Figure 1: RFI filter or EMI filter concept

A Radio Frequency Interference (RFI) filter is an electronic device designed to suppress radio frequency noise currents or conducted RFI signals present on power and/or signal lines, which could interfere with radio communication equipment or systems and other RF-sensitive electrical and electronic devices. The radio frequency spectrum is a subset of the electromagnetic spectrum, encompassing frequencies from 30 kHz to 300 GHz.  RFI filters are designed to target and suppress interference within this radio frequency range. Sometimes, the RFI filters are also referred to as electromagnetic interference (EMI) filters. 

The purpose of the RFI/EMI filter is to filter out high-frequency noise currents (conducted RFI or EMI) present on the power lines and signal lines. This prevents these conducted noise currents from entering or exiting the filtered device, allowing the device to function smoothly without disrupting other connected equipment (figure 2). The RFI filters ensure the proper operation of radio communication systems, prevent data corruption and signal quality degradation, and also prevent the malfunction of RF-sensitive electrical and electronic devices.

Figure 2: RFI/EMI filter used in power line to prevent conducted noise currents from entering or exiting the filtered device

Note: When high-frequency or radio-frequency noise currents flow through power lines or wires, they can act as unintentional antennas, radiating electromagnetic signals that could interfere with the operation of nearby communication systems or electronic devices (referred to as radiated EMI). Also, when acting as unintentional antennas, the wires can pick up electromagnetic signals from the environment and convert them into electrical noise currents, leading to conducted EMI/RFI on the lines. By suppressing the noise currents on the wires or lines, the RFI/EMI filter blocks both conducted EMI and radiated EMI (caused by conducted noise currents on the lines). 

The RFI/EMI filters are connected to signal lines and the input (power entry point, or POE) and output power lines of the equipment to attenuate noise currents. They can be used as a separate device or integrated with electrical and electronic equipment. These filters help the connected electrical or electronic equipment to operate reliably without causing interference to nearby devices, thereby enhancing the electromagnetic compatibility (EMC) of the equipment.

Construction and working principle of RFI/EMI Filters:

Generally, the RFI or EMI filters' design utilizes passive circuit elements such as capacitors, inductors, and in some cases resistors to attenuate both conducted and radiated RFI/EMI. Active filters are also available that utilize active elements such as e.g., transistor, Op-amp, etc.

                         Figure: Typical single-phase EMI/RFI filter electrical circuit (passive filter)This filter basically works as a low-pass filter that suppresses high-frequency noises on power lines or signal lines while allowing desired frequency currents to reach the equipment or load. The filter diverts the noise currents on the lines to the ground while allowing the desirable clean current to reach the connected device. Some RFI or EMI filters are also available in high-pass, band-pass, and band-stop filter configurations. 

Usually, these filters are housed in metal enclosures to shield the filter circuit from exposure to any radiated interference. The metal enclosure shield of the filter also blocks any internally generated radiated EMI from reaching the outside environment.

Types of RFI/EMI filters include:

• Single-Phase Filters: Designed for filter noises in single-phase power supply systems.
• Three-Phase Filters: Designed for three-phase power systems, handling higher power loads.
• Signal Line Filters: Protect data, telephone, and audio/video communication lines.
• DC filters
• EMI Feedthrough Filters
• EMC/EMI Chokes
• PCB Mount Filters: Available in surface mount and through-hole configurations for direct mounting on printed circuit boards.

Applications: 

RFI/EMI filters are used in various applications to suppress interference and ensure the proper functioning of electronic devices and systems. These applications include:

• Consumer Electronics: Televisions, radios, and home appliances
• Switch Mode Power Supplies (SMPS)
• Power supplies
• Telecommunication Equipment: For ensuring clear signal transmission and reception
• Computers and Peripherals
• Home Appliances: examples, microwave ovens, refrigerators, and washing machines
• Industrial Machinery
• Medical Devices
• Uninterruptible Power Supplies (UPS) Systems
• Variable Frequency Drives (VFD)
• Test and Measurement Devices
• EMC Testing Chambers
• Energy Management Systems
• Military and Aerospace Systems, etc.

By using RFI/EMI filters in these applications, it is possible to enhance the reliability, performance, and electromagnetic compatibility (EMC) of various electronic and electrical equipment.