Passband flatness and band-edge selectivity in microwave filters with finite quality-factor resonators can be improved by the synthesis of lossy filters. This paper demonstrates the extension of this technique to a lossy diplexer by means of resistive coupling. A dual-mode stub-loaded resonator (SLR) junction and a fork-like feedline are used in the diplexer to address the challenge of independently controlling the external coupling from the common port to the two channel filters and therefore enable flexible realization of the channel bandwidth. The coupling matrices with resistive couplings for the lossy diplexer are generated. For verification, a microstrip lossy diplexer operating at 1.91 and 2.6 GHz was designed and tested. The flatness of the passband has been significantly improved, with a reduction of the passband insertion loss variation from 1.4/1.2 dB to 0.66/0.63 dB for the low/high band. The measured results are in good agreement with the simulations as well as the theoretical responses from the coupling matrix. This was also experimentally compared with a reference diplexer without resistive couplings.
"A Microstrip Lossy Diplexer with Flat Channel Passbands," Progress In Electromagnetics Research M,
Vol. 90, 99-108, 2020. doi:10.2528/PIERM20011605
1. Basti, A., A. Perigaud, S. Bila, S. Verdeyme, L. Estagerie, and H. Leblond, "Design of microstrip lossy filters for receivers in satellite transponders," IEEE Transactions on Microwave Theory and Techniques, Vol. 62, No. 9, 2014-2024, 2014. doi:10.1109/TMTT.2014.2337285
2. Szydlowski, L., A. Lamecki, and M. Mrozowski, "Design of microwave lossy filter based on substrate integrated waveguide (SIW)," IEEE Microwave and Wireless Components Letters, Vol. 21, No. 5, 249-251, 2011. doi:10.1109/LMWC.2011.2119471
3. Basti, A., S. Bila, S. Verdeyme, A. Perigaud, L. Estagerie, and H. Leblond, "Comparison of two approaches for the design of microstrip lossy filters," Proceedings of the 43rd European Microwave Conference, 21-24, Nuremberg, Germany, Oct. 2013.
4. Miraftab, V. and M. Yu, "Advanced coupling matrix and admittance function synthesis techniques for dissipative microwave filters," IEEE Transactions on Microwave Theory and Techniques, Vol. 57, No. 10, 2429-2438, 2009. doi:10.1109/TMTT.2009.2029625
5. Miraftab, V. and M. Yu, "Generalized lossy microwave filter coupling matrix synthesis and design," IEEE MTT-S International Microwave Symposium Digest, 627-630, Atlanta, GA, Jun. 2008.
6. Williams, A. E., W. G. Bush, and R. R. Bonetti, "Predistortion techniques for multicoupled resonator filters," 1984 IEEE MTT-S International Microwave Symposium Digest, 290-291, 1984.
7. Yu, M., W.-C. Tang, A. Malarky, V. Dokas, R. Cameron, and Y. Wang, "Predistortion technique for cross-coupled filters and its application to satellite communication systems," IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 12, 2505-2515, 2003. doi:10.1109/TMTT.2003.820172
8. Yu, M. and V. Miraftab, "Shrinking microwave filters," IEEE Microwave Magazine, Vol. 9, No. 5, 40-54, 2008. doi:10.1109/MMM.2008.927636
9. Ni, J., W. Tang, J. Hong, and R. H. Geschke, "Design of microstrip lossy filter using an extended doublet topology," IEEE Microwave and Wireless Components Letters, Vol. 24, No. 5, 318-320, 2014. doi:10.1109/LMWC.2014.2309089
10. Hunter, I., A. Guyette, and R. D. Pollard, "Passive microwave receive filter networks using low-Q resonators," IEEE Microwave Magazine, Vol. 6, No. 3, 46-53, 2005. doi:10.1109/MMW.2005.1511913
11. Qiu, L. F., L. S.Wu, W. Y. Yin, and J. F. Mao, "A flat-passband microstrip filter with nonuniform-Q dual-mode resonators," IEEE Microwave and Wireless Components Letters, Vol. 26, No. 3, 183-185, 2016. doi:10.1109/LMWC.2016.2525017
12. Guyette, A., I. Hunter, and R. Pollard, "The design of microwave bandpass filters using resonators with nonuniform Q," IEEE Transactions on Microwave Theory and Techniques, Vol. 54, No. 11, 3914-3922, 2006. doi:10.1109/TMTT.2006.884627
13. Meng, M. and I. Hunter, "The design of parallel connected filter networks with non-uniform Q resonators," 2012 IEEE MTT-S International Microwave Symposium Digest, 1-3, 2012.
14. Mateu, J., A. Padilla, C. Collado, M. Martinez-Mendoza, E. Rocas, C. Ernst, and J. M. O. Callaghan, "Synthesis of 4th order lossy filters with uniform Q distribution," 2010 IEEE MTT-S International Microwave Symposium Digest, 568-571, 2010. doi:10.1109/MWSYM.2010.5517741
15. Gao, B., L.-S. Wu, and J.-F. Mao, "Flat-passband substrate integrated waveguide filter with resistive couplings," Progress In Electromagnetics Research C, Vol. 62, 1-10, 2016. doi:10.2528/PIERC15111503
16. Shang, X. B., Y. Wang, W. Xia, and M. J. Lancaster, "Novel multiplexer topologies based on allresonator structures," IEEE Transactions on Microwave Theory and Techniques, Vol. 61, No. 11, 3838-3845, 2013. doi:10.1109/TMTT.2013.2284496
17. Chuang, M. L. and M.-T. Wu, "Microstrip diplexer design using common T-shaped resonator," IEEE Microwave and Wireless Components Letters, Vol. 21, No. 11, 583-585, 2011. doi:10.1109/LMWC.2011.2168949
18. Wu, Y., Y. Wang, and L. Sun, "Independently controllable external coupling for resonant junctions in diplexers," 2018 IEEE MTT-S International Microwave Symposium Digest, 1068-1071, 2018. doi:10.1109/MWSYM.2018.8439295
19. Hong, J. S. and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, NY, USA, 2001. doi:10.1002/0471221619
20. Miraftab, V. and M. Yu, "Generalized lossy microwave filter coupling synthesis and design using mixed technologies," IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 12, 3016-3027, 2008. doi:10.1109/TMTT.2008.2008267
21. Zhang, X. Y., J. X. Chen, Q. Xue, and S. M. Li, "Dual-band bandpass filters using stub-loaded resonators," IEEE Microwave and Wireless Components Letters, Vol. 17, No. 8, 583-585, 2007. doi:10.1109/LMWC.2007.901768
22. HFSS high frequency structure simulator, USA, [Online], Available: www. ansys. com.