I was playing with my VNA yesterday and noticed in the Smith chart view of a surface mount device that the trace went from an open at 30 KHz, through inductive to some capacitance at 6 GHz. Nothing new.
As I placed the marker on the spiraling trace and moved it along, I found two interesting spots along the way.
At one spot, the real and imaginary values became the same positive number and that yielded a value for the part of about 120 nH which is what I think this part was.
As I kept moving the marker along into higher frequencies, the real and imaginary numbers became the same number but the imaginary number became negative and the device was a capacitor.
I'm very used to all of this behavior but my question is about the point where the real and imaginary values were equal and the parts value seemed rational. I ran into this early in my career when trying to figure out why a SMD didn't act as I expected in an RF circuit. Not only were the PCB parasitics unexpected but the value of the SMD I was trying to use to tune the circuit wasn't working for me. I struggled for weeks on the problem until I figured out how far the value of a SMD varies over frequency. I'm curious about the real and imaginary numbers becoming equal and the value of the part becoming that listed on the bag.