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18 DESIGN007 MAGAZINE I MARCH 2022 between 4–5 mils due to the proximity of the two planes. 2. Return Path Loop Area Return path discontinuities (RPDs) can cre- ate large loop areas that increase series induc- tance, degrade signal integrity, and increase crosstalk and electromagnetic radiation. Small discontinuities, such as vias and non-uniform return paths on a bus, are becoming an impor- tant factor for the signal integrity and timing of high-speed systems. RPDs produce impedance discontinuities due to the local return induc- tance and capacitive changes. Impedance dis- continuities create reflected noise, contribute to differential channel to channel noise, and may promote mode conversion. Common mode radiation is the result of parasitics in the circuit that emanate from the unwanted voltage drops in the conduc- tors. As the signal follows the transmission line, capacitive coupling between the trace and plane conductors completes the loop and displacement current flows through the capacitance which returns to the source. e common mode current that flows through the ground impedance produces a voltage drop in the digital logic ground system and generates magnetic radiation. To control common mode radiation, it is important to minimize the common mode ground voltage at the source. Also, good grounding minimizes noise sources by pre- senting common mode currents with a low impedance path to ground potential. Return- ing signal currents tend to stay near their sig- nal conductors, falling off in intensity with the square of increasing distance. If the return path of a common mode current is far from the sig- nal path, then the common mode current will radiate. However, if you engineer the return path to be near the source current, then the loop area will be small and therefore the com- mon mode current will not radiate. 3. Planar Coupling With the continuous trend to smaller feature sizes and faster signal speeds, planar capaci- tor laminate or embedded capacitor materials (ECM) are becoming a cost-effective solution for improved power integrity. is technol- ogy provides an effective approach for decou- pling high-performance ICs whilst also reduc- ing electromagnetic interference. Coupling the planes very close together creates high capacitance. Embedded capacitance technology allows for a very thin dielectric layer (0.24–2.0 mil) that provides distributive decoupling capaci- tance and takes the place of conventional discrete decoupling capacitors over 1 GHz. Unfortunately, standard decoupling capacitors have little effect over 1 GHz and the only way to reduce the AC impedance of the power dis- Table 1: Embedded capacitor materials available in the iCD Dielectric Materials Library.