1.26–2-GHz Miniaturized Tunable Bandpass Filter with Constant Absolute Bandwidth and Wide Stopband Using Two-Path Electrical Complementary Coupling

In this paper, a miniaturized third-order tunable bandpass filter (BPF) with constant absolute bandwidth (ABW) and wide stopband is proposed. By using two-path electrical complementary coupling, the coupling coefficients almost match the ideal curve for constant ABW in wide tuning range. Then, a tiny bandwidth variation and simple tuning operation are obtained. Wide stopband is achieved from the slow-wave effect of patch-loaded transmission line. Besides, multilayer structure is used to implement couplings at different layers for independent coupling control. Based on the mechanisms above, a third-order tunable BPF covering the range from 1.26 to 2.00 GHz is designed and fabricated. The measured 3-dB bandwidth is 278±5 MHz with the insertion loss of 2.42–3.05 dB, showing a good passband performance with constant ABW. Besides, the stopband is up to 10 GHz with a rejection level more than 20 dB. The proposed tunable BPF exhibits a miniaturized size (i.e., 0.0054 λg², where λg is the microstrip guided wavelength at 1.26 GHz).