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Troubleshooting Distributed Power Systems: Measuring Impedance Using VNAs (2-Port) (Part 4 of the on


As Part 3 of this video series explained, recently introduced vector network analyzers (VNAs) have made it possible for these frequency domain instruments to measure the impedance of many types of power components and circuits. In that video, Steve Sandler discussed the benefits of using VNAs to measure the impedance of devices encountered in distributed power systems, providing details on how to perform and interpret one-port measurements. In this video, the focus shifts to making and interpreting two-port impedance measurements, particularly those in which the device under test is connected “in shunt through” with the VNA ports.


Shunt through, wideband measurements can be made from approximately 100 microohms to a few ohms. This is the measurement performed by designers for the power distribution network (PDN) assessment of VRMs and POLs. This technique may also be used to measure the impedances of batteries, dc-dc converters, EMI filters, and other functions. Setup requirements such as 4-wire connections, a common-mode transformer, dc blockers, and ac versus dc coupling are explained in this video. Also discussed is the use of a preamp to measure impedances below 1 milliohm. In addition, the video presents impedance measurement examples such as a POL’s output, a motherboard’s PDN and a 250-microohm resistor.

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