Novel design and analysis of high roll-off rate ultra-wideband bandpass filter with nine reflection zeros and four transmission zeros

Abstract

This paper presents the novel design and experimental validation of a compact high roll-off rate ultra-wideband bandpass filter(HRUWB-BPF) with a ninth-order, allowing flexibility in fractional bandwidth variation, to meet the requirements of emerging wireless communication systems operating in the L-band and S-band radar and satellite links while enabling multi-standard operation in broadband software-defined and cognitive radio RF front ends. The proposed design incorporates four transmission zeros and nine reflection zeros for the first time, resulting in sharp skirt selectivity and an enhanced fractional bandwidth. A hybrid configuration is introduced by employing cascaded coupled-line sections combined with a step-impedance open stub, which significantly improves the bandwidth response and selectivity factor. A detailed mathematical analysis based on even–odd mode theory is carried out, and closed-form expressions are derived to precisely estimate the positions of the transmission and reflection zeros, offering analytical insight into the filter mechanism. To validate the design, a prototype was fabricated and measured. The experimental results demonstrate a center frequency of 2.5 GHz, 3 dB fractional bandwidth of 108%, roll-off rates of 307.7 dB/GHz and 169 dB/GHz at the lower and upper edges, respectively, and selectivity factor of 0.945 measured at the $-3\mathrm{dB}$ and $-20\mathrm{dB}$ passband edge frequencies. These performance metrics confirm that the proposed HRUWB-BPF is highly suitable for modern wireless communication systems.