Where is the Return Path?
At DC, the return path is a straight line (path of least resistance), but as frequency increases, we cannot simplify and just say “the path of least resistance” anymore. It is in fact the path of least impedance. At the low to medium-high frequency of the digital high-speed spectrum, inductance is the dominating part of impedance. Therefore, we can somewhat simply say that the return path is the path of least inductance. In today’s circuit boards, this path is usually in the ground plane right under the signal conductor. This deviates only if there are obstacles in the form of a slotted or split ground plane or where vias perforate the ground plane enough to cut the return path. At RF frequencies (this includes ultra-high-speed digital), capacitance becomes dominant. Thus, the signal will typically find its return path as the path of least capacitance. This makes things extremely complicated as the path of “least capacitance” is not as predicable as for inductance. It may very well inc
