A Compact Tunable Bandpass Filter Using Switchable Varactor-Tuned Dual-Mode Resonator

A novel switchable dual-mode resonator is presented for a compact tunable bandpass filter (BPF) with a wide frequency tuning range and a constant absolute bandwidth (ABW). The dual-mode resonator switches between a low-band state and a high-band state with p-i-n diodes to overcome the limited tunability of varactor diodes. The bandpass frequency can be easily tuned by controlling two resonant frequencies of each state using the varactor diodes and the ABW remains constant by tapped-line input and output coupling. To verify the presented structure, a 2-pole tunable BPF is fabricated and measured, which shows an in-band insertion loss of 2.3–5.8 dB maintaining a constant 3-dB ABW of 110 MHz with a tuning range from 360 MHz to 970 MHz.