The Evolution of EMC During a job interview, a young candidate when asked what he understood by EMI replied: "Whenever something goes wrong with a piece of electronic equipment and you cannot explain why - that's EMI!" A good explanation, but perhaps a little more depth is needed. Radio Frequency Interference (RFI) was the precursor to Electromagnetic Interference (EMI). Regulations concerning RFI have existed since the early 50's. These regulations were primarily concerned with interference to radio and TV, hence the name Radio Frequency Interference. Due to increasing problems of RFI in a wide range of environments, more specific requirements were produced. The International Electrotechnical Committee (IEC) through CISPR (International Special Committee on Radio Frequency Interference) developed recommendations to test and measure interference. As RFI and its propagation became better understood and with the expansion of the usable frequency spectrum, EMI (Electromagnetic Interference) became the new buzzword. Military disciplines were concerned, not only with EMI emissions emanating from their equipment, but also with the susceptibility of their sensitive electronic equipment to EMI in the environment. Electromagnetic Compatibility (EMC) encompasses both the emissions and the immunity (or susceptibility) portion of EMI. The proliferation of sensitive electronic equipment within the commercial environment made EMC the new concern. Electromagnetic Compatibility EMI can be described as the degradation of a device or system caused by an electromagnetic disturbance. An electromagnetic disturbance is any electromagnetic phenomena which may degrade the performance of a device, equipment or system, or adversely affect living or inert matter. An example of EMI affecting living matter is the current controversy regarding portable cellular telephones causing brain tumors. Therefore, an electromagnetic disturbance can be an unwanted signal or even a change in the propagation medium itself. A change in the propagation medium can attenuate the signal and have a direct effect on the level of disturbance. EMC, on the other hand, can be described as the ability of different pieces of electrically operated equipment to work in close proximity to each other without causing any mutual interference. EMC therefore implies the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to any other equipment in that environment. EMC is a twofold occurrence and consists of emissions and immunity. First, EMC implies that the equipment will not generate unacceptable interference emission levels which could cause interference (the emissions portion); and second, EMC implies that the equipment's intrinsic immunity levels are such that it can tolerate ambient levels of interference without degradation of performance (the Immunity portion). Therefore, EMC means that a device must be capable of operating in all modes in the environment for which it was designed without degrading its own performance or that of any nearby equipment. Sources of Electromagnetic Interference An Electromagnetic Environment can be described as the electromagnetic conditions existing at a given location. The EMI environment includes interference emanating from natural sources like lightning and atmospheric static to the various man-made sources of interference such as vacuum cleaners, washing machines, power tools, computers, cellular phones, mobile radios and even electronic toys. Natural sources can be either terrestrial or extraterrestrial in nature. Man-made sources include intentional or unintentional radiators (see figure 1 below). Within the scope of man-made noise sources we can break it down even further into Intersystem interference and Intra-system interference. Inter-system interference is EMI in a system caused by an electromagnetic disturbance generated by another system; whereas Intra-system interference is self-generated EMI present in a system. There is very little that can be done to prevent electromagnetic energy generated from natural interference sources. However, natural sources do not create that much of a problem except for perhaps, surges and spikes on power lines induced by lightning strikes. It is also very difficult to prevent EMI from intentional sources of electromagnetic energy. Cellular telephones and two-way radios are a major problem and can create havoc for example in hospital environments. It is therefore crucial that electronic equipment be made immune or less susceptible to environmental interference. However, the major source of all interference is generated from unintentional manmade sources. This is due to the vast amount of electrical and electronic equipment in use. The Three Elements of an EMI Problem There are three essential elements to any EMC problem. There must be an EMI source or an electromagnetic disturbance, a receptor or "victim" that cannot function properly due to the electromagnetic phenomenon, and a path between them that allows the source to interfere with the receptor. This is shown in figure 2 below. Each of these three elements must be present (although they may not readily be identified) at the same time in order to have an electromagnetic disturbance or EMI. EMC problems can be solved by identifying at least two of these elements and eliminating or attenuating the interference from one of them. Characteristics of an EMI Source Interference signals are established whenever electrons move. Therefore, any current flow may cause either direct coupling to other circuits or radiated fields, which may in turn couple unwanted