Line filter

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A Line Filter is an electronic device that is placed between an electronic equipment and a line external to it, to attenuate conducted radio frequencies (RFI, a.k.a.: Electro-Magnetic Emissions, EMI) between the line and the equipment.

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

[edit] 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

[edit] 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 an 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

[edit] References

  • Schaffner EMC Manufacturer of EMI filters
  • Electronic Specialists Application notes
  • Corcom Manufacturer of stand-alone filters and filtered power entry modules
  • Qualtek Manufacturer filtered power entry modules
  • Schurter Manufacturer EMI filters and filtered power entry modules
  • Panasonic Manufacturer of PCB level filters
  • Epcos Manufacturer of PCB level filters
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