Dual-Band Bandpass Filter Using a Single Short-Ended Dual-Mode Resonator with Adjustable First Passband

In this paper, a novel dual-band bandpass filter (BPF) based on a single short-ended dual-mode resonator (SEDMR) is presented. According to the voltage distribution of the resonator, two pairs of loaded open stubs, inside and outside of the resonator, are utilized to tune the center frequency and the external quality factor of the first passband respectively, while there is no influence on the second passband. Meanwhile, source-load coupling is introduced to produce transmission zeros to improve the passband selectivity and band-to-band isolation. For demonstration, a dual-band filter working at 1.52 GHz for GPS and 3.5 GHz for WiMax is designed, fabricated and measured with insertion losses of 1.47 and 0.95 dB. In addition, two transmission zeros, introduced by source-load coupling, located at 2.19 and 2.67 GHz between the first and second passband improve passband selectivity and band-to-band isolation better than 50 dB.