In this letter, an interdigital band-pass filter is proposed for out-of-band rejection improvement. Seven quarter-wavelength resonators are employed to form the passband. Three extra transmission zeros (TZs) on both sides of the passband are implemented by introducing source-load coupling and dumbbell defected ground structures (DGS). As demonstrated by the measured results, out-of-band rejection and selectivity are improved by these three TZs, and good performances are achieved. The proposed method of increasing out-of-band rejection is feasible and applicable in the design of modern microstrip filters.
2. Jankovic, N., et al., "A compact dual-band bandpass filter using folded quarter-wavelength resonators," 2012 42nd European Microwave Conference (EuMC), IEEE, 360-363, 2012.
3. Jankovic, N., V. Radonic, and V. Crnojevic-Bengin, "Compact tri-band bandpass filter based on quarter-wavelength resonators," Journal of Electromagnetic Waves and Applications, Vol. 27, No. 6, 750-757, 2013.
4. Yang, Z., et al., "Design of a compact dual-band dual-mode microstrip filter with an adjustable transmission zero," 2012 IEEE MTT-S International Microwave Symposium Digest (MTT), IEEE, 1-3, 2012.
5. Wang, L., et al., "Dual-band bandpass filter using stub loaded resonators with multiple transmission zeros," 2010 9th International Symposium on Antennas Propagation and EM Theory (ISAPE), IEEE, 1208-1211, 2010.
6. Cameron, R. J., "Advanced coupling matrix synthesis techniques for microwave filters," IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 1, 1-10, 2003.
7. Zhou, M., X. Tang, and F. Xiao, "Compact dual band bandpass filter using novel E-type resonators with controllable bandwidths," Microwave and Wireless Components Letters, IEEE, Vol. 18, No. 12, 779-781, 2008.
8. Sun, S., L. Zhu, and H. H. Tan, "A compact wideband bandpass filter using transversal resonator and asymmetrical interdigital coupled lines," Microwave and Wireless Components Letters, IEEE, Vol. 18, No. 3, 173-175, 2008.
9. Pozar, D. M., Microwave Engineering, 4th Ed., John Wiley & Sons Inc., USA, 2012.
10. Cameron, R. J., C. M. Kudsia, and R. R. Mansour, Microwave Filters for Communication Systems: Fundamentals, Design, and Applications, John Wiley & Sons Inc., USA, 2007.
11. Hong, J.-S. and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, John Wiley & Sons Inc., USA, 2001.
12. Montejo-Garai, J. R., "Synthesis of N-even order symmetric filters with N transmission zeros by means of source-load cross coupling," Electronics Letters, Vol. 36, No. 3, 232-233, 2000.
13. Amari, S., "Direct synthesis of folded symmetric resonator filters with source-load coupling," IEEE Microwave & Wireless Components Letters, Vol. 11, No. 6, 264-266, 2001.
14. Boutejdar, A., et al., "A simple transformation of improved WLAN band pass to low pass filter using defected ground structure (DGS), defected microstrip structure (DMS) and multilayer-technique," Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 12, No. 1, 111-130, 2013.
15. Khan, M. T., et al., "Tuning of end-coupled line bandpass filter for 2.4GHz using defected ground structure (DGS) parameters," Business Engineering and Industrial Applications Colloquium (BEIAC), 2013 IEEE, 131-134, 2013.