Whenever I used a VNA to measure s-paramerers, I would do a calibration, and that would remove them from having any impact on the end measurement results. Why is it that even after a VNA calibration for 2 port shunt measurement, you still have to use an isolator? Why isnt the calibration good enough?
Thanks!
Fantastic answer! Thanks!!!
That's a really good question, Ryan. Thanks for asking it!
You can use the calibration to remove the error, but the result will be very poor. Say the ground loop resistance is 20mOhms. So when you calibrate, the short Calibration subtracts 20mOhms from the measurement. The sensitivity to errors (like say contact resistance) is one. A 1 mOhm change in contact resistance, for example, shows up as a 1 mOhm error. Adding the coaxial isolator divides the 20mOhms by the CMRR of the isolator prior to the calibration and the sensitivity becomes 1/CMRR. If CMRR IS 100dB, you can see the error from the 1mOhm contact resistance change is now 100nOhms. Huge difference.
So the answer is that we become much less sensitive to the changes in path resistance using the coaxial transformer.