A high selectivity wideband bandpass filter (BPF) is proposed based on a stepped impedance open-stub loaded ring resonator. The resonant modes can be adjusted by the parameters of the stepped impedance stubs and eventually to tune the bandwidth conveniently. The even resonant modes can be tuned lower than the first odd resonant mode that is excited by the half wavelength resonator. Due to the analysis of the electric field distributions on the resonator, three adjacent resonant modes can be employed to form a wide bandwidth by electric coupling between input/output feeding line and the resonator. In addition, by introducing two more resonant modes in the passband using the parallel-coupled feeding open stubs, the selectivity is further improved. The related coupling topology is theoretically analyzed and experimentally verified. As the design example, a wideband BPF centered at 3.24 GHz with a 3-dB fractional bandwidth of 58.3% is fabricated and measured. The experimental results fit the simulations well.

提出了一种基于步进阻抗的开环负载环形谐振器的高选择性宽带带通滤波器（BPF）。谐振模式可以通过阶跃阻抗桩的参数进行调整，最终可以方便地调整带宽。可以将偶数谐振模式调谐为低于由半波长谐振器激发的第一奇数谐振模式。由于分析了谐振器上的电场分布，可以采用三个相邻的谐振模式通过输入/输出馈线与谐振器之间的电耦合来形成较宽的带宽。另外，通过使用并联耦合的馈通开口短截线在通带中引入两个以上的谐振模式，选择性进一步提高。对相关的耦合拓扑进行了理论分析和实验验证。作为设计示例，制造并测量了一个中心频率为3.24 GHz，3dB分数带宽为58.3％的宽带BPF。实验结果很好地拟合了仿真。