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Design007-Mar2022

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16 DESIGN007 MAGAZINE I MARCH 2022 The Coupling Coup Coupling on a multilayer PCB may be a good or bad thing. On one hand, close cou- pling of signal traces to reference planes and differential pair signals is the best way to pre- vent common mode radiation and to mitigate electromagnetic (EM) emission. But on the other hand, close coupling of unrelated signal traces can bring us grief with unintentional crosstalk caused by overlapping EM fields. In this month's column, I will look at where close coupling should be used and where it should be avoided. ere are five common situations where coupling can influence the signal and power integrity of a design: 1. Signal to reference plane coupling. 2. Return path loop area. 3. Planar coupling, 4. Differential pair signal coupling. 5. Induced crosstalk. 1. Signal-to-reference Plane Coupling e first rule of stackup design is that all sig- nal layers should be adjacent to, and closely coupled to, an uninterrupted reference plane, which creates a clear return path. Closely cou- pling a signal trace to the reference plane mini- mizes the loop area and reduces inductance. Also, reducing the dielectric height will result in a large reduction in crosstalk without hav- ing a negative impact on available space on your board. However, one must be aware that reducing the dielectric height will require an increase in trace width to accommodate the desired impedance of the transmission line. is could become an issue for a densely routed design. So, typically a ~3 mil thick dielectric is selected for microstrip configurations. Figure 1 illustrates the variation of dielectric thick- ness and trace width. For stripline configura- tions, the dielectric thickness generally falls Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Coupling of signal-to-reference plane (simulated by the iCD Stackup Planner).

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