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