S22 output reflection parameter derivation

For S22 "output reflection coefficient" measurement it is explain in text that:


b2 = S21*a1 + S22*a2

a1 = 0 because ZS=Z0 sets Vs=0

so....... S22 = b2/a2

my question is, is Vs zero from the perspective of looking at port 2?

What I dont understand is how can Vs be 0 since Zs and Zo are perfectly matched doesnt that suggest that there is signal/wave flow from source to input of two port device?

The only way I could come close to making this make sense was to envision that since Zs = Zo no part of the wave will be seen on the output side of the two port device so because of that from the port 2 side of the VNA Vs looks like 0.

Can anyone explain it better or scrutinize my rationale for accuracy?

Hi,

you perform some excitation just on port 2 and measure the reflection at port 2. At port one there is no excitation signal. Just this part from port 2, but this is not reflected at port 1 because it is matched.

So you are saying the excitation current on port 2 side is not reflected at port 1 because Zs = Z0?

Exactly, and that's the definition of s-parameters .
Nevertheless, in reality you will have some mismatch -> reflection!