Wideband microstrip bandpass filters with asymmetrically-loaded interdigital coupled lines

Asymmetrically-loaded interdigital coupled lines are proposed for innovative design of wideband microstrip bandpass filters with excellent out-of-band performance in both lower and upper stopbands. It is realized by properly terminating one of outer coupling lines with a two-stage open-ended stub. In this way, the lower and upper stopbands can be greatly enhanced by adjusting the stepped-impedance ratio of such a stub. Based on the analysis for a six-port three-line network, the resultant two-port network of the proposed interdigital coupled-lines can be derived in terms of the simple impedance or admittance parameters. Taking advantage of their tightened coupling degree and self-inclusive transmission zeros, they are utilized to implement a class of compact wideband filters. Two filters with one and two half-wavelength resonators are designed, aiming at achieving a desired passband in the frequency range 2.8 to 5.1 GHz and good out-of-band rejection in DC to 2.8 GHz and 5.1 to 10.0 GHz. Measured results of the two fabricated filters have evidently verified the predicted wideband filtering performance.