It's the inductance that matters

Last evening, I attended a seminar given by Bruce Archambeault, an IBM distinguished engineer and overall EMC authority. His topic, “PCB Decoupling for Effective Power Integrity and EMI control” sheds light on the physics of PCBs, how adding capacitors can help reduce emissions, and why inductance leads to EMI and power-integrity problems.

Archambeault explained how vias in a PCB act as antennas that radiate energy from a board. Those emissions can couple into other parts of a circuit, making their way to I/O pins where they can escape and cause interference with other devices.

Photo courtesy of Endicott Interconnect Technologies

Through measurements and simulations, Archambeault also explained that you need capacitors to supply charge to IC when they need it, but placement of those capacitors is critical. The loop distance from a capacitor to an IC has inductance that can reduce the current from a capacitor or an IC. The longer the loop, the greater the inductance. He also demonstrated how capacitors combine with PCB inductance to form resonant circuits and that the effect or loop inductance is frequency dependent. Thus, you should minimize the distance from a capacitor to a via. You should also place a capacitor on the side of the PCB closest to the power planes to minimize inductance.

Archambeault’s 90-min. presentation, sponsored by the Boston chapter of the IEEE EMC Society, gave engineers insight as to how the physics of a PCB affects power  integrity and EMI. He livens his presentations with a little humor, too.

I recommend this seminar to anyone. Archambeault is a regular presenter at the IEEE EMC Symposium. I’ll make it a point to see more of his papers and presentations.