Q: How to troubleshoot and fix EMC failures in pre-compliance testing?
Answer
EMC failure troubleshooting methodology: (1) Identify emission type: conducted (150 kHz-30 MHz) vs radiated (30 MHz-1 GHz). (2) For conducted: LISN measurement points to power leads – add X-capacitor and common-mode choke. (3) For radiated: near-field probe scan (H-field and E-field) to locate the emission source on the PCB. (4) Isolate the source: turn off peripherals one by one to identify which section causes emissions. (5) Common fixes: SMPS: add RC snubber on MOSFET ringing. Clock: add series termination or move the crystal away. Motor PWM: add EMI filter on motor wires. Cables: add ferrite clamp. (6) Check ground connection: floating grounds can cause unexpected emissions. (7) Add shielding: solder-down EMI can on identified noise source. (8) Verify fix: use spectrum analyzer with peak detector in max-hold mode. Document the fix in the BOM. Common misconception: more shielding always helps – sometimes it creates resonance. Always measure after each change.
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