A compact dual-mode balun bandpass filter with improved out-of-band performance is proposed based on a ring resonator. In order to excite the two degenerate modes and obtain filtering balun performance, three parallel-coupled lines are placed at three excitation ports along the ring resonator. By properly arranging the angles between the excitation arms, four transmission zeros can be realized to improve stopband performance. Finally, a prototype of the designed balun bandpass filter is implemented and fabricated. The measured results show a good agreement with those obtained from the simulation, thus verifying our proposal with good in-band filtering and out-of-band performance.

基于环形谐振器，提出了一种具有改善的带外性能的紧凑型双模巴伦带通滤波器。为了激发两种简并模式并获得滤波平衡-不平衡转换器性能，将三条平行耦合线放置在沿着环形谐振器的三个激励端口处。通过适当地布置激励臂之间的角度，可以实现四个传输零，以提高阻带性能。最后，实现并制造了所设计的巴伦带通滤波器的原型。测量结果表明与仿真结果吻合良好，从而以良好的带内滤波和带外性能验证了我们的建议。