Electromagnetic Interference and Compatibility

Why it matters

Electromagnetic compatibility means a device performs acceptably in its electromagnetic environment while limiting disturbance to other equipment. It involves both emissions and immunity, not immunity alone.

Coupling paths

An interference problem has a source, a coupling path and a susceptible receiver. Paths include common-impedance coupling through shared returns, capacitive coupling from changing voltage, inductive coupling from changing current, and radiated fields. Conducted interference travels through connected conductors; radiated coupling occurs through fields and need not be in air.

Mitigation

Reduce noise at the source through appropriate switching edges and local decoupling. Minimize high-current loop area, provide continuous intentional return paths, separate sensitive circuits from noisy nodes, and use suitable filters and shield terminations. Twisted pairs reduce magnetic pickup by limiting loop area; differential signaling also depends on balance and common-mode rejection. Grounding choices are frequency- and topology-dependent; a long wire is not an ideal RF ground.

Useful equations

An ideal inductive coupling estimate is v_induced = M di/dt, where M is mutual inductance in H. Capacitive coupling current is i = C_c dv/dt. Shielding effectiveness for electric-field amplitude is SE = 20log10(E_before/E_after) dB under comparable measurement conditions. These are simplified estimates, not a compliance test.

Worked example

A switching-current loop couples M = 10 nH to a nearby measurement loop. Current rises by 2 A in 20 ns; assume a linear ramp.

di/dt = 2/(20×10^−9) = 1×10^8 A/s. The induced-voltage magnitude is M di/dt = (10×10^−9)(1×10^8) = 1 V during the ramp. Reducing mutual inductance to 1 nH reduces it to 0.1 V under the same assumptions. Polarity follows the chosen loop orientation and Lenz’s law.

For a separate shielding measurement, reducing field amplitude from 1 V/m to 0.01 V/m gives SE = 20log10(100) = 40 dB.

Common mistakes

Calling EMC only resistance to interference; treating DC continuity as proof of a good high-frequency return; confusing common-mode and differential-mode noise; or substituting photon energy for an EMI coupling calculation. Product-specific compliance requires the applicable test method, setup, limits and standard edition, not these illustrative calculations.

Quick check

1. What are the three parts of an interference problem?
2. What is the induced voltage for M = 2 nH and di/dt = 10^8 A/s?
3. Does EMC include emissions?

Answers: 1. Source, path and susceptible receiver. 2. 0.2 V. 3. Yes, emissions and immunity.