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54 DESIGN007 MAGAZINE I OCTOBER 2025 board design system. Most CAD systems can define a power and ground area that appears as a solid graphic representation. When an area of copper is retained on the outer surface of the circuit board, it will likely be positioned to service ground isola- tion of specific components or a group of related components. For mixed-function applications, the designer may divide the power and ground planes on an inner circuit layer to isolate differing voltage and ground potentials. Experts in this domain note that frequency signals can radiate EMI if they aren't routed carefully on the circuit board. Not only can the length and configura- tion of the conductors be a problem, but conductor and via stubs can also act as an antenna. Another source of EMI is the signal return path, which opti- mally should be on an adjacent reference plane. If the return path is blocked in any way, the signal will radiate even more noise as it seeks a path back to its source. Two issues that can affect product per- formance are crosstalk and switching noise: • Crosstalk (effects of electromagnetic coupling): High-speed transmission lines spaced too close together may inadvertently couple, with one signal overpowering the other, creating crosstalk. This can result in the victim signal mimicking the characteristics of the aggressor signal and not performing its intended task. Not only is this a problem with side-by-side conductors, but also with those routed in par- allel on adjacent layers of the board. This type of crosstalk is known as broadside coupling and is why multilayer circuit board design- ers are advised to alternate the horizontal and vertical routing directions on adjacent layers. D ES I G N E RS N OT E B O O K • Switching noise (ground bounce): With sev- eral components switching between high and low states on a circuit board, the voltage level may not return all the way to the ground potential (as it should) when it switches to low. If the voltage level of the low state bounces too high, the low signal state may be falsely interpreted as a high state. When this hap- pens simultaneously, it may result in false or double-switching (bounce) and disrupt the operation of the circuit. Planar Capacitor Design The general value range for the embedded pla- nar capacitor is contingent on the dielectric mate- rial selected for separating the conductive foil lay- ers (Figure 2). Basic Planar Capacitor Using the dielectric material sandwiched between opposing copper planes will provide significant capacitance with very low inductance. The dielec- tric constant of the selected material, material thick- ness, and the total area determine the resulting level of capacitance. Distributed Planar Capacitor Considered the simplest solution and commonly used to replace discrete external power supply decoupling capacitors, the planar capacitors rely on closely spaced power and ground planes that are separated by a thin dielectric layer. Polyimide (PI) Film Planar Capacitors Polyimide film dielectrics are typically used to sepa- rate the circuit board layers dedicated to the power F i g u re 2 : C o m p a r i n g c a p a c i t a n c e ra n g e fo r d i e l e ct r i c m ate r i a l s . ▼