Sharp-rejection broadband microstrip bandpass filters using loaded open-loop resonator

A broadband microstrip bandpass filter using loaded open-loop resonators is proposed. The basic block of the proposed filter is a triple-mode resonator consisting of a halfwave resonator and a loaded open loop. The first three resonances are of even, odd, and even modes, sequentially. The proposed resonator also provides two transmission zeros beside the passband for a sharp cut-off response. Two bandpass filters using the proposed resonator have been designed, fabricated, and tested to validate the design concept. For the first bandpass filter, from 2.3 to 3.9 GHz, the measured return loss is greater than 12 dB, and the measured insertion loss is less than 1.5 dB. The maximum group delay variation is 0.7 ns. Furthermore, there are two transmission zeros at 1.55 and 4.6 GHz for a sharp cutoff response. For the second bandpass filter, from 2.3 to 3.9 GHz, the measured return loss is greater than 10 dB, and the measured insertion loss is less than 2.5 dB. The roll-off rates are 136 dB/GHz and 167 dB/GHz at the lower and the higher stopbands, respectively.