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Line filter

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Title: Line filter  
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Subject: Switched-mode power supply, Filter (signal processing)
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Line filter

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

References

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