I receive many emails with questions about measuring power distribution networks (PDNs), but these two are very common. Why do I calibrate the 2-port measurement with a 1 shunt resistor, and, why do I use DC blockers on both ports? In this article Ill provide responses to both of these questions. The measurement setup in Figure 1 is an example where I used both the 1 calibration and the inclusion of the DC blockers
Figure 1: Voltage regulator test board with a pair of SMA connectors connected to the regulator output and a pair of SMA connectors connected to a 1 calibration resistor (R3). Image courtesy AEI Systems.
Why are DC blockers used on both ports?
The second question is easier to answer than the first, so lets answer that one first. There are two reasons for the use of the DC blockers. First, the standard 2-port measurement connects both instrument ports (typically 50) to the voltage regulator module (VRM) being tested. These ports result in a load current of Vout/25 and this can be significant in comparison to the device loading. For example, measuring a 2.5V low power voltage regulator the port loading would add 100mA of output current. This 100mA can be more than the maximum load of the device, can overload the regulator, and can greatly alter the regulator performance, which is load current dependent.