Microwave Filters for Communication Systems: Fundamentals, Design and Applications

CHAPTER 5

ANALYSIS OF MULTIPORT MICROWAVE NETWORKS

Any filter or multiplexer circuit can be viewed as a combination of multiport subnetworks. Figure 5.1 illustrates a circuit that consists of a five-port network connected to several two-port networks. The overall circuit in this case is a four-channel multiplexer. The five-port network is the combining manifold junction. Each filter could be also viewed as a cascade of two-port or three-port subnetworks. In this chapter, we discuss several matrix representations of multiport microwave networks. Also, we present various techniques to analyze linear passive microwave circuits that are formed by connecting any number of multiport networks. As an example we apply these techniques, at the end of this chapter, to demonstrate step by step the evaluation of the overall scattering matrix of a three-channel multiplexer.

The most commonly used matrices to describe a network are the [Z], [Y], [ABCD], and [S] matrices [1–3]. The [Z], [Y], and [ABCD] matrices relate the voltage and current at the various ports by describing the network at the lumped-element level. The scattering matrix [S] and the transmission matrix [T] relate the incident and reflected normalized voltages at the various ports. While at microwave frequencies, the voltage and currents cannot be measured, the [Z], [Y], and [ABCD] matrices are often used to provide a physical insight into the equivalent circuit of the microwave network. In RF design, the most commonly quoted parameters ...