Synthesis-Applied Highly Selective Tunable Dual-Mode BPF With Element-Variable Coupling Matrix

This paper proposes a unified synthesis methodology and the related architecture for fully passband-tunable dual-mode filters with wide tuning range. The methodology and architecture relate the well-developed frequency-fixed synthesis technologies to tunable filters, and can be applied to any tunable filter design and analysis by an element-variable coupling matrix. This general methodology accounts for bandwidth (BW), center frequency (CF), and return loss (RL) (passband ripple or stopband rejection) tuning, and helps to implement the passband-tunable filter. For validation, a novel tunable dual-mode bandpass filter (BPF) based on the architecture is presented. The proposed filter has the ability to arbitrarily construct the two-pole passband within the frequency tuning range as predicted. In addition, three transmission zeros (TZs) are generated by the proposed structure, which results in a highly selective passband and a reconfigurable stopband. A tunable filter with a tuning range from 0.8 to 1.2 GHz is designed and fabricated. The measured filter presents an elliptic response during the frequency tuning. The 50% CF tuning range (0.8–1.42 GHz) with a constant absolute BW1 dB and a BW1 dB tuning range from 50 to 500 MHz at the fixed CF are measured. Good agreement among matrix calculation, circuit model simulation and measurement demonstrates the validity of the proposed method and structure. In addition, different states of the stopband with the same passband are obtained to further broaden its application.