使用耦合谐振电路理论设计微带交指滤波器具有较好的灵活性, 然而当衬底厚度和中心频率增加到一定程度时, 由传统的50 Ω抽头线得到的外部品质因数Qe达不到理论值。为了增大实际设计中的Qe, 同时匹配源和负载, 本文将传统的抽头结构改进为一段50 Ω微带线与一段较细微带线组合的类SIR抽头; 此外, 为了减小微带线阶跃变化带来的回波损耗, 将类SIR抽头改进为一段50 Ω微带线与一段较细微带线渐变式过渡的抽头。最终, 分别采用类SIR抽头和渐变式抽头设计了一个Ka波段微带交指滤波器。仿真表明: 使用类SIR抽头和渐变式抽头得到的Qe都能与理论值相匹配, 且带渐变式抽头的滤波器相比于带类SIR抽头的滤波器, 其回波损耗减小8.31 dB, 插入损耗减小0.16 dB。从而验证了所提出的两种抽头结构改进的可行性。

According to the theory of coupled resonant circuits, the design of the microstrip interdigital filter is quite flexible. However, when the substrate thickness and the center frequency increase to a certain value, the external quality factor Qe cannot reach the theoretical value when using the traditional 50 Ω tapped-line. In order to increase the Qe in the actual design and match the source and load, the traditional tapped-line structure was improved to an SIR-like tapped-line, with the combination of a 50 Ω microstrip line and a narrower microstrip line. To reduce the return loss caused by the step change of the microstrip line, the SIR-like tapped-line was improved to a gradual tapped-line with the combination of a 50 Ω microstrip line and a narrower microstrip line. Then, taking a Ka-band filter as an example, the microstrip interdigital filter was designed by using the SIR-like tapped-line and the gradual tapped-line, respectively. The simulation indicates that both the Qe of the SIR-like tapped-line and the gradual tapped-line can match the theoretical values. In addition, compared with the filter with the SIR-like tapped-line, the return loss of the filter with a gradual tapped-line is reduced by 8.31 dB, and its insertion loss is reduced by 0.16 dB. The feasibility of the proposed two kinds of tapped-line structure improvement is verified.