An independently tunable dual-band bandpass filter using open-loop resonators

Abstract

Based on open-loop resonators (OLRs), an independently tunable dual-band (ITDB) bandpass filter (BPF) is presented in this paper. The top layer of the proposed ITDB-BPF is composed of transmission feed lines and OLRs in a stepped-impedance resonator (SIR) structure. The bottom layer is constructed by OLRs in an asymmetrical SIR with a defect-grounded structure. To obtain the required frequencies and bandwidths, the sizes of resonators and feed lines are realized by defining the coupling coefficients and the external quality factors. As to the proposed ITDB-BPF, the first center frequency is set to be 2.45 GHz for Wireless Local Area Network systems. The second center frequency is selected to be 3.5 GHz for a Worldwide Interoperability for Microwave Access systems. The measurement results show that the insertion losses are 2.9 and 2.6 dB for 2.4 and 3.5 GHz, respectively. The minimum return losses are 14.5 dB for 2.4 GHz and 22.8 dB for 3.5 GHz. The stopband rejections of the lower band from dc to 1.84 GHz and the upper bands from 4.34 to 6 GHz are both greater than 30 dB. The center frequencies of the two passbands can be tuned independently by adjusting different resonators which makes the design of dual-band BPF more flexible than conventional ones.

Keywords:

• dual-band bandpass filter
• open-loop resonator
• asymmetrical stepped-impedance resonator
• transmission zero
• defect-grounded structure

Additional information

Funding

This work was supported by National Science Council, Taiwan, under the [grant number NSC101- 2221-E-259-037], [grant number NSC102-2221-E-259-033].