APPLICATION NOTE
1
Introduction
This document explains Measuring S parameters in PSpice using Probe measurements. It explains obtaining S
parameter for a passive circuit (a band pass filter) and an active circuit (transistor circuit).
S parameter Theory
S-parameters measure the ratio of the incident and reflected signal. The incident signals are defined as a1 and
a2. The reflected signals are defined as b1 and b2. The incident and reflected signals are related to voltages and
currents at ports 1 and 2 by
(1)
(2)
(3)
(4)
The scattered waves are related to the incident waves by the following set of linear equations:
(5)
(6)
Or, in matrix form as;
The Sij coefficients are dimensionless ratios; for most applications, the characteristic impedance of the system,
Z0, is 50 ohms. S11 is the input reflection ratio and is defined as the ratio of the input port reflected wave to the
input incident wave. If the incident wave at the output, a2, is set to zero, then the equations reduce to b1 = S11a1
and b2 = S21a1. Using the defining equations, this reduces to
