A two-port power distribution network (PDN) measurement is arguably the most accurate method of measuring low-value impedances, such as the output of point of load (POL) regulators and power planes. One limitation of the two-port measurement is that it places the operating voltage at both ports of the vector network analyzer (VNA), which are typically 50 Ohms. Typical network analyzers are limited to 500mW or 1W at these ports (The OMICRON Bode 100 used in this analysis, for example, is limited to 500mW at the source and 1W at CH1 or CH2).
Assuming the AC signals in the measurement are very small, the power ratings of the ports limit the absolute maximum DC voltage that can be measured to approximately 5V. Most POL applications are 3.3V or lower, so that this is not a problem for the output side of the POL. However, many POLs operate at input voltages above 5V, which would damage the front end of the analyzer if we directly connected the input voltage to the analyzer. In this case, the PDN measurement can still be performed, though some adjustment to the setup is needed.