High selectivity wideband BPF with very wide stopband characteristic

This paper describes the design and evaluation of a highly selective wideband microstrip bandpass filter with a near brick-shaped transmission response and a very wide stopband characteristic. The proposed filter structure excites multimode resonances that combine to realize a wideband filter response and excited too are transmission zeros that create a highly selective filter with wideband suppression in the upper and lower stopbands. The filter configuration comprises electromagnetically coupled resonators that are stub loaded. The input and output feedlines are interdigitally coupled to the resonators. Measured results confirm the low-loss and via-free wideband filter exhibits an elliptical response with a wide stopband with a rejection greater than 30 dB. The selectivity factor and stopband performance of the proposed filter is better than that obtained with the high-temperature superconductor (HTS) filters. Design of the filter is relatively simple and easy to manufacture using standard PCB technology. There is good correlation between the simulation and measured results. The proposed wideband bandpass filter is suitable for applications in high interference environments and cognitive radio systems.