By Eric Dontigney | Updated Mar 24, 2022

Photo by Piotr Wytrazek via iStock/GettyImages.

Background

Electromagnetic interference (EMI) refers to unwanted electrical or magnetic signals that degrade the performance of electronic equipment. Broadly, EMI is divided into narrowband emissions—typically human‑made and confined to a small portion of the spectrum, such as the hum from power lines—and broadband emissions that span a wide frequency range and can be either natural (e.g., lightning) or man‑made (e.g., computer processors).

Sources of EMI

EMI can arise from multiple origins:

• Internal sources – impedance mismatches, voltage fluctuations, or wiring interactions within a device.
• External sources – cosmic events like solar flares, nearby power or telephone lines, household appliances, and power cords.
• Power‑line‑borne interference – a significant portion of EMI travels along mains wiring to connected equipment.

Effective EMI mitigation relies on dedicated filters that are either standalone devices or integrated modules designed to attenuate these unwanted signals.

EMI Filters

Most EMI occurs at high frequencies, where rapid oscillations (e.g., thousands of cycles per second) introduce noise into sensitive circuits. EMI filters employ a synergy of two core components:

• Capacitors – specifically shunting capacitors that divert high‑frequency currents away from the main signal path while allowing the desired alternating current to pass.
• Inductors – miniature electromagnets that store energy in a magnetic field, thereby opposing rapid voltage changes and reducing overall interference. When arranged in series, the inductors progressively attenuate the signal, often grounding residual noise.

Optimally, an EMI filter can reduce high‑frequency interference to negligible levels, effectively “shorting” it to ground. These filters are ubiquitous across industries, protecting equipment such as laboratory instruments, radio systems, computers, medical devices, and military hardware.