S-Parameters （Scattering Parameters）

In a multiplex connection, this is a parameter that expresses the mutual relationship between the magnitude and phase of each input and output signal.

For example, if there are two ports, S11 is the ratio of the signal that goes out from port 1 and returns to port 1, and S21 is the ratio of the signal that goes out from port 1 and reaches port 2. S11, S12, S21, S22 consists of four The front side of the two numbers is the receiving end, and the back side is the sending end. If there are four ports, there are a total of 16 elements, S11, S12, S13, S44. There are also things called mixed-mode S-parameters for checking the quality of differential transmission.
As a measurement system, it is measured using a network analyzer, and the degradation of the signal output to the target destination (insertion loss), the magnitude of the signal returning to itself (reflection/ return loss), Includes elements representing output to a location different from the destination (crosstalk), etc.
By modeling the S-parameters of each element and incorporating them into a circuit simulation such as H-SPICE, it is used to simulate the movement of the entire circuit.