Wideband microstrip quasi‐elliptic function bandpass filter with high out‐of‐band rejection

Abstract

Presented in this article is a novel and compact quasi‐elliptic function bandpass filter based on microstrip technology that exhibits a high out‐of‐band rejection over a very wide frequency range. The design comprises stub‐loaded half‐wavelength resonators that are electromagnetically coupled to resonant structures that are interdigitally coupled to the input and output feed‐lines. Perturbation introduced by the presence of the stub causes the resonators to act as a multimode device. In addition, the stub introduces two transmission zeros to significantly improve the filter's passband selectivity and suppress harmonic and spurious modes. This configuration yields a wideband response with low insertion loss of 0.73 dB, high return‐loss <20 dB, and out‐of‐band rejection level >40 dB across 2‐4.5 GHz and 5.74‐16 GHz. The planar filter structure can be easily manufactured using standard PCB technology.