Since no impedance path is perfectly flat, we need to make some design adjustments. This article aims to help you make compromises that have the minimum impact on the system performance. The source impedance should match the impedance of the transmission line.
This is a basic premise of S parameter measurements and of all RF equipment in general. The source impedance (most often 50 ohms) is connected to a coaxial cable that has an impedance matching the source, and the load is terminated into the same impedance. This results in a perfectly flat impedance looking either from the source to the load or from the load to the source.
The output impedance of a voltage regulator can be considered to be a source while the PCB planes can be represented as a transmission line. The far-end decoupling capacitors are the load.