A line filter is the kind of electronic filter

Electronic filter

Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...

 that is placed between an electronic equipment and a line external to it, to attenuate conducted radio frequencies -- RFI, also known as electromagnetic interference

Electromagnetic interference

Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

 (EMI) -- between the line and the equipment.

In particular, an AC Line Filter is used between the AC power line and the equipment.

Types of line filters

• A Line Filter may be incorporated in a connector. For example:
  • An AC Line Filter may be incorporated in a modular IEC power inlet connector or Power Entry Module
    Power entry module
    A power entry module ' is an electromechanical component used in electrical appliances, integrating the appliance inlet with other components such as:* a switch, possibly including integrated or remote bowden cable actuation...
    
    
  • A telephone line filter may be incorporated in a modular RJ11 connector
• A line filter may be mounted on a PCB
• An AC line filter may be a stand-alone device, chassis mounted inside the equipment
• A facility AC Line Filter is mounted inside a room or cabinet, at the point where the AC power comes in

Characteristics of line filters

• A line filter may be used to attenuate EMI in either direction. For example:
  • Emissions: It may be used to reduce the unintentional conducted emission from the equipment, to a level sufficiently low to pass regulatory limits (such as FCC part 15). For example, in switching power supplies.
  • Immunity: It may be used to reduce the level of EMI entering the equipment, to a level sufficiently low not to cause any undesired behavior. For example, in equipment used in Radio Transmitter facilities

• The attenuation of Line filters is measured in 2 areas:
  • Common Mode - attenuation to signals that appear identically on each of the wires going through the filter
  • Differential Mode - attenuation to signals that appear on just one of the lines
• For each Mode, the attenuation is characterized over the frequency spectrum, and measured in dB