What is an EMI | RFI Filter?

what is an emi, rfi filter, emi rfi noise filtersElectro Magnetic Interference (EMI) and Radio Frequency Interference (RFI) is the radiation or conduction of radio frequency energy (or unwanted electronic noise) produced by electrical and electronic devices at levels that interfere with operation of adjacent equipment. Frequency ranges of most concern are 10kHz to 30MHz (conducted) and 30MHz to 1GHz (radiated).

Sources of EMI include:

  • Motors
  • Fans
  • Electronics
  • Household Appliances
  • Switching Power Supplies
  • Lightning
  • Relays/Switches
  • Computers
  • Wi-Fi Devices
  • ARC Welders
  • Other Power Sources


Victims of Electrical Noise Include:

  • Computers
  • Medical Equipment
  • TV’s
  • Radios
  • Electronic Control Equipment
  • Telephone/Telecom/Data Equipment
  • Any Electrical Circuit


What Causes EMI/RFI?

The most common sources include components such as switching power supplies, relays, motors and triacs. These devices are found in a wide variety of equipment used in industrial, medical, white goods, and building HVAC equipment.

What are the Types of EMI | RFI?

An Electrical or Electronic Device Emits RFI in Two Ways:
  • Radiated RFI is emitted directly into the environment from the equipment itself.
  • Conducted RFI is released from components and equipment through the power line cord into the AC power line network. This conducted RFI can affect the performance of other devices on the same network.


How can EMI be Controlled?

Radiated RFI is usually controlled by providing proper shielding in the enclosure of the equipment.

Conducted RFI can be attenuated to satisfactory levels by including a power line filter in the system.

The filter suppresses conducted noise leaving the unit, reducing RFI to acceptable levels. It also helps to lower the susceptibility of the equipment to incoming power line noise that can affect its performance.

How do EMI | RFI Noise Filters Work?

Consisting of a multiple-port network of passive components arranged as a dual low-pass filter, the RFI filter attenuates radio frequency energy to acceptable levels, while permitting the power frequency current to pass through with little or no attenuation. Their function, essentially, is to trap noise and to prevent it from entering or leaving your equipment.

Selection of the most suitable RFI power line filter can best be based on the type of power supply or input impedance of the equipment and the mode of the offending RFI noise.

What are Power Line Noise Modes?

RFI is conducted through a power line in two modes. Asymmetric or common mode noise occurs between the line and ground. Symmetric or differential mode is measured from line to line.

Common Mode: Also known as line-to-ground noise measured between the power line and ground potential.

Differential Mode: Also known as line-to-line noise measured between the two (line and neutral) power conductors. Power line filters are designed to attenuate either one or both modes of noise. The need for one design over another will depend on the magnitude of each noise type present. The attenuation is measured in dB (decibels) at over a wide band of signal frequencies.

What are the Circuit Configurations of EMI | RFI Power Line Filters?

Typical types of EMI | RFI power line filters are designed for a specific type of signal and the devices in which they will be installed. The wide variation in devices and equipment that benefit from EMI filtering necessitate a range of standard solutions, as well as an expanse of customization capabilities. The following are a few types of EMI | RFI Power Line Filters.

Single-Phase Filters

A single-phase EMI | RFI power line filter is designed for AC or DC power lines with a positive or negative, or dual, signal/power path. This type of filter is installed in-line with the power/signal lines, allowing DC and AC signals to pass without attenuation, while heavily attenuating signals from 10kHz to 30MHz. These types of filters are used in single-phase motor drives, power supplies, office equipment, and test and measurement equipment, amongst other applications. Some single-phase filters are optimized for specific applications, such as their DC performance, medical equipment requirements, industrial safety requirements and other standards.

Three-Phase Filters

Three-phase filters are similar to single-phase filters except that the filter is designed to filter three signal/power lines for three-phase power and motor systems. There are some three-phase filters that also include filtering on the neutral line for applications that require it. Three-phase filters are useful as main input filters for industrial equipment, machine tools, machinery and automation systems. Depending on the leakage performance of a filter, they may even be used with some medical devices and equipment.

DC Filters

DC filters are designed specifically for filtering DC power and control lines. This could be for protecting solar panels, photovoltaic charging/converting systems, battery charging and conditioning systems, DC motor drives and inverter/converters. Though similar to AC EMI | RFI filters, DC EMI | RFI filters are optimized for passing only DC signals and are typically rated for higher DC voltages and currents. These filters are useful in preventing premature aging and protection of solar panels due to conducted emissions, such as HF stray and leakage currents.

EMI/RFI suppression and mitigation technologies are essential for meeting today’s rigorous compliance standards and regulations. Of the available EMI/RFI suppression technologies, EMI/RFI filters are a versatile and effective solution for a wide range of applications, which can be readily customized to target specific EMI/RFI suppression requirements during the design or troubleshooting phase of product development. There are many factors to consider when selecting an ideal EMI/RFI suppression solution, and leveraging the experience of Curtis Industries can help optimize the product development and integration process.