Narrow Bandpass Filters Using Microstrip Lines Loaded with Asymmetric Bandstop Resonator Pairs

This paper is focused on the analysis and design of narrowband bandpass filters based on the microstrip transmission lines loaded with the stepped impedance resonators (SIRs). Equivalent circuit models are used for the analysis of the SIR-loaded microstrip transmission lines. It is shown that a microstrip transmission line loaded with two asymmetric SIRs shows two transmission zeros with a passband between them. The middle passband is incorporated in the design of bandpass filters with transmission zeros on the upper and lower sidebands for out-of-band response suppression. An analytical method using the filter theory is developed for designing the desired filtering responses. The validity of the presented analysis and design method is verified by fabricating and measuring a filter prototype.