Electromagnetic Interference
Understanding EMI
EMI is one of the most common and frustrating problems in electronic system design. An RF product that works perfectly on the bench may fail EMI compliance testing, requiring redesign of shielding, filtering, grounding, and layout. Designing for EMI from the start is far more cost-effective than fixing problems after prototyping.
EMI Types
- Conducted emissions: Noise on power and signal cables. Measured with a LISN and spectrum analyzer.
- Radiated emissions: Noise radiated through space. Measured with an antenna in an anechoic chamber or OATS.
- Conducted susceptibility: Immunity to noise injected on cables.
- Radiated susceptibility: Immunity to external RF fields.
EMI Mitigation
- Shielding: Metal enclosures prevent radiation escape. Seal all openings.
- Filtering: EMI filters on power and signal lines suppress conducted noise.
- Grounding: Proper ground design prevents ground loops and common-mode noise.
- Layout: Minimize loop area, separate analog/digital, use ground planes.
Frequently Asked Questions
What is EMI?
EMI is unwanted electromagnetic energy that disrupts electronic systems. It can be conducted (through wires) or radiated (through space). Sources include switching power supplies, digital circuits, transmitters, and natural events. Compliance testing is mandatory.
What are FCC and CE EMI limits?
FCC Part 15 (US) and CE EN 55032 (EU) specify maximum allowable conducted and radiated emissions from electronic products. Class A is for industrial use; Class B (stricter) is for residential. Failure to comply prohibits sale in those markets.
How is EMI measured?
Conducted emissions: current probe or LISN on power cable, measured with EMI receiver. Radiated emissions: calibrated antenna at 3m or 10m distance in anechoic chamber or open-area test site. Measurements compared to regulatory limits.