EMC basics: neither emit nor receive
If you first meet electromagnetic compatibility at the certification test, it is too late. A few habits at the design stage prevent most problems.
EMC goes both ways. Emission (EMI): your board must not send out noise. Immunity (EMS): it must not malfunction from external noise.
The three elements of noise
Source → path → victim. Breaking any one of the three is enough.
| Source | Measure |
|---|---|
| Switching converters, motor PWM | Minimise loop area, adjust switching speed |
| Clocks, fast signals | Series resistors, route over ground |
| Cables | Easily become antennas → filter at the connector |
Design habits
- An unbroken ground plane
- Small loops for fast currents
- Filters and ESD protection next to the connector on every line that leaves the board
- No fast signals at the board edge; ring the edge with ground vias (via stitching)
- Fix unused MCU pins as output LOW or with a pull-down
Measurement
Pre-compliance checks start with a near-field probe and a spectrum analyser to find where the noise is coming from. Test standards are defined: CISPR 25 for automotive, the IEC 61000 series for industrial.