One of the attractive aspects of a 2-port probe is the browser type functionality to measure PDN impedance of any board.
In lecture "2-Port Impedance Measurement Part-2", you measure the PDN impedance by putting the 2-port 4-point (Kelvin) across two capacitor pads. This means either the capacitors were removed, or the pads were added extra during design time.
(1) Can we measure the PDN impedance without removing the decoupling capacitors, since we may not have the luxury to remove them. We will be measuring the PDN impedance or capacitor impedances? How much error does measuring across capacitors create instead of just the pads?
(2) For the 4-point probe, we need two capacitor pads side-by-side, correct?
You bet, I am sure many others will benefit from your questions ;)
Thanks Steve! I'll check out the EDICON online video.
1) If you want to measure the capacitor, say for modeling purposes, remove the cap for the measurement.
If you measure PDN impedance on a capacitor, the results will be distorted, but probably fine for measuring the VRM impedance or NISM
If you want to measure the plane impedance, a pair of pads side by side works. There will still be some coupling, which is why Istvan Novak recommends 2-sided measurement (others have papers on single sided measurement). The most important thing is to measure and simulate at the same locations and the same way.
Eventually the measurement will run out of fidelity, while the simulation still has plenty of fidelity to offer . Our goal is to get confidence in the model so that we can trust the simulator when we get tot hat point.
I did cover this topic in my EDICON online presentation a week or so ago. You should be able to watch that on-demand now and I even showed the 4-pad measuremenrt.