Bandwidth control of reconfigurable bandpass filter by automatic coupling adjustment techniques

Reconfigurable bandpass filters that can adapt to many frequency bands will be one of the key parts in the future mobile terminals. A multi-bit reconfigurable resonator using tapped transmission line and RF Switches has been proposed and two-stage bandpass filters (BPF) have been developed. However, bandwidth control of the reconfigurable BPF remains as an important issue from the application point of view. This paper proposes reconfigurable coupling methods that automatically adjust coupling between two resonators in the BPF synchronized to the change of center frequency. Two-bit reconfigurable BPFs were fabricated according to the proposed design principle and their performances are compared with traditional capacitance coupling method. It is shown that the proposed methods can reduce the bandwidth change among four center frequencies to 30 to 36 % of the traditional capacitance coupling method.