Ultra-wideband bandpass filter using multi-stub-loaded ring resonator

A compact ultra-wideband (UWB) bandpass filter (BPF) using a multi-stub-loaded ring resonator is presented. The resonant frequencies of the proposed resonator can be tuned freely by changing the lengths of the corresponding stubs. By utilising the proposed multimode resonator, four resonant frequencies are evenly allocated in the UWB range and two transmission zeros are introduced at the lower and upper stopbands. Parallel-coupled feedlines with defected ground structures were utilised to provide sufficient external coupling. To validate the design theory, an UWB BPF operating in the frequency range from 3.1 to 10.6 GHz was designed, analysed and fabricated. Measurements showed good agreement with simulations.