A Widely Tunable Compact Bandpass Filter Based on a Switched Varactor-Tuned Resonator

This paper presents a compact tunable bandpass filter (BPF) based on a switched varactor tuned resonator with a wide frequency-tuning range and a constant absolute bandwidth (ABW). The proposed switched varactors are adopted to overcome the limited tunability of varactor diodes by switching the stubs. The pass-band can be easily tuned by controlling two resonant frequencies of each state. After the separation of the two resonant frequencies, the ABW of the filter is maintained constant by tuning the external coupling of the tapped-line input and output matching capacitors. To verify the proposed structure, a two-pole tunable BPF has been fabricated and measured. It exhibits a low in-band insertion loss of 1.4-2.9 dB, maintaining a constant 3-dB ABW of 75 MHz with a large tuning range from 255 to 725 MHz (1:2.85). The proposed two-pole filter shares the resonator for the low- and high-band states, and therefore the overall size is only 0.2λ
_g
×0.14λ
_g
. In addition, this two-pole tunable filter is extended to a four-pole tunable one as an estimation of the measured results by using the simulated results. They have the wide frequency tuning ratios of 2.85 and 2.92, with the low insertion losses, respectively.