Design of Dual-Band Bandpass Filter With Simultaneous Narrow- and Wide-Bandwidth and a Wide Stopband

This paper presents a novel dual-band bandpass filter with narrow and wide passband simultaneously and a wide stopband. The dual-band bandpass filter is implemented using a stepped-impedance ring loaded resonator (SIRLR). The properties of the SIRLR are analyzed in detail based on the even- and odd-mode equivalent circuits. Resonant modes, excited by SIRLR, can be controlled and used to form a narrow passband at 2.4 GHz with bandwidth ratio of 8% and a wide passband from 3 to 5 GHz, with insertion losses less than 1.5 dB for both two passbands. Three transmission zeros are created near the passband edge to improve the passband selectivity and achieve a sharp cutoff skirt (over 60 dB/GHz). In addition, for wide stopband consideration, rectangular stub loaded resonators (RSLRs) are tapped on the input and output ports of above dual-band filter without destroying the dual-band performances. Thus, the proposed dual-band filter with a wide stopband region from 5 to 20 GHz with a rejection level of 25 dB is also achieved. The measured results show a good agreement with the simulated results.