Design of a Microstrip Three-State Switchable and Fully Tunable Bandpass Filter With an Extra-Wide Frequency Tuning Range

In this study, a new method and structure are proposed to design of a switchable and fully tunable microstrip bandpass filter (BPF). The central frequency and bandwidth (or the resonance frequency of the resonators, coupling coefficient between resonators, and external quality factor of the I/O resonator) of the proposed BPF can be efficiently controlled using varactor diodes as tuners. In addition, the proposed BPF is designed to have three operating states, each with a different frequency tuning range. The three operating states can be switched between independently by introducing three pairs of switching diodes in the input and output feeding structures. Because of the ability to switch among the three operating states, an extra-wide frequency tuning range can be achieved. For demonstration, a two-pole switchable and tunable BPF was designed and implemented using microstrip technology. The experimental and simulation results were consistent and indicate that a wide frequency tuning range from 0.54 to 2.4 GHz, corresponding to a fractional tuning range of 127%, and a nearly constant absolute bandwidth of 82 MHz are achieved. In addition, the proposed switchable and tunable BPF is small (0.24λ
_g
× 0.15λ
_g
).