A varactor-tuned microstrip bandpass filter (BPF) with independently tunable center frequency and bandwidth is proposed in this paper. The proposed BPF with a simple configuration is composed of a half-wavelength transmission line with both ends short-ended and a T-shaped transmission line. Meanwhile, two varactors are inserted symmetrically in the middle section of the half-wavelength transmission line to adjust the resonant frequency. The T-shaped transmission line is connected to the half-wavelength transmission line by a lumped capacitor. In addition, two inductors loaded symmetrically in the feed line are employed to control the coupling coefficient. It is convenient to adjust the frequency and bandwidth of the filter independently by using only three varactors, which simplifies the circuit structure greatly. The predicted results on S parameters are compared with the measured ones, and a reasonable agreement is achieved.
2. Cheng, C.-C. and G. M. Rebeiz, "A three-pole 1.2-2.6-GHz RF MEMS tunable notch filter with 40-dB rejection and bandwidth control," IEEE Trans. on Microw. Theory and Tech., Vol. 60, No. 8, 2431-2438, 2012.
3. Yang, T. and G. M. Rebeiz, "Tunable 1.25-2.1-GHz 4-pole bandpass filter with intrinsic transmission zero tuning," IEEE Trans. on Microw. Theory and Tech., Vol. 63, No. 5, 1569-1578, 2015.
4. Chiou, Y.-C. and G. M. Rebeiz, "A tunable three-pole 1.5-2.2GHz bandpass filter with bandwidth and transmission zero control," IEEE Trans. on Microw. Theory and Tech., Vol. 59, No. 11, 2872-2878, 2011.
5. Renedo, M. S. and R. G. Garcia, "A tunable combline filter with continuous control of center frequency and bandwidth," IEEE Trans. on Microw. Theory and Tech., Vol. 53, No. 1, 191-199, 2005.
6. Wang, Y., F. Wei, H. Xu, and X.-W. Sh, "A tunable 1.4-2.5GHz bandpass filter based on single mode," Progress In Electromagnetics Research, Vol. 135, 261-269, 2013.
7. Chiou, Y.-C. and G. M. Rebeiz, "Tunable 1.55-2.1GHz 4-pole elliptic bandpass filter with bandwidth control and > 50 dB rejection for wireless systems," IEEE Trans. on Microw. Theory and Tech., Vol. 61, No. 1, 117124, 2013.
8. Huang, X.-G., J.-Q. Zhang, Y.-Q. Lin, and Q.-Y. Xiang, "Design of a six-pole tunable band-pass filter with constant absolute bandwidth," 2016 Progress in Electromagnetic Research Symposium (PIERS), 3507-3510, Shanghai, China, August 8-11, 2016.
9. Zhang, X., C. Chen, M. Li, W. Chen, and J. Cai, "Tunable tri-band bandpass filter using varactor-tuned stub-loaded resonators," 2016 Progress in Electromagnetic Research Symposium (PIERS), 4228-4232, Shanghai, China, August 8-11, 2016.