为了实现三带通滤波器的小型化、高边缘选择性、通带独立可调以及电路设计简单化，提出了一种具有高选择性的三通带滤波器( BPF) 设计方法．该滤波器基于中间层插入导体平面的双面平行带线，由分别位于顶层和底层的 2 个谐振器组成，并由它们谐振产生 3 个不同的通带， 第 1 和第 3 个通带的频率由顶层的枝节加载型阶跃阻抗谐振器控制，第 2 个通带的频率由底层的半波长谐振器控制; 顶层谐振器采用交叉耦合结构以产生传输零点，提高滤波器的边缘选择性．最后设计加工了一个工作在 2．38、 3．40 和 5．15 GHz的三通带滤波器并进行了实验，实验结果和仿真结果吻合良好，从而验证了文中所提设计方法的有效性．

 Proposed in this paper is a novel design method of tri- band bandpass filter (BPF) with compact structure,high skirt selectivity,individually- controlled passbands and simple circuit.The proposed filter,which is based on adouble- sided parallel stripline with an inserted conductor plane in the middle of the substrate,consists of tworesonators respectively located on the top and the bottom layers,which produce three passbands.The frequencies ofthe first and the third passbands are controlled by the stub- loaded stepped- impendence resonators on the top layer,while the frequency of the second passband is controlled by the half- wavelength resonators on the bottom layer.Moreover,transmission zeros are generated by employing a cross coupling structure for the top- layer resonator,thusimproving the skirt selectivity of the filter.Finally,a tri- band filter working at the frequencies of 2.38, 3.40 and5.15GHz is implemented and tested.The tested results accord well with the simulated ones,which means that theproposed design method is effective.