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18 DESIGN007 MAGAZINE I OCTOBER 2025 • Suppress Edge-fired Emissions With Plane Offset Techniques: Resonant emis- sions typically originate from fringing fields at board edges. While reducing plane sepa- ration is effective for ground/power pairs, it's not always feasible for multilayer stacks. An alternative is to shrink power planes slightly (~200 mil) relative to ground planes. This shifts the fringing field inward, reducing edge radiation and improving EMC performance. Dampening resonance with R-C terminators may also improve plane resonance. • At DC and low frequencies, inductance is negligible and can often be disregarded. However, as signal frequencies and edge rates rise, parasitic capacitance and induc- tance begin to dominate. • Each capacitor includes an equivalent series inductance (ESL), which causes its impedance to climb at higher frequencies. • Capacitors exhibit their lowest impedance at their self-resonant frequency. • The inductance introduced by the capaci- tor's mounting configuration significantly influences circuit behavior. • For high-layer-count stackups, it's optimal to place decoupling capacitors on the same side of the board as the IC, minimizing via length and loop area. • Beyond a few hundred megahertz, only on-die capacitance and the intrinsic planar capacitance between tightly coupled power and ground planes can effectively suppress PDN impedance. • The return current path plays a pivotal role in power integrity. Discontinuities and high- impedance paths can severely impact signal integrity and power delivery in high-speed designs. DESIGN007 Resources Beyond Design by Barry Olney: "The Impact of PDN Impedance on EMI," "The Curse of the Golden Board," "The Target Impedance Approach to PDN Design." Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integrity software, incorporating the iCD Stackup, PDN, and CPW Planner. You can download the software at www.icd.com.au. To read past columns, click here. B E YO N D D ES I G N Spectral integrity is established through the precise control of frequency-domain behavior across a system's signal paths. " " Spectral integrity is established through the pre- cise control of frequency-domain behavior across a system's signal paths. Its structure is built on a com- bination of physical layout, material properties, and electromagnetic design principles. At its core are transmission line geometries that preserve wave- form fidelity, dielectric materials that maintain consis- tent impedance across frequencies, and return path continuity that minimizes mode conversion and radi- ated emissions. Decoupling strategies, PDN reso- nance control, and stackup symmetry all contribute to a stable spectral profile. When these elements are harmonized, the system maintains clean spec- tral content—minimizing distortion, reflections, and noise coupling—ensuring reliable performance in high-speed digital and mixed-signal environments. Key Points • Maintaining AC impedance within acceptable limits across the entire bandwidth is essential to minimize unwanted radiation and ensure compliance with EMC standards. • Decoupling capacitors fulfill two distinct yet interdependent functions—one temporal, one spectral—that together ensure robust power integrity.