This chapter describes tunable circuits in CMOS and BiCMOS technologies. First, active and passive basic components are briefly described: MOS and bipolar transistors, and passive components like MOM capacitors, and transmission lines. Slow-wave transmission lines are described and compared to their microstrip lines counterparts, in terms of electrical performance and footprint. Next, tunable components are introduced,varactors and switches, anddigital tunable capacitors. Then, tunable transmission lines are described. Some examples of tunable inductances are also highlighted. Finally, two families of tunable circuits are addressed, phase shifter and VCOs, respectively. Both circuits are used in many systems for beam-forming/steering concerning phase shifters, and transceivers concerning VCOs, respectively. Many design examples are given. Topologies based on the use of lumped components (varactors and inductances) are compared to those based on distributed components (tunable transmission lines). In the middle, hybrid approaches mixing lumped and distributed components can lead to very efficient solutions, both in terms of electrical performance and footprint.

The chapter introduces the Microwave Liquid Crystal Technology which features unique properties for reconfigurable systems for mm-wave communications. After an intrioduction of the material's properties, different implementations of components and systems are compared and discussed such as phase shifters, tunable filters and steerable antenna systems. These LC-based components are implemented for wide frequency range from about 10 GHz up to THz. Characterization, modelling and simulation are key for the design of such components,. Therefore, suited methodology is presented. Additionally, anliterature review on available realizations and technologies is given.