Hello, I'm currently working on an assessment of the NISM method in my lab where I am measuring the phase margin of a POL using both NISM and Bode plot methods to see how similar the results are. I'm running into some trouble trying to make the measurement on some of my buck converter POL's. Some of these converters measure very low impedances at low frequencies (<10mOhm). When looking for the impedance peak (where the DC-DC converter stops being able to keep the impedance low and the MLCC's take over) I will typically find that the peak can be 30mOhms or less.
Looking at the Q(Tg) plot, I find that whenever the impedance is less than about 30mOhms, it becomes incredibly noisy. I have tried increasing signal power, increasing number of points, reducing receiver BW and none of these allow me to get a clean Q(Tg) plot. I find that there are peaks that show up, but as I lower the receiver BW, increase number of points, or increase signal power in an attempt to increase the fidelity of Q(Tg), I get multiple large peaks concentrated near the same frequency. Below is an image I took of a POL. My setup is a Bode 100 with the P2102A 2-port probe, a J2113A differential amplifier on the receiver port, and a P2130A DC block on the input to the J2113A. I know from performing transient step loads and looking at the response to assess the damping ratio, that the actual phase margin can't be only 5.8 degrees. Besides this one example below, I have measured other DC-DC's where the impedance peak is even lower than this plot, and I can't obtain any meaningful Q(Tg) plot.
Overall, how do I overcome this issue of a noisy Q(Tg) when the impedance peak near the crossover frequency of the POL ends up being at a low impedance below 30mOhms? I also have a J2112A that I have experimented performing a 3-port impedance analysis with as well, but I run into the exact same issue, and I have not been able to determine a method to properly calibrate the VNA when using the J2112A since voltage is needed for calibration. Any advice/tips/pointers would be very helpful as so far I am having trouble overcoming this limitation.