ssh工作电脑的时候总会打成：“cd 工作电脑”，我感受到了一个plan9的幽灵在游荡

Finally understood why does a 2-port measurement on a network analyzer can reliably find the impedance of a Power Distribution Network. The RF impedance between power and ground is nearly zero on a PCB with many capacitors, diverting 99% current coming out from port 1 to ground, meanwhile port 2 is in parallel, but its 50 Ω input impedance is large and gets almost no current, no voltage drop. Like Kelvin sensing but instead of 1 MΩ, 50 Ω is large enough. And since port 2 receives almost no power, S21 goes to zero and impedance equation simplifies to a straight line Z = 25 Ω x S21, really neat. #electronics