A compact frequency-tuned bandpass filter (BPF) with sharp rejection characteristic is presented. It is composed of a trans-directional (TRD) coupled line and two short-circuited stubs. By changing the capacitor values of the coupled line and the electrical lengths of the short-circuited stubs, a frequency-tuned BPF with sharp rejection is obtained. For verification, a prototype tuned from 1.0 GHz to 1.6 GHz (46.2%) is designed and fabricated. The measured results show that the proposed structure exhibits the return loss of more than 17 dB, the insertion loss of 1.4 dB, and the 3-dB fractional bandwidth (BW) of 43.2-50%. Sharp rejections are also obtained, agreeing well with the simulation results.
2. Yang, T. and G. M. Rebeiz, "Tunable 1.25–2.1-GHz 4-pole bandpass filter with intrinsic transmission zero tuning," IEEE Trans. Microw. Theory Techn., Vol. 63, No. 5, 1569-1578, May 2015.
3. Tsai, H., T. Huang, and R. Wu, "Varactor-tuned compact dual-mode tunable filter with constant passband characteristics," IEEE Trans. Compon., Packag., Manuf. Technol., Vol. 6, No. 9, 1399-1407, Sept. 2016.
4. Zhang, S., Z. Chen, Q. Chu, and S. Member, "Compact tunable balanced bandpass filter with novel multi-mode resonator," IEEE Microw. Wireless Compon. Lett., Vol. 27, No. 1, 43-45, Jan. 2017.
5. Lin, F. and M. Rais-Zadeh, "Continuously tunable 0.55–1.9-GHz bandpass filter with a constant bandwidth using switchable varactor-tuned resonators," IEEE Trans. Microw. Theory Techn., Vol. 65, No. 3, 792-803, Mar. 2017.
6. Shie, C., J. Cheng, S. Chou, and Y. Chiang, "Transdirectional coupled-line couplers implemented by periodical shunt capacitors," IEEE Trans. Microw. Theory Techn., Vol. 57, No. 12, 2981-2988, Dec. 2009.
7. Liu, H. M., S. J. Fang, and Z. B. Wang, "Modified coupled line trans-directional coupler with arbitrary power divisions and its application to a 180◦ hybrid," IET Microw. Antennas Propag., Vol. 9, No. 7, 682-688, 2015.
8. Data Sheet-SMV1265 Series Hyperabrupt Junction Tuning Varactors, Skyworks Solutions Inc., Woburn, MA, USA, 2011.
9. Data Sheet-SMV2020 Series Hyperabrupt Junction Tuning Varactors, Skyworks Solutions Inc., Woburn, MA, USA, 2011.