A 10.23–15.7-GHz Varactor-Tuned Microstrip Bandpass Filter With Highly Flexible Reconfigurability

This article demonstrates a 10.23–15.7-GHz varactor-tuned microstrip bandpass filter (BPF) with fully controllable center frequency, bandwidth, and transmission zeros. The concept of cascading a tunable low-pass unit together with a tunable high-pass unit is adopted to achieve the reconfigurable BPF. A comprehensive loss analysis is carried out to determine the preferable circuit topologies for low-pass filter (LPF) and high-pass filter (HPF), respectively. This not only overcomes the limitation of low quality factor of varactor at high frequency, but also contributes to a highly flexible reconfigurability of the BPF. A reconfigurable LPF using defected ground structure (DGS) and a quasi-elliptic HPF using microstrip technology are then proposed to demonstrate the improved BPF performance at high frequency. The measured results show that the designed BPF is capable of operating in five working modes, with a center frequency tuning range from 10.23 to 15.7 GHz and a fractional bandwidth (FBW) tuning range from 32.48% to 95.76%. The minimum in-band insertion loss reaches 2.55 dB. A steep cutoff characteristic and ultrawide stopband with high rejection level are successfully achieved and are well preserved during the whole tuning process, demonstrating the superiority of the designed BPF.