signals into other circuits. Sources of interference can be characterized by their frequency, bandwidth and amplitude. The frequency spectrum line chart shown in figure 3 depicts the frequency range with respect to EMC. The frequency range of concern for EMI from electrical apparatus is the harmonic range from 50 Hz (fundamental of the mains frequency) to 2 kHz (the 40th harmonic of the mains frequency), the conducted range from 150 kHz to 30 MHz and the radiated emission range from 30 MHz to 300 MHz and higher. Although we now only measure conducted emissions for the EU from 150 kHz, there is discussion of increasing the spectrum and starting from as low as 9 kHz. This could make it more difficult for manufacturers of SMPS, Inverters and SCR circuits to comply with the regulations. The propagation medium of EMI below 30 MHz tends to be mains-borne or conducted. The interference travels along the power cord or signal lines from the source to the receptor or victim circuit. The conducted interference is not easily attenuated over distance. The radiated portion of EMI emissions is borne as an electromagnetic wave, propagating through the air or any other non-conducting media. Generally, the higher the EMI in the frequency spectrum, the more easily it will radiate. EMI and EMC are becoming more of a problem due to the trend to produce equipment in smaller packages operating at very high speeds and processing rates. The use of higher speed switching logic increases emissions from printed circuit boards. Also the use of devices with low operating voltages and currents, packaged more closely together, increases the potential for intra-system interference and reduced immunity (increased susceptibility). Bandwidth One cannot sufficiently classify EMI disturbances in terms of frequency content only. The character of the signal also needs to be examined. The character of the interference signal can be described as either narrowband or broadband in nature. Classification according to bandwidth is determined from the ratio of the EMI signal to a reference bandwidth. This ratio can be given in a manner derived from both the measuring receiver bandwidth and the characteristics of the disturbance signal. Therefore, an electromagnetic emission is classified as broadband if while tuning the measuring bandwidth over a range of two impulse bandwidths (IBW) around its center frequency, a change in peak response of 3 dB or less is detected. If a change of 3 dB or more is depicted the signal would be determined to be narrowband. Therefore, bandwidth is a function of the measuring receiver. Unintentional radiating sources usually exhibit wide bandwidths (broadband noise) and usually have high magnitudes. Deliberate noise sources like cellular telephone, Radio and TV transmitters, etc., will generate narrowband signals. Examples of broadband noise sources are typically brush motors, inverter circuits, SCR circuits and more. Single frequency signals can be described as narrowband. A sine wave is a pure tone and contains only one single frequency. However, a square wave such as produced by a digital switching circuit or pulse, contains more than one frequency, comprising of the fundamental frequency and harmonics. Each harmonic, therefore, represents a narrowband source. Military Standards and some of the older CISPR standards referred to test procedures and limits using Narrowband and Broadband detectors. However, the EU European Norm (EN) standards specify the use of Quasi-peak and Average detectors for measuring EMI levels. A Quasi-peak detector has specified electrical time constants which, when regularly repeated identical pulses are applied to it, delivers an output voltage which is a fraction of the peak value of the pulses, this fraction increasing towards unity as the pulse repetition rate is increased. The quasi-peak EMI detection system is probably the fairest way of assessing interference as it is based on the annoyance factor of the interfering signal. The higher the repetition rate of the interfering signal (i.e. the higher the annoyance factor) the longer and higher the detector stays charged, therefore, the higher the level recorded. As the repetition rate of the interfering signal increases, the quasi-peak level approximates the peak level. The Average detector gives the average value of the envelope of an applied signal. The average value must be taken over a specified time interval. A constant signal like a clock signal will be measured with an average detector. Typically, a broadband signal will be detected as quasi-peak and a narrowband signal as average. Common Mode and Differential Mode Interference Electromagnetic disturbances can appear in the form of Common-Mode (CM) and Differential-Mode (DM) voltage and current components. Differential mode also called symmetrical noise or interference occurs when noise currents travel between live and neutral (or return). The differential mode voltage components are measured between the phase conductors. Differential mode signals are usually used to convey the desired information and do not usually cause that much interference as the EMI fields generated by differential currents oppose each other (180o out of phase)causing a cancellation effect. Common mode signals, on the other hand are usually the major source of EMI from power & transmission (all I/O) cables. They cause the cables to behave as monopole antenna. These currents flow from the phase and neutral conductors to ground (earth). The circuit for the common mode component is completed by the stray impedance (capacitance) to ground.