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EMI and EMC Basics
Learn2026-01-14

EMI and EMC Basics

#electronics#emi#emc

Overview

This chapter explains common sources of radiated and conducted emissions and practical techniques to reduce EMI and ensure EMC compliance at the board level.

Prerequisites

  • Basic PCB layout knowledge and signal routing experience

Learning objectives

  • Identify typical EMI sources on a board and their coupling mechanisms
  • Apply filtering, shielding, and grounding strategies to reduce emissions
  • Prepare a design for basic pre‑compliance checks

Tools & materials

  • Spectrum analyser (or pre‑compliance tool), oscilloscope with broadband probe, basic shield materials

Hands-On Mini Task

  1. Measure a switching regulator's radiated noise with a near‑field probe or improvised loop and identify dominant noise bands.
  2. Add input/output LC filtering, a PCB ground pour, and common-mode choke where appropriate; re-measure and compare.

Expected result: reduced peak emissions in measured bands and a clear understanding of which mitigation steps were most effective.

Sources and coupling mechanisms

  • Switching nodes: fast edges create broad-spectrum radiated emissions.
  • Loops and cables: act as antennas for radiated and conducted emissions.
  • Common-mode currents: can couple into chassis and cables causing long-range interference.

Mitigation techniques

  • Minimise loop area: place decouplers and route returns to reduce emitted fields.
  • Filtering: use LC input/output filters and common-mode chokes for cable interfaces.
  • Shielding: enclosures and gaskets for sensitive circuits or emissions-critical components.

Pre-compliance checks

  • Use a near-field probe and a spectrum analyser to find hotspots before formal testing.
  • Test cables and connectors as part of the setup; fix conducted issues early with series ferrites.

Worked example — reducing switching regulator emissions

  1. Measure baseline with near-field probe; identify dominant frequency bands.
  2. Add a common-mode choke on the input and a Pi filter on the output; place X/Y capacitors where appropriate.
  3. Re-measure and compare spectra; document improvements and remaining peaks for targeted work.

Troubleshooting

  • If adding filters causes instability, revisit regulator compensation and loop stability.
  • Excessive emissions from connectors often indicate poor grounding or cable routing; re-route and add common-mode suppression.

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