What do the subscripts of S parameters represent?

S-parameters (scattering parameters) use a standard naming convention like Sxy, where the subscripts 'x' and 'y' refer to the network ports being measured. Specifically, 'x' is the port where the signal exits the network (measurement port), and 'y' is the port where the signal enters the network (stimulus port). For example, S21 measures the signal coming out of port 2 when a stimulus is applied to port 1, which represents forward gain or insertion loss. S11 measures the signal coming out of port 1 when a stimulus is applied to port 1, representing reflection. The subscripts do not refer to time, frequency, or data quality; they simply map out the specific physical connections (ports) on the device under test.