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68 DESIGN007 MAGAZINE I MAY 2025 • Shielding: Placing a grounded shield between conductors can block electric fields and reduce coupling. • Signal voltage: Reducing the voltage levels of the aggressor signals can weaken elec- tric fields and decrease coupling. • Dielectric material: Increasing the dielec- tric constant slows down the signal propa- gation, reducing interaction and improv- ing the containment of the field. Energy can also radiate from the board edges. e energy stored between the con- ducting planes works with the energy stored in the decoupling capacitors. When energy is needed, electromagnetic waves travel through the decoupling capacitors and the cavity between the planes. Reflections of these waves send energy back to the point where it's required. However, when a wave reaches the edge of the board, the open circuit causes the wave to reflect, doubling its voltage. Different frequency wavefronts reach the board edge at different times, and each reflection doubles the wave's amplitude. ere are various approaches to reducing radi- ation edge effects from the PCB. In many cases, energy can be reflected, possibly creating addi- tional internal cavity resonance effects and cou- pling to internal vias, also resulting in increased radiation. However, termination along the board edges reduces the resonance peaks. In practice, this means approximating a continuous structure with resistors, spaced around the perimeter, that are AC coupled with a ceramic capacitor of suf- ficient capacitance to allow the resulting imped- ance to appear predominately resistive at and above the lowest frequency of interest. Effective mitigation techniques, including proper spacing, shielding, and strategic ter- mination along board edges, are essential for maintaining signal integrity and minimizing radiated emissions. By addressing these chal- lenges, engineers can design PCBs that per- form reliably even in demanding electromag- netic environments. Key Points • e characteristic impedance represents the ratio of the electric field intensity to the magnetic field intensity as they propa- gate along the line. • ere is negligible radiation for traces situ- ated between conducting planes in a strip- line configuration. • On a typical outer (microstrip) layer, even a small portion of energy escaping from each trace can collectively produce a sig- nificant radiated field. • Electromagnetic fields are totally blocked by a solid plane. • Radiation or field coupling occurs only in areas where the wave amplitude is chang- ing. is happens at the leading edge as it moves down the line. When the rise time increases, the radiating area grows larger. • Crosstalk is caused by capacitive and inductive coupling. • Energy can radiate from the board edges. e energy stored between the conduct- ing planes works together with the energy stored in the decoupling capacitors. • When a planar wave reaches the edge of the board, the open circuit causes the wave to reflect, doubling its voltage. • Effective mitigation techniques, includ- ing proper spacing, shielding, and stra- tegic termination along board edges, are essential. DESIGN007 Resources • Beyond Design columns by Barry Olney: "Elec- tromagnetic Fields Parts 1 & 2," "Fringing Fields," "Dampening Plane Resonance With Termination" • "Fast Circuit Boards" by Ralph Morrison 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. The software can be downloaded at www.icd.com.au. To read past columns, click here.