Whitepaper

EMC Best Practices: Part II

Published by Astrodyne TDI

This application note from Astrodyne TDI, authored by Field Applications Engineer David Bourner, is the practical companion to Part I and translates the electromagnetic physics discussed there into actionable design guidance. It begins by framing the frequency domains relevant to conducted emissions (150 kHz to 30 MHz) and radiated emissions (30 MHz to 1 GHz and beyond), noting that managing conducted emissions through good board design makes radiated emission control significantly easier. The document then covers system partitioning as the foundational strategy: treating each enclosure as a three-dimensional conductive shield, keeping enclosure gaps below one-tenth of the wavelength of the highest emission frequency, and ensuring all penetrations are properly filtered or shielded. A contrasting pair of grounding examples illustrates how star-point grounding creates large inductive loops that both generate and receive radiated EMI, while tight conductor pairing — whether via twisted wire pairs externally or trace-over-ground-plane configurations on PCBs — minimizes loop area and reduces both emissions and susceptibility. Additional topics covered include shielded twisted pair cable and proper 360-degree shield termination, differential signaling as a means of rejecting common-mode noise (demonstrated with LTSpice simulations), placement of X and Y capacitors and ferrites around switched-mode power supplies, layered passive filtering strategies using inductors, MOVs, and TVS devices, and PCB stackup design principles including the importance of adjacent ground return planes, via stitching, and impedance-controlled connectors and test points.