S Parameter

The S-parameter is a parameter that describes the characteristics of a high-frequency network, and is an important parameter for signal integrity, similar to the Z-parameter and Y-parameter in circuit theory. However, since the measurement of Z and Y parameters is not suitable for high-frequency applications due to the presence of open and short circuits, the S-parameters are used to characterize the network.

As shown in the figure below, when port 2 is matched, two S parameters S11 and S22 can be defined, S11 is the reflection coefficient and S21 is the transmission coefficient.

The formulas for the reflection coefficient S11 and transmission coefficient S21 of the S-parameter

For common two-port interconnect structures, four S parameters can be defined:

• Where S11 and S22 are called insertion loss and reflect the ability of the signal to pass through the transmission line network;
• There are two additional parameters S21 and S12 called return loss that reflect the reflection of the signal on the transmission line network.

Different formulas for calculating S11,S22,S21,S12 for S-parameters