EMI / EMC Basics
Electromagnetic interference sources, coupling paths, and mitigation techniques.
Reference
Coupling paths
| Path | Mechanism | Mitigation |
|---|---|---|
| Conducted | Noise rides on power/signal wires | Common-mode chokes, filters, ferrite beads |
| Radiated | Antenna-like structures emit EM | Shielding, ground planes, reduce loop area |
| Inductive | Changing current creates H-field | Twisted pairs, shorter loops, magnetic shielding |
| Capacitive | Changing voltage creates E-field | Separation, guards, balanced signaling |
Common sources
- Switching power supplies (SMPS) — sharp edges at f_sw and harmonics.
- Digital clocks / crystals — strong peaks at fundamental + harmonics.
- Brushed motors, relays — arcing at contacts.
- Fluorescent lighting ballasts, CFL/LED drivers.
- RF transmitters (WiFi, BT, LTE, walkie-talkies).
- ESD events from people/furniture.
Design practices
- Slow the edges: adding small series resistors on fast signals reduces high-frequency spectral content.
- Minimize loop area: high-speed return currents follow the signal trace — keep ground reference nearby.
- Continuous ground plane: avoid splits under fast traces — impedance discontinuities radiate.
- Filter at the connector: common-mode chokes + TVS + ferrite as signals enter/exit the enclosure.
- Shield cables and terminate shields at both ends for HF (one end for LF to avoid ground loops).
- Spread spectrum (SSCG) on clocks reduces peak EMI at the cost of slight jitter.
Standards (shortlist)
- FCC Part 15 (US)
- Class A (industrial) & Class B (residential) emission limits
- CISPR 22 / EN 55022
- European radiated + conducted limits (ITE)
- IEC 61000-4-x
- Immunity series (ESD, surge, burst, RF)
- CE marking
- Requires EMC Directive 2014/30/EU conformance
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