Two-Path Electric Complementary Coupling Scheme of Miniaturized Tunable BPFs With Constant Absolute Bandwidth and Wide Stopband

In this paper, a two-path electric complementary coupling scheme is proposed for tunable bandpass filters (BPFs) with constant absolute bandwidth (ABW) and wide stopband. Based on the coupled structure consisting of anti-parallel coupled line and series capacitor, such a two-path electric coupling scheme is introduced with complementary bandwidth variations. Since the proposed electric coupling shows the same convex function as the ideal curve, a constant ABW is achieved during a wide tuning range. Meanwhile, the capacitively loaded transmission line introduces the intrinsic slow-wave effect to improve the stopband performance. Moreover, the multi-layer coupled structure is proposed for the high-order tunable filter design within a miniaturized circuit size. For demonstration, second- and third-order tunable BPFs with constant ABW and wide stopband is designed and fabricated. Besides, a third-order BPF with source-load coupling is proposed and implemented to further improve the passband selectivity. All the tunable BPFs are measured with less than 2.5% ABW variation during a tuning range of more than 45%. The stopbands are up to 7.1 minimum tunable center frequency with a rejection level of more than 16 dB. The proposed BPFs have merits of constant ABW, wide stopband, wide tuning range, and miniaturized size, simultaneously.