Quasi-Elliptic Higher-Order Tunable Bandpass Filter With Constant Absolute Bandwidth Using Synchronously Tuned Dual-Mode Resonators

Designing a higher-order tunable bandpass filter (BPF) with constant absolute bandwidth (ABW) and identical passband response at every passband frequency tuning state is considered a challenge. In this paper, we propose quasi-elliptic higher-order tunable BPFs that use synchronously tuned dual-mode resonators, which can achieve identical passband frequency response and constant ABW. To achieve identical passband frequency response and constant ABW, all frequency-dependent coupling coefficient and external quality factor are physically realized using series capacitor and shunt varactor. To achieve quasi-elliptic response, transmission zeros are generated at lower and upper sides of passband through cross-coupling between dual-mode resonators. For experimental validation, a prototype of 4th and 6th-order BPFs are designed and fabricated. Experimental results reveal that the passband center frequency of the 4th-order tunable BPF can be tuned from 1.35 to 1.75 GHz (25.82%) with identical passband return loss (RL) of 20 dB and 1-dB constant ABW of 92 ± 1.3 MHz. Similarly, the passband center frequency of 6th-order BPF can be tuned from 1.38 to 1.77 GHz (27.76%) with an identical passband RL of 20 dB and 1-dB constant ABW of 98 ± 1.5 MHz.