Background Today, voltage regulator modules (VRM) (and power supplies) 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 is 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.
It has been well discussed that the 2-port shunt-through impedance measurement is the gold standard for measuring a VRM’s (or power supply's) output impedance in the microOhm and milliOhm region . However, it is not always possible to make these measurements with direct coaxial connectors designed into the PCB or Device Under Test (DUT).
What do we get with a 2-port probe?
We want repeatability, which is not possible with any of the connections to the device under test (DUT), as shown in Figure 1 below. This only comes with a connector or a probe. Achieving good and repeatable calibration with 2-port measurements is not possible with any of the solutions shown below. Further, most PCBAs are not designed with proper probe points to make 2-port measurements. Lastly, the inductance of these measurements ultimately limits the noise floor of the measurement setup.
Figure 1 - Why you cannot use Clip Leads or Soldered Wires 
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 Picotest P2102A, you can quickly characterize multiple VRMs to ensure stability or even check if your model is accurate during your initial PDN design.
Figure 2 - Depiction of Picotest P2102A 2-Port Probe
The 2-port shunt-through impedance testing method is the gold standard for measuring a VRM's output impedance and the control loop gain (phase) stability performance.
The P2102A browser probe allows you to very simply, quickly, and accurately make 2-port impedance measurements that help better design your PDN and provide quick GO/NO-GO testing. When impedances are measured, not only can we determine a VRM’s small signal stability, but we can also determine what the power supply distribution network is composed of. We can even create a highly accurate model of the VRM from the impedance measurement that includes time domain, frequency domain, and even EMI-related (EMC) data. We can tell which parts of the impedance are based on control loop performance and which parts are based on printed circuit board and/or decoupling performance.
As a final thought that I always like to teach folks, it is that "noise follows impedance, ALWAYS!" If you remember that, impedance measurements can tell you a lot about where the noise in your PDN and system is residing. This premise will always help you as a power integrity engineer.
If you want to learn more about how to do 2-port impedance measurements, feel free to reach out to Signal Edge Solutions for details on our power integrity training. You can also check out a video on 2-port impedance calibration with the Bode 100 and Picotest P2102A probe on our YouTube channel HERE.