Issue link: https://iconnect007.uberflip.com/i/1481368
58 DESIGN007 MAGAZINE I OCTOBER 2022 possibly in the tens of nHs, the noise across it was low because the dI/dt rate of current change was even smaller. Fast-forward about 50 years; Figure 2 shows the front and back of a CPU module from the late 2010s 2 . I designed the power distribution network for this board that consumed hun- dreds of watts, had 20+ layers, and used sev- eral hundred bypass capacitors. e board had multiple solid ground layers and multiple power planes for the high-current supply rails. e power and ground planes in close proxim- ity produced very low inductance in the tens of picohenries range, which was necessary to counter the high dI/dt of the chip. Large power planes provide not only lower inductance, but they are also necessary to keep the DC voltage drop low. However, power planes come with some downsides as they produce resonances that can interfere with both the power delivery, or most likely, with our high-speed signaling. We know that signal traces will resonate if we don't terminate them properly. Even with proper terminations at the ends, additional reactances along the signal- ing channel can create quarter-wave or half- wave resonances 3 . Traces are one-dimensional transmission lines, exhibiting a series of modal resonances associated with their length. Traces are one-dimensional, because we must keep the trace width and dielectric separation much smaller than the shortest wavelength of inter- est. In contrast, planes are two-dimensional resonators and rectangular plane pairs exhibit modal resonances both along their length and width 4,5 . As an illustration, Figure 3 shows a simulated impedance surface created by the standing-wave pattern on a 2:1 aspect ratio rectangular plane pair. Depending on the resonance frequencies and the functionality of our circuit on the board, the standing waves and resonances can create Figure 2: CPU module from the late 2010s. a b Figure 3: Impedance magnitude showing two-dimensional standing-wave pattern on a rectangular pair of power/ground planes.