RF and Microwave Circuit Design

5S-Parameters

5.1 Introduction

Scattering parameters, usually abbreviated to S-parameters, are network parameters used to specify the performance of linear networks and devices at microwave frequencies.

At low frequencies (that is below microwave), the properties of multi-port linear networks are normally expressed in terms of Z-parameters, Y-parameters, or ABCD parameters. These parameters, which are defined in the appendices of this chapter, require standard terminating impedances such as short circuits and open circuits, which are readily provided in practice at low frequencies. At higher (microwave) frequencies, the provision of short or open circuits is not so easy. At high frequencies, short circuits tend to have residual inductance, and open circuits have residual capacitance, and so to use them as reference terminations requires that these stray reactances are determined from subsidiary experiments.

Also, at low frequencies (below microwave), measurement of current and voltage associated with the characterization of linear networks is relatively straightforward, and consequently the measurement of Z, Y, and ABCD parameters is relatively easy. But at microwave frequencies, current and voltage are more difficult to measure, and so the accurate determination of Z, Y, and ABCD parameters is again correspondingly more difficult. For these reasons, a different approach is adopted at microwave frequencies and the network is specified in terms of the microwave power which ...