Tunable high Q narrow-band triplexer

A switchless bank of three tunable bandpass filters is demonstrated which can be used as a triplexer over a large tuning range. Three high Q narrow-band evanescent-mode cavity filters are integrated in a single substrate using an input feed-line network to achieve multi-band operation. Using shorted feed-lines for magnetic field coupling allows the out of band impedance to be dominated by the reactance of the feed-lines. The feed-lines are then designed to have high out of band impedance over the tuning range. A three-way switchless filter bank is presented in the 1.7 GHz-3.4 GHz range maintaining less than 4 dB loss for 14 MHz-31 MHz 3-dB bandwidth over most of the tuning range. The filter bank is fabricated in one low-loss Rogers TMM3 substrate using standard PCB processing, and commercially available 0.5 mm thick piezoelectric actuators are used for the electrical tuning.