A narrowband bandpass filter with wide upper stopband and high frequency selectivity

In this paper, a narrowband bandpass filter with wide upper stopband and high selectivity at X-band is presented. The proposed filter consists of hybrid λ/2 and dual-mode resonators. Different from the conventional even-odd mode method, the proposed filter has a dual-mode resonator which can be equivalent to separate λ and λ/2 resonators. Three transmission zeros can be produced near the core passband so as to improve the frequency selectivity and to suppress the upper stopband. The Measured result shows that the proposed filter has center frequency of 10.05 GHz with fractional bandwidth of 3.24%, and has an insertion loss of 3.8 dB. In addition, the double-sided selectivity with 25 dB skirt slope steepness is 1.67 and the stopband can be extended to 27 GHz with the suppression at the level of more than 20 dB.