In the measurement of active devices, it is impractical to attach RF connectors to the actual device terminals. By embedding the device in the transmission line structure, we can then measure the device's S-parameter with a change of reference plane as shown in Figure 9.
Figure 9: Different measurement planes
The S-parameter matrix measurement, , at these two planes is related to the S-parameter
of the device under test, S, by
In case of two-port network,