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54 The PCB Design Magazine • April 2015 the cross sectional area and length of the cop- per planes. From equation 1 below, capacitance can be calculated where C = capacitance in pF, eo = permittivity of free space (0.225pF/inch), er = dielectric constant, A = area (in 2 ), and S = spacing between the planes (inches) [3] . Here we see that the best way to increase the capacitance is by decreasing the spacing between the planes or increasing the area of the metal layers. (1) According to Ott [2] , the effective radius in which capacitance can be utilized is calculated by the equation below, where r = radius in inch- es, t = rise/fall time of switching edge (ns), and er = dielectric constant. By looking at this equa- tion, we can conclude that the effective area (r) decreases when either the switching edge be- comes faster (t decreases), or the dielectric con- stant of the medium increases. (2) article Figure 1: The baseline stack-up for experiment 1. Power plane pairs are spaced 3 mils apart. EFFECTIvE DECoUPLING RADIUS continues

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