基于阶跃阻抗谐振器的四阶频率可调滤波器设计

Design of Fourth-Order Frequency Tunable Bandpass Filter Using Varactor-Loaded Step-Impedance Resonators

提出一种基于阶跃阻抗谐振器的紧凑型四阶四零点频率可调滤波器设计。滤波器使用变容二极管加载的阶跃阻抗谐振器(VL-SIR),有效拓宽了滤波器的调谐范围。在VL-SIR之间引入交叉耦合,从而在滤波器通带近端产生一对传输零点,显著提高了滤波器的选择性。同时,引入源和负载端的交叉耦合,在通带远端生成一对传输零点,以提高带外抑制。此外,采用基于VL-SIR的频变耦合结构来实现恒定绝对带宽。仿真结果与实测结果吻合良好。测量结果显示,可调滤波器频率调谐范围为0.78~1.15 GHz,3 dB绝对带宽约为55±3 MHz,滤波器的回波损耗大于10 dB,插入损耗约为2.8~3.3 dB。

In this article,a compact fourth-order frequency tunable Bandpass Filter(BPF)with four Transmission Zeros(TZs)based on Varactor-Loaded Step-Impedance Resonators(VL-SIRs)is presented.With the utilization of VL-SIRs,the Tuning Range(TR)of the proposed fourth-order BPF is improved.Besides,by introducing cross coupling between VL-SIRs,a pair of TZs close to the passband are produced and the selectivity of the filter is enhanced significantly.Another pair of TZs are generated to improve the out-of-band rejection by using source-load coupling.Moreover,the Frequency-Dependent Coupling(FDC)structures based on VL-SIRs are employed to realize constant Absolute Bandwidth(ABW).The simulated and measured results are presented and show good agreement.The measured results exhibit a tuning range from 0.78 GHz to 1.15 GHz with a 3 dB constant ABW of about 55±3 MHz,the return loss of the filter is greater than 10 dB and the insertion loss is about 2.8 dB to 3.3 dB.