Ultra-wideband Bandpass Filter Using Symmetrical Step-impedance Resonators

This paper presents a compact ultra-wideband (UWB) bandpass filter (BPF) using symmetrical step-impedance resonators (SSIR). The operating frequency range is chosen to fall within the lower UWB spectrum of 3.1 GHz to 5 GHz. The passband has a centre frequency is 4 GHz. The proposed BPF generates a single passband located at the desired frequency through a single filter circuitry. The highly selective passband effect is obtained by the step-impedance resonators configuration in a similar topology as the classical comb filter. The proposed step-impedance topology consists of several microstrip sections: a main 50 Omega T-shape transmission line and a fork-shape SSIR cascaded with a shunt stub. A ring resonator with an inner circular aperture is further integrated at the end terminate of the two shunt stubs. Within the step-impedance configuration, the topology and position of the fork-shape SSIR is critically important in achieving the desired passband response. The proposed compact ultra-wideband bandpass filter is prototyped using FR4 substrate with a dielectric constant of 4.7, thickness of 1.6 mm, and loss tangent of 0.027. The fabricated structure has a miniature dimensional size of 35 mm by 39 mm. The best matched return loss Si is observed at 4.22 GHz with value less than -20 dB. The passband insertion loss S-21 is greater than -1.5 dB. Attenuation is greater than 14 dB for the lower and upper stopband respectively. The passband bandwidth is around 1.1 GHz at -10 dB level. The measured filter response showed good agreement with the simulation results. Both simulation and measurement data are presented and discussed.