What is EMC and how does EMI couple?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Electromagnetic compatibility (EMC) is the ability of equipment to function in its electromagnetic environment without introducing intolerable disturbances to anything else. Electromagnetic interference (EMI) requires a source, a coupling path and a victim; coupling is conductive, capacitive (electric field), inductive (magnetic field) or radiative. EMC regulations limit both emissions and required immunity.^[1]^[2]

Why are common-mode currents so important?

Differential-mode currents on a small loop radiate weakly; common-mode currents on cables act as monopole antennas and radiate strongly. Paul’s estimate for common-mode radiation from a cable of length L carrying current I_c at frequency f, at distance r, is E ≈ 1.257×10⁻⁶·f·I_c·L/r V/m: just microamps of common-mode current can exceed Class B limits.^[2]

What are the key design principles?

Minimize loop areas of high-di/dt currents; keep return paths adjacent.
Use solid reference planes; avoid slots under fast signals.
Filter every cable entering or leaving the enclosure at the boundary.
Control edge rates and clock harmonics; use spread-spectrum clocking where allowed.
Shield where necessary, minding apertures (see shielding).

^[1]

Which standards apply?

US unintentional radiators: FCC Part 15 Subpart B. International multimedia equipment: CISPR 32 emissions and CISPR 35 immunity; automotive components: CISPR 25; immunity test methods: IEC 61000-4 series. See CISPR standards and immunity and ESD.^[3]^[4]

Frequently asked questions

What is the difference between EMI and EMC?

EMI is the interference; EMC is the discipline and property of avoiding it.

Why do cables cause most EMI failures?

They are efficient antennas for common-mode currents.

Is EMC only about radiated emissions?

No; it also covers conducted emissions and immunity to RF, ESD, surges and transients.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Electromagnetic Compatibility Engineering”. H. W. Ott, Wiley. 2009.
“Introduction to Electromagnetic Compatibility, 2nd ed.”. C. R. Paul, Wiley. 2006.
“47 CFR Part 15: Radio Frequency Devices”. U.S. Federal Communications Commission, via eCFR. current.
“CISPR 32: Electromagnetic compatibility of multimedia equipment — Emission requirements”. IEC / CISPR. edition 2.0, 2015 and amendments.

Categories: EMC and EMI

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