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EMI cases generally include a source of interference, a path that couples the EMI to other circuits, and a target referred to as the “victim” whose performance is degraded by the source EMI. EMI can first be categorized as continuous interference as opposed to transient interference. Continuous interference occurs when the source emits an uninterrupted signal composed of the source’s fundamental frequency and associated harmonics. Continuous interference can be further subdivided by frequency band. Frequencies from a few Hz up to 20 KHz are classified as audio. Sources of audio interference include power supply hum and associated wiring, transmission lines and substations, audio processing equipment such as audio power amplifiers and loudspeakers, and demodulation of high frequency carrier waves such as those seen in FM radio transmission. Radio Frequency Interference (RFI) occurs in a frequency band from 20kHz to a constantly increasing limit defined by advancing technology. Sources of RFI include wireless and radio frequency transmissions, television and radio receivers, industrial, scientific and medical equipment, and high frequency circuit signals such as those in microprocessors, microcontrollers, and other high speed digital equipment. Transient EMI arises when the source emits a short duration pulse of energy rather than a continuous signal. Sources include switching electrical circuitry, e.g., inductive loads such as relays, solenoids and electric motors. Other sources are electrostatic discharge (ESD), lightning, nuclear and nonnuclear electromagnetic pulse weapons, and power line surges. Repetitive transient EMI can be caused by electric motors, gasoline engine ignition systems and continuous digital circuit switching.
