A Miniaturized UWB Bandpass Filter Employing Multi-Stub-Loaded Short-Circuited Stepped Impedance Ring Resonator

- This article presents a novel microstrip line topology implementation of an ultra-wideband (UWB) bandpass filter. The proposed topology is a shortcircuited stepped-impedance ring resonator with an open-circuit stub and a short-circuit stub loaded at the central vertical position of the low impedance of this resonator, respectively. Within the UWB spectrum, five modes are allocated under weak coupling, and to increase filter selectivity, two transmission zeros are added. It is possible to extend the upper stopband and provide sufficient external coupling by utilizing interdigitalcoupled lines. To demonstrate the design theory, a prototype is designed and manufactured on a 0.8 mm thick substrate of the affordable F4B-2, and its performance is verified. Measurements show a 3-dB bandwidth span from 2.8 to 9.5 GHz, insertion loss of 0.36 dB, and return loss better than 13 dB.