Constant absolute bandwidth six-pole quasi-elliptic tunable bandpass filter using synchronously tuned dual-mode resonators

This paper presents the design of a 6^th-order tunable bandpass filter (BPF) that achieves constant absolute bandwidth (ABW) and transmission zeros (TZs) across a wide frequency tuning range. The proposed tunable BPF employs three synchronously tuned dual-mode resonators, along with a frequency-dependent coupling structure realized using a T-network consisting of series and shunt capacitors. To achieve constant ABW, the tunable external quality factors are implemented using series capacitor and shunt varactor. As an illustrative example, a 6^th-order BPF is fabricated and measured, results revealing that the center frequency of BPF can be tuned from 1.39 GHz to 1.81 GHz (26.25 %) with two TZs on the lower and upper side of the passband, 1-dB constant ABW of 62 ± 1.5 MHz and return loss > 12 dB in every tuning state.