A novel method for a dual-band filter and quad-channel diplexer design is presented in this paper. This method, by altering the gap between resonators, realizes a transformation from bandpass to dual-band for the filter and diplexer. At first, a high selectivity bandpass filter (BPF) with four controllable transmission zeros (TZs) is designed. Then altering the gap between resonators, a band gap is generated and utilized to split the passband of the proposed BPF into two bands, which transforms the BPF to a dual-band filter with narrow passband separation. The center frequency and bandwidth of the new dual-band filter are controllable by adjusting the frequency and width of band gap. Based on the dual-band filter, a quad-channel diplexer with stepped impedance T-junction is designed, and it can be transformed to a wideband diplexer. For demonstration, the dual-band filter and quad-channel diplexer are fabricated and measured.
2. Zhang, X.-S., Y.-J. Zhao, H.-W. Deng, L. Zhang, and W. Chen, "High selectivity dual-mode bandpass filter with source-loaded coupling," Progress In Electromagnetics Research Letters, Vol. 18, 187-194, 2010.
3. Guo, L., Z.-Y. Yu, and L. Zhang, "Design of a dual-mode dual-band filter using stepped impedance resonators," Progress In Electromagnetics Research Letters, Vol. 14, 147-154, 2010.
4. Weng, M.-H., S.-K. Liu, H.-W. Wu, and C.-H. Hung, "A dual-band bandpass filter having wide and narrow bands simultaneously using multilayered stepped impedance resonators," Progress In Electromagnetics Research Letters, Vol. 13, 139-147, 2010.
5. Thomas, J. B., "Cross-coupling in coaxial cavity filters --- A tutorial overview," IEEE Trans. Microwave Theory Tech., Vol. 51, No. 4, 1368-1376, 2003.
6. Ma, Z., T. Asano, and Y. Kobayashi, "Theory for the design of a filter having one cross coupling path to realize transmission zeros," IEICE Transactions on Electronics, Vol. 86, No. 8, 1690-1698, 2003.
7. Chu, Q. X., F. C. Chen, Z. H. Tu, and H. Wang, "A novel crossed resonatorand its applications to bandpass filters," IEEE Trans. Microwave Theory Tech., Vol. 57, 1753-1759, 2009.
8. Qiu, F., L. Z. Zhang, and B. Wu, "High selective dual-band microstrip filter using zero-degree coupled SIR feed lines," 2014 3rd Asia-Pacific Conference on IEEE Antennas and Propagation (APCAP), 731-733, 2014.
9. Feng, W., X. Gao, W. Che, and Q. Xue, "Bandpass filter loadedwith open stubs using dual-mode ring resonator," IEEE Microwave and Wireless Components Letters, Vol. 25, No. 5, 295-297, May 2015, doi:10.1109/LMWC.2015.2410174.
10. Wu, H. W., S. H. Huang, and Y. F. Chen, "Design of new quad-channel diplexer with compact circuit size," IEEE Microwave and Wireless Components Letters, Vol. 23, No. 5, 240-242, 2013.