I also want a solution, but I think the measurement is far from straightforward. So far what I've found are (Disclaimer: I'm a sysadmin, not a qualified hardwar

I also want a solution, but I think the measurement is far from straightforward. So far what I've found are (Disclaimer: I'm a sysadmin, not a qualified hardware engineer, it's just for fun): * Using a Vector Network Analyzer and do a S22 (S11 on a two-port VNA) measurement directly at the output driver - it's exactly what we want by definition. But the measurement is not passive and you are connecting the output of the VNA to a "hot" output driver, guess what could possibly go wrong? Both the chip and the VNA can be damaged if it's not done properly. It's quite a headache for testing RF amplifiers. Even if the setup is a success, everything else connected to the output driver can distort the measurement. But I believe it's easier on low power logic circuits. You add a DC-block to the VNA, get a test fixture and do Open/Short/Load calibration, then solder the output driver on the fixture and turn it on, drive the output high, measure it, drive the output low, measure it again. But it certainly needs to be done on a test fixture in a controlled setting, not an actual live board. I'm not sure if the approach I just described can work yet, I'll do some experiments in the future. * Weng-Yew Chang, et al. proposed output impedance measurement by current injection using two reproposed wideband oscilloscope current probe (Tektronix CT-6, $1000) and a standard VNA. You hook both probes at the output side of the driver, connect them to the VNA, and do your measurements as usual [0]. Apparently it's already the standard practice for measuring DC power supplies (e.g. see PicoTest Signal Injector manual [1]), and they simply applied the same trick on output driver. * Richard Allred proposed a method at DesignCon 2014 [2]: First, get a multi-GHz oscilloscope, get a Time Domain Reflectrometer, do a lot of TDR measurements while the board is unpowered. Then, power on the board, measure the real output signal on your oscilloscope. Next, solder a shunt resistor at the output, repeat the measurement. Finally, do a lot of linear algebra on the measured waveform using previous TDR data to remove the influence ("de-embedded") of the interconnection (cable, board) from the signal, and do more algebra to get the complex impedance of the output driver on the Smith Chart. Expensive equipment and a considerable amount of work. But Allred's work is used on 10 GHz+ systems, and it's designed to be applied directly on the board, without the use of a test fixture, and avoiding a that can potentially upset the output. [0] https://sci-hub.tw/10.1109/APMC.2009.5384220 https://sci-hub.tw/10.1109/APMC.2009.5384220 [1] https://www.picotest.com/images/download/injector-manual-1.1.pdf https://www.picotest.com/images/download/injector-manual-1.1... [2] https://www.sisoft.com/elearning/secure/files/2-WE4_PracticalMethodForMeasuringDigital.pdf https://www.sisoft.com/elearning/secure/files/2-WE4_Practica...