Novel Compact Ultra-Wideband Bandpass Filter Employing Short-circuited Stubs with Coupled Stepped-Impedance Resonator

A novel compact ultra-wideband (UWB) bandpass filter is proposed to meet federal communications commission (FCC)´s spectrum mask. Shunt short-circuited stubs and the parallel coupled stepped-impedance resonators (SIRs) are used to achieve a fraction passband from 3.1 GHz to 10.6 GHz. The equivalent circuit of the newly proposed filter is established, while the corresponding chebyshev-type transfer function is derived for filter synthesis. Compared with formerly proposed UWB filter without short-circuited stubs, the new one significantly improves the selectivity and out-of-band characteristics to completely satisfy the FCC spectrum mask. In addition, the use of the short-stubs effectively enlarged the coupling gaps (from 0.05 mm to 0.15 mm) to successfully alleviate the requirement on fabrication precision. For verification, the proposed filter is synthesized, simulated and fabricated in microstrip line. The good agreement between the measured and predicted results validates the effectiveness of newly proposed synthesis theory. Meanwhile, the designed filter exhibits very compact size, good characteristics of low insertion loss, sharp skirts, flat group delay and good stopband (especially in lower one) as well.