(Received: 2019-05-15, Revised: 2019-06-23 , Accepted: 2019-07-09)
An electronically tunable multi-passband filter using one varactor diode is implemented based on transmission line stub method for passing several most favourable applications through multiple operating bands. In this paper, the filter is designed on Rogers RT/Duroid 5880-substrate and its input and output ports are terminated by 50 Ω microstrip feed line. The filter passbands consist of low pass filter (LPF) with tunable cut-off frequency which can reach 0.94 GHz, then several tunable bandpass filters (BPFs) that can cover the following frequency ranges BPF1 (1.94 – 3.33 GHz), BPF2 (3.83 – 4.23 GHz), BPF3 (4.53 – 5.56 GHz) and BPF4 (6.83 – 7.48 GHz) with insertion loss (IL) of |S21| ≤ 3 dB. The designed filter is the binomial type with 3 elements that are implemented in three shunt stubs with the middle stub being shorted. A parametric study was conducted for the optimum location of the varactor diode and an external DC biasing circuit introduced to produce the required reverse biasing for the varactor diode and its effect was considered. The demonstrated filter is investigated using the high-frequency structure simulator (HFSS). The measured scattering parameters’ S11 (reflection coefficient) and S21 (transmission coefficient) results show good agreement with the simulated values.

[1] P. W. Wong and I. Hunter, "Electronically Tunable Filters," IEEE Microwave Magazine, vol. 10, no. 6, pp. 46–54, 2009.

[2] J. Uher and W. J. R. Hoefer, "Tunable Microwave and Millimeter-wave Band-pass Filters," IEEE Transactions on Microwave Theory and Techniques, vol. 39, no. 4, pp. 643–653, 1991.

[3] I. C. Reines et al., "A Low Loss RF MEMS Ku-band Integrated Switched Filter Bank," IEEE Microw. Wirel. Components Lett., vol. 15, no. 2, pp. 74–76, 2005.

[4] C. A. Hall, R. C. Luetzelschwab, R. D. Streeter and J. H. Vanpatten, "A 25 Watt RF MEM-tuned VHF Bandpass Filter," Proc. of IEEE MTT-S International Microwave Symposium Digest, pp. 503–506, 2003.

[5] J. H. Park, H. T. Kim, Y. Kwon and Y. K. Kim, "Tunable Millimeter-wave Filters Using a Coplanar Waveguide and Micromachined Variable Capacitors," Journal of Micromechanics and Microengineering, vol. 11, no. 6, pp. 706–712, 2001.

[6] A. Pothier et al., "Low-loss 2-Bit Tunable Bandpass Filters," IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 1, pp. 354–360, 2005.

[7] E. Fourn et al., "MEