EMI/EMC Academy ~35 min Interactive

Common-Mode Chokes & Filtering

When the source is as quiet as it'll get, filtering removes the rest. The key insight: most EMI is common-mode, so the hero component is the common-mode choke — which passes your signal untouched while blocking the noise riding on both conductors.

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Learning objectives. Distinguish common-mode from differential-mode noise; select and place a CM choke; recognize π/T/L filter topologies and Y-caps (with safety); use ferrite beads correctly; and measure a filter's real insertion loss on the bench.

Noise on a two-wire interface comes in two flavors. You must know which one you have, because the cure is different.

Differential-mode (DM). Current flows out on one conductor and back on the other — equal and opposite. It's the intended signal path; DM noise is usually your own supply ripple. Cured by DM filtering (X-caps, series inductors). Radiates relatively little because the two currents cancel.
Common-mode (CM). Noise current flows the same direction on both conductors, returning through ground/chassis/free space. It's the main cause of radiated failures because the currents add and the cable becomes an antenna. Cured by the CM choke and Y-caps to chassis.
DM (oppose → cancel) CM (same dir → add) returns via ground/free space → radiates
A CM choke presents high impedance to the red (common) currents but near-zero to the green (differential) signal.

A CM choke is not a perfect inductor — it has a self-resonant peak, after which parasitic capacitance takes over and impedance falls. Choose a part whose impedance peak sits at your problem frequency. Drag the slider to move the peak.

Illustrative |Z| curve. Match the peak to the emission you measured in Module 4.

Line filters combine components into π, T, or L topologies. The rule of thumb: face a high source/load impedance with a shunt capacitor, and a low impedance with a series inductor. A π filter (C–L–C) suits high-impedance environments on both sides; an L filter suits a high/low pairing.

Y-caps (line/neutral → chassis) shunt CM noise to ground. They are safety-critical: use only certified Y-rated capacitors and respect leakage-current limits. Never substitute an ordinary cap here.
Ferrite beads — common mistakes: using one on a high-DC-current rail (it saturates and does nothing), or on a signal that needs the bandwidth (it slows the edge and breaks SI). Pick the bead's impedance at the noise frequency, not its "value."
A ferrite over a whole cable is a snap-on CM choke — quick to try on the bench with a current probe. If the peak drops, design in a proper CM choke; if it doesn't, the emission isn't CM current on that cable.

For a simple LC low-pass, the corner is fc = 1 / (2π√(LC)). Set the corner about a decade below your problem frequency for meaningful attenuation.

Key takeaways. Identify CM vs DM, then pick a CM choke whose impedance peaks at your problem frequency; add Y-caps (certified) and use ferrites by impedance, not value. Next: when filtering isn't enough, we shield.
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