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# S paremter of cascaded system

#### ogologol

##### Newbie
First, sorry for my bad English,
I have a question about the s parameter of the cascades system.
Assuming that there are two system A,B as shown in the figure and each has an s-parameter measured at 50ohm port,
I understand that the s-parameter of the A,B cascaded system can be calculated by signal flow graph analysis using the s-parameter of A and B.

However the s-parameter of system A,B are obtained with 50ohm port impedance attached to the system.
Why can I ignore this 50ohm port impedance when A and B are cascaded?
S-parameter is a value obtained when port impedance is attached in parallel to the port,
it is confusing because it seems to be used as if it is a response characteristics between port and port without port impedance.

The termination impedances are not part of the networks defined by the S-parameters, they are external to them. So, when you cascade these networks, you wouldn't include them in the cascade, only append them at the edges.

My understanding is that measurement devices like VNA are dealing with signal voltage and phase. Firmware contains constants for input/output impedance (Zport=50 Ohm) and some formulas to convert voltage/phase to S-parameters/point on a Smith Chart/VSWR etc.

Another way to think about it: assume we converted S-parameters to impedance and it is purely real, e.g., 133 Ohm at some frequency band. This means that it acts as a 133 Ohm resistor. We can put these 133 Ohm resistor in series and it doesn't matter if measurement device port impedance was 50 Ohm. Hope this makes sense.

Hi, check this out. Formulas for cascading scattering parameters. BR, -e