Scattering parameters, popularly known as S-parameters deals with incident and reflected power waves in linear systems. S-parameters are complex numbers. Smith charts are used to represent s-parameters, even CAD tools and VNC also.
Consider a 2-port network shown in Figure 1.
S-parameters for this 2-port network is defined as
and are normalized incident voltages, and and are normalized reflected voltages
power incident on the input port of the network
power reflected from the input port of the network
power incident on the output port of the network (or power reflected from load)
power reflected from the output port of the network
Using these incident and reflected voltages, S-parameters of 2-port network are given below
Input reflection coefficient,
should be as small as possible to ensure that very small power is reflected back.
Similarly output reflection coefficient,
For a passive device the reflecting power is always lower than incident power. Therefore the reflection coefficients, and , are always smaller than unity for passive devices.
For passive devices, and
Forward transmission gain,
Reverse transmission gain,