Three-bit reconfigurable bandpass filter using branch-line type variable resonator

Current mobile terminals are installed with multiple RF circuits that cover all frequency bands assigned for the system. In order to make efficient use of frequency spectrum and to reduce circuit scale of terminals, reconfigurable RF filters are necessary to flexibly change RF frequencies. This paper proposes a new two-stage reconfigurable bandpass filter (BPF) having eight-frequency (three-bit) selection capability. It consists of branch-line switched type variable resonators. A prototype on a printed circuit board with high dielectric constant substrate has been fabricated and evaluated in 2 GHz bands. It shows very close performances to the design results with respect to insertion loss, center frequency and bandwidth. Low insertion loss of less than 1 dB is achieved among the eight frequencies.