ABSTRACT
In some cases, engineers—both designers of electromagnetic interference (EMI) devices and others—call EMI black magic. Most of the energy in the stop band of the filter is reflected to its source. This fact is often overlooked in both standard-filter and EMI technology. EMI is any unwanted signal from either the power line or the equipment, and this must be removed to prevent a magnetic field from interfering with closely associated equipment or to stop a malfunction of the equipment containing the filter. The regular filter designer is able to use software filter-design tools or may even develop the filter design from mathematical derivation techniques using pole placement and ladder synthesis. In contrast, most EMI filter manufacturers design only the low-pass filters needed for the required EMI attenuation. Rarely do they build bandpass or other conventional filters. Specifications are another subject that often creates uncertainty and controversy. Some test specifications unnecessarily complicate the design and make it overspecified.
