Hi,

I read Ken's paper on differential s-parameter test bench/measurement but I have a few questions regarding it.  I am fairly new to this High Frequency area and am still grasp some of the concepts.

A single-ended 50 ohm s-parameter test bench is very straight forward and I understand what is going on.  After reading Ken's paper, I kind of see what he is doing with the diff + common mode conversion using the balun transformer but still a little fuzzy about it.  Some questions that come to mind are:

1. If each of the ports (Pid, Pic, Pod, Poc) are 50 ohms, then is the differential impedance seen by the DUT a 100 ohms?  Does the DUT also see a 50 ohms common mode impedance to ground?

2. Does the other parametric analysis like Zin (mag) in the results window display the correct differential impedance Z11 of the DUT?

3. It is possible that I may have gooffed on this but I found a difference in measuring the -3dB bandwidth using S21 (differential) versus using 2 Vac sources, (+0.5V and -0.5V), with 2x 50 ohms source (= 100 ohm differential) and 2x 50 ohms load (= 100 ohm differential) and plotting dB[(Vop-Von)/(Vip-Vin)].  This doesn't make sense to me.  Any ideas why?

Thank you for your help,
whome.