Q: What are the common sources of radiated EMI emissions and how to suppress them?
Answer
Common EMI sources in embedded systems: (1) Switching power supplies: SMPS switching transients (1-100 MHz) are the dominant source in most embedded designs. (2) Digital clock harmonics: square waves contain odd harmonics up to GHz. (3) Motor PWM: high di/dt switching causes conducted and radiated emissions. (4) Crystal oscillators: fundamental and harmonics radiate directly. (5) Data lines with fast edges (USB HDMI). Suppression methods: (1) Shielding: metal enclosure with RF gasket around seams. (2) Filtering: pi-filter (C-L-C) at the power input. (3) Ferrites: clip-on ferrites on cables. (4) Decoupling: multiple bypass caps on IC power pins. (5) Edge rate control: use series resistors on gate drivers to slow MOSFET edges (trade-off with switching loss). (6) Spread spectrum clock generator (SSCG) to spread peak energy.
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