A novel 3-pole bandpass filter (BPF) based on microstrip loaded ring resonators (LRRs) is proposed. Each resonator is composed by a closed-loop transmission line and a short-circuited stub. By properly adjusting the impedance and the electrical length of each resonator, the proposed circuit may be made compact (over 93.7% smaller than a conventional ring resonator) and its stopband may be extended simultaneously. Each resonator exhibits an area of 0.0727 λg x 0.079 λg where λg is the guided wave length. A BPF at the center frequency of f0=1.9 GHz with stopband extended up to 7.8 GHz (=4f0) is developed showing good agreement between simulation and experimental results.
2. Lu, X., X Guo, S. Jin, X. Zhang, H. Peng, M. Li, B. Wei, and B. Cao, "UHF band ultra-narrowband superconducting filter with double U-type secondary coupling structure and Wireless Components Letters," IEEE Microwave, Vol. 19, 707-709, 2009.
3. Zagoya-Mellado, I., A. Corona-Chavez, and I. Llamas-Garro, "Miniaturized metamaterial filters using ring resonators," IEEE MTT-S International Microwave Workshop Series on Signal Integrity and High-speed Interconnects, IMWS 2009, 45-48, 2009.
4. Liang, C., W. Chang, and C. Chang, "Compact microstrip band-pass filters using miniaturized quarter-wavelength resonators," 2010 Asia-Pacific Microwave Conference Proceedings (APMC), 263-266, 2010.
5. Deng, P., S. Lin, Y. Lin, C. Wang, and C. Chen, "Microstrip bandpass filters with dissimilar resonators for suppression of spurious responses," 2005 European Microwave Conference,, Vol. 2, Oct. 2005.
6. Afkhami, A. and M. Tayarani, "Spurious response suppression in hairpin filter using CSRR merged in the filter structure," Progress In Electromagnetics Research C, Vol. 11, 137-146, 2009.
7. Vidhya, K. and T. Jayanthy, "Design of microstrip hairpin band pass filter using defected ground structure and open stubs," International Conference on Information and Electronics Engineering, Vol. 6, 268-272, 2011.
8. Corona-Chavez, A., M. J. Lancaster, and T. S. Hieng, "HTS quasi-elliptic filter using capacitive-loaded cross-shape resonators with low sensitivity to substrate thickness," IEEE Transactions on Microwave Theory and Techniques, Vol. 55, 117-120, 2007.
9. Hong, J.-S. and M. J. Lancaster, "Microstrip Filters for RF/Microwave Applications," John Wiley & Sons, Inc., 2001.