CHAPTER THIRTEEN
Tunable and Reconfigurable Filters
Electronically tunable or reconfigurable microwave filters have attracted more and more attention for research and development, because of their increasing importance in improving the capability of current and future wireless systems [1]. Thinking ahead to future cognitive radio and radar applications, it is certain that electronically reconfigurable microwave filters will play an even more important role in wireless systems.
In general, to develop an electronically reconfigurable filter, active switching or tuning elements such as semiconductor p-i-n and varactor diodes, RF MEMS, or other functional material-based components including ferroelectric varactors (see Table 13.1) need to be integrated within a passive filtering structure. Since microstrip filters can conveniently facilitate this kind of small size integration, there has been increasing interest in developing tunable or reconfigurable filters based on microstrips [2–36]. These filters may be classified as tunable combline bandpass filters [2–9]; RF MEMS tunable filters [10–16]; piezoelectric transducer (PET) tunable filters [17–19]; tunable high-temperature superconductor (HTS) filters [20–22]; reconfigurable UWB filters [23,24]; tunable dual-band filters [25]; tunable bandstop filters [26–30]; reconfigurable/tunable dual-mode filters [31–36]; reconfigurable bandpass filters based on a switched delay-line approach [37]; and wideband bandpass filter with reconfigurable ...
