A lowpass filter with sharp transition and wide stopband using a novel coupling stepped-impedance triangular resonator is presented. The L-C equivalent circuit is developed for designing this type filter and analyzing the mechanism for improving roll-off and rejection property. The stopband width, passband edge, roll-off rate and overall suppression level are affected by coupling capacitance. The effect of coupling capacitance is analyzed using calculated frequency response. Coupling triangular stubs provide adequate coupling capacitance resulting in balance among transition property, stopband width and suppression level easily. A single LPF unit is designed and fabricated with cutoff frequency of 860 MHz. The single LPF unit exhibits 40-dB suppression level from 1.11 GHz to 2.28 GHz. A cascaded LPF with three asymmetric units provides 40-dB suppression level from 1.1 GHz to 6.76 GHz, and roll-off rate of 154 dB/GHz with compact size as small as 0.23λg × 0.05λg, where λg is guided wavelength at cutoff frequency.
1. Cao, H. L., W. B. Ying, H. Li, and S. Z. Yang, "Compact lowpass filter with ultra-wide stopband rejection using meandered-slot dumbbell resonator," Journal of Electromagnetic Waves and Applications, Vol. 26, No. 17--18, 2203-2210, 2012. doi:10.1080/09205071.2012.729501
2. Cao, H., W. Ying, H. Li, and S. Yang, "Compact lowpass filter with wide stopband using novel windmill resonator," Journal of Electromagnetic Waves and Applications, Vol. 26, No. 17--18, 2234-2234, 2012. doi:10.1080/09205071.2012.732024
3. Hayati, M., S. Roshani, S. Roshani, and F. Shama, "A novel miniaturized Wilkinson power divider with nth harmonic suppression," Journal of Electromagnetic Waves and Applications, Vol. 27, No. 6, 726-735, 2013. doi:10.1080/09205071.2013.786204
4. Yang, R. Y., Y. L. Lin, C. Y. Hung, and C. C. Lin, "Design of a compact and sharp-rejection low-pass filter with a wide stopband," Journal of Electromagnetic Waves and Applications, Vol. 26, No. 17--18, 2284-2290, 2012. doi:10.1080/09205071.2012.733495
5. Li, L., Z. F. Li, and J. F. Mao, "Compact lowpass filters with sharp and expanded stopband using stepped impedance hairpin units," IEEE Microw. Wireless Compon. Lett., Vol. 20, No. 6, 310-312, 2010. doi:10.1109/LMWC.2010.2047457
6. Hsieh, L. H. and K. Chang, "Compact elliptic-function low-pass filters using microstrip stepped-impedance hairpin resonators," IEEE Trans. on Microw. Theory and Tech., Vol. 51, No. 1, 193-199, 2003. doi:10.1109/TMTT.2002.806901
7. Velidi, V. K. and S. S. Sanyal, "Sharp roll-offlowpass filters with wide stopband using stub-loaded coupled-line hairpin unit," IEEE Microw. Wireless Compon. Lett., Vol. 21, No. 6, 303-303, 2011.
8. Ma, K. and K. S. Yeo, "New ultra-wide stopband low-pass filter using transformed radial stubs," IEEE Trans. on Microw. Theory and Tech., Vol. 59, No. 3, 604-611, 2011. doi:10.1109/TMTT.2010.2095031
9. David, M. P., Microwave Engineering, 3rd Ed., New York, 2005.
10. Hong, J. S. and M. J. Lancaster, "Microstrip Filters for RF/Microwave Applications," Wiley, 2001.