EMI (electromagnetic interference) is omnipresent. It is generated by switching of electrical current from a variety of sources including electronic power supplies. Conducted EMI is when the noise travels along electrical conductors and radiated EMI is when noise travels through the air as magnetic fields or radio waves.
Many electronic systems include an EMI filter, either as a separate device, or embedded in circuit boards. The function of the EMI filter is to reduce high frequency electronic noise that interferes with other devices.
An EMI filter for a power supply consists, in most cases, of passive components, including capacitors and inductors, that form LC circuits. The inductors filter out undesired high frequency currents, while enabling DC or low frequency currents to pass through. The capacitors provide a low impedance path to divert the high frequency noise away from the input of the EMI filter, either back into the power supply, or into the ground connection.
Unlike EMI filters, EMI shields help protect electronics from electromagnetic interference by interposing metal sheets shaped to fit electronic housings or enclosures. Note that while aluminum, copper, and steel are strong and rigid, thin metal sheets can nonetheless deform under the pressures that are required for sealing and permit leakage to and from electronic circuits.