RF and Microwave Circuit Design

10Switches and Phase Shifters

10.1 Introduction

A phase shifter is essentially a two-port network through which the transmission phase can be changed either continuously or in discrete steps, but without significant change in the transmission loss or in the match of the network. Phase shifters in which the transmission phase can be changed continuously are known as analogue phase shifters, and those in which the phase can be changed in discrete steps are known as digital phase shifters.

Phase shifters have many important applications in modern RF and microwave communication systems, notably for use in phase modulation circuits and phased array antennas. Digital phase shifters have wider application than their analogue counterparts and it is digital phase shifters that will be the main focus of this chapter.

Digital phase shifters require the use of high-frequency control components that can be biased to provide ON and OFF states, where the two states correspond ideally to zero and infinite impedance. In practical circuits the control components are connected to provide the functions of high-frequency switches. Figure 10.1 depicts a single pole single throw (SPST) switch, and its implementation in microstrip.

With an SPST switch, transmission from port 1 to port 2 is controlled by opening and closing the switch. In microstrip, this function is achieved by mounting a control component across a gap in the microstrip line, so that switching the control component ON corresponds ...