Line filter. A line filter (aka EMC filter, EMI filter, RFI filter) is an electronic filter that is placed between the mains electricity input and internal circuitry of electronic equipment to attenuate conducted radio frequencies radio frequency interference (RFI), also known as electromagnetic interference (EMI). Often it is either integrated into the power entry module or as a separate module (similar to the photo).

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 60320 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.
 For DC grids, photovoltaic applications and EV charging applications, DC filters are used.
 AC line filters are very commonly used with inverters (aka drives, frequency converters). The switching semiconductors in the inverters cause energetic noise in high frequencies, that can cause EMI.

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 two 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.
 AC line filters typically comply to IEC 60939 - Passive filter units for electromagnetic interference suppression

See also
 AC-line filter capacitor

References

Further reading
 R. Phukan, S. -Y. Chen, D. Dong, R. B. G. Mondal, H. Krupp and S. Nielebock, "Design of a Three-Phase Three-Level Back-To-Back Bridge Interconnection Based Filter Scheme," in IEEE Journal of Emerging and Selected Topics in Power Electronics. doi: 10.1109/JESTPE.2023.3266154
 R. Phukan et al., "Enhanced Three-Phase AC Common-Mode Filter With Optimized Damping Network for VFDs," in IEEE Transactions on Industry Applications.doi: 10.1109/TIA.2023.3261859
 R. Phukan et al., "Characterization and Mitigation of Conducted Emissions in a SiC based Three-level T-Type Motor Drive for Aircraft Propulsion," in IEEE Transactions on Industry Applications.doi: 10.1109/TIA.2023.3252521
 R. Phukan et al., "Optimized DC-AC EMI Filter Design for DC-Fed High Speed SiC-Based Motor Drive," 2022 IEEE Energy Conversion Congress and Exposition (ECCE), Detroit, MI, USA, 2022, pp. 1-8.doi: 10.1109/ECCE50734.2022.9947974
 X. Zhao et al., "Design of Rogowski Coil Current Sensor Integrated with Busbar and Gate Driver for 211 kW SiC-Based Three-Level T-Type Inverter," 2022 IEEE Energy Conversion Congress and Exposition (ECCE), Detroit, MI, USA, 2022, pp. 1-7.doi: 10.1109/ECCE50734.2022.9947570
 R. Phukan and R. Burgos, "Alternate Filter Structures for Circulating Current and Conducted Noise Mitigation," in IEEE Transactions on Power Electronics, vol. 37, no. 12, pp. 14052-14056, Dec. 2022. doi: 10.1109/TPEL.2022.3198501
 R. Phukan, X. Zhao, P. Asfaux, D. Dong and R. Burgos, "Investigation of Staggered PWM Scheme for AC Common Mode Current Minimization in SiC-Based Three-Phase Inverters," in IEEE Transactions on Transportation Electrification, vol. 8, no. 4, pp. 4378-4390, Dec. 2022.doi: 10.1109/TTE.2022.3198528
 R. Phukan, D. Nam, D. Dong and R. Burgos, "Design Considerations for a Modular 2-Stage LCLC Filter for Three Phase AC-DC Interleaved Converters," 2022 IEEE Transportation Electrification Conference & Expo (ITEC), Anaheim, CA, USA, 2022, pp. 517-522.doi: 10.1109/ITEC53557.2022.9813883
 R. Phukan et al., "A Compact Integrated DM-CM Filter with PCB Embedded DC Current Sensor for High Altitude High Current Applications," 2022 IEEE Transportation Electrification Conference & Expo (ITEC), Anaheim, CA, USA, 2022, pp. 923-928.doi: 10.1109/ITEC53557.2022.9814074
 S. Ohn et al., "A Scalable Filter Topology for $N$-Parallel Modular Three-Phase AC–DC Converters by an Arrangement of Coupled Inductors," in IEEE Transactions on Power Electronics, vol. 37, no. 11, pp. 13358-13367, Nov. 2022. doi: 10.1109/TPEL.2022.3179396
 R. Phukan et al., "Design of an Indirectly Coupled Filter Building Block for Modular Interleaved AC–DC Converters," in IEEE Transactions on Power Electronics, vol. 37, no. 11, pp. 13343-13357, Nov. 2022. doi: 10.1109/TPEL.2022.3179346
 R. Phukan, S. Ohn, S. Nielebock, G. Mondal, D. Dong and R. Burgos, "Comparison Between Interconnected Filter Blocks for Three-Phase AC–DC Interleaved Converters," in IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 10, no. 5, pp. 5956-5968, Oct. 2022. doi: 10.1109/JESTPE.2022.3179335
 R. Phukan, S. Ohn, D. Dong and R. Burgos, "An Approach to Localize Circulating Current for Three Phase Interleaved AC-DC Converters," 2021 IEEE Applied Power Electronics Conference and Exposition (APEC), Phoenix, AZ, USA, 2021, pp. 1848-1853.doi: 10.1109/APEC42165.2021.9487128
 R. Phukan, D. Nam, D. Dong, R. Burgos, G. Mondal and S. Nielebock, "Highly Integrated Monolithic Filter Building Block for SiC based Three-Phase Interleaved Converters," 2021 IEEE Applied Power Electronics Conference and Exposition (APEC), Phoenix, AZ, USA, 2021, pp. 2876-2882.doi: 10.1109/APEC42165.2021.9487139
 R. Phukan, S. Ohn, D. Dong, R. Burgos, G. Mondal and S. Nielebock, "Fault Tolerant Operation of Interleaved Converters using a Bypass Switch Arrangement," 2021 IEEE Applied Power Electronics Conference and Exposition (APEC), Phoenix, AZ, USA, 2021, pp. 2657-2661.doi: 10.1109/APEC42165.2021.9487217
 S. Ohn et al., "Modeling of $N$-Parallel Full-SiC AC–DC Converters by Four Per-Phase Circuits," in IEEE Transactions on Power Electronics, vol. 36, no. 6, pp. 6142-6146, June 2021. doi: 10.1109/TPEL.2020.3033708
 R. Phukan, S. Ohn, D. Dong, R. Burgos, G. Mondal and S. Nielebock, "Evaluation of Modular AC Filter Building Blocks for Full SiC based Grid-Tied Three Phase Converters," 2020 IEEE Energy Conversion Congress and Exposition (ECCE), Detroit, MI, USA, 2020, pp. 1835-1841.doi: 10.1109/ECCE44975.2020.9236265
 S. Ohn et al., "Modular Filter Building Block for Modular full-SiC AC-DC Converters by an Arrangement of Coupled Inductors," 2020 IEEE Energy Conversion Congress and Exposition (ECCE), Detroit, MI, USA, 2020, pp. 4130-4136. doi: 10.1109/ECCE44975.2020.9236309
 R. Phukan, S. Ohn, D. Dong, R. Burgos, G. Mondal and S. Nielebock, "Design and Optimization of a Highly Integrated Modular Filter Building Block for Three-Level Grid Tied Converters," 2020 IEEE Energy Conversion Congress and Exposition (ECCE), Detroit, MI, USA, 2020, pp. 4949-4956.doi: 10.1109/ECCE44975.2020.9235895
 R. Phukan, L. Wei and J. Hu, "A Low Profile Gate Drive Power Supply," 2019 IEEE Applied Power Electronics Conference and Exposition (APEC), Anaheim, CA, USA, 2019, pp. 3394-3399.doi: 10.1109/APEC.2019.8722106

Mains power connectors
Wireless tuning and filtering
Tone, EQ and filter