnetic environment. The electromagnetic environment is composed of both radiated and conducted energy. EMC therefore has two aspects, emission and susceptibility.
Susceptibility is the capability of a device or circuit to respond to unwanted electromagnetic energy (i.e., noise). The opposite of susceptibility is immunity. The immunity level of a circuit or device is the electromagnetic environment in which the equipment can operate satisfactorily, without degradation, and with a defined margin of safety. One difficulty in determining immunity (or susceptibility) levels is defining what constitutes performance degradation.
Emission pertains to the interference-causing potential of a product. The purpose of controlling emissions is to limit the electromagnetic energy emitted and thereby to control the electromagnetic environment in which other products must operate. Controlling the emission from one product may eliminate an interference problem for many other products. Therefore, it is desirable to control emission in an attempt to produce an electromagnetically compatible environment.
To some extent, susceptibility is self-regulating. If a product is susceptible to the electromagnetic environment, the user will become aware of it and may not continue to purchase that product. Emission, however, tends not to be self-regulating. A product that is the source of emission may not itself be affected by that emission. To guarantee that EMC is a consideration in the design of all electronic products, various government agencies and regulatory bodies have imposed EMC regulations that a product must meet before it can be marketed. These regulations control allowable emissions and in some cases define the degree of immunity required.
EMC engineering can be approached in either of two ways: one is the crisis approach, and the other is the systems approach. In the crisis approach, the designer proceeds with a total disregard of EMC until the functional design is finished, and testingâor worse yetâfield experience suggests that a problem exists. Solutions implemented at this late stage are usually expensive and consist of undesirable âadd ons.â This is often referred to as the âBand Aidâ approach.
As equipment development progresses from design to testing to production, the variety of noise mitigation techniques available to the designer decreases steadily. Concurrently, cost goes up. These trends are shown in Fig. 1-1. Early solutions to interference problems, therefore, are usually the best and least expensive.
The systems approach considers EMC throughout the design; the designer anticipates EMC problems at the beginning of the design process, finds the remaining problems in the breadboard and early prototype stages, and tests the final prototypes for EMC as thoroughly as possible. This way, EMC becomes an integral part of the electrical, mechanical, and in some cases, software/firmware design of the product. As a result, EMC is designed intoâ and not added ontoâthe product. This approach is the most desirable and cost effective.
If EMC and noise suppression are considered for one stage or subsystem at...
