How does EMI filter (Electromagnetic Filters)work?

Most electronics contains an EMI filter, either as a separate device, or embedded in circuit boards.  Its function is to reduce high frequency electronic noise that may cause interference with other devices.  Regulatory standards exist in most countries that limit the amount of noise that can emitted.

EMI, or Electro-Magnetic Interference, is defined as unwanted electrical signals and can be in the form of conducted or radiated emissions.  Conducted EMI is where the noise travels along the electrical conductors and radiated EMI is where the noise travels through the air as magnetic fields or radio waves.