App Note: 2-Port Impedance Measurement using the P2102A Probe and Bode 100 VNA

By Benjamin Dannan and Steven Sandler, Picotest.com


High speed Printed Circuit Board (PCB) design requires well designed Power Delivery Networks (PDN) to support today's FPGAs and custom mixed-signal ASICs. The PDN contains important impedance information that can tell a designer how a system will react to dynamic currents and the impact of PCB layout. If we consider the PDN as a transmission line between the Voltage Regulator Module (VRM) and the load (ASICs), then a starting point for a good PDN design is the VRM.


Today VRMs need to supply power to multiple VDD cores to support FPGAs and/or custom ASICs using multi-gigabit ethernet, PCIe, and DDR memory interfaces. With that being said, vendor information for a VRM’s output impedance is not available and not always accurate when it is supplied. Further, measuring ultra-low impedance on multiple VRMs or multi-topology DC-DC regulators is a challenge for any design engineer.


The 2-port shunt-through impedance measurement is the gold standard for measuring a VRM’s output impedance in the microOhm and milliOhm region [1]. However, it is not always possible to make these measurements with direct coaxial connectors designed into the PCB or Device Under Test (DUT). Therefore, when a designer makes these types of measurements with a Vector Network Analyzer (VNA), the method of connecting the DUT requires attention to detail to ensure inductance and various error sources are minimized to allow an accurate measurement. To get the most out of your VNA, you need to use the right probes and accessories to ensure your particular measurement application is successful. With a browser probe like the P2102A you can quickly characterize multiple VRMs to ensure stability or even check if your model is accurate during your initial PDN design...


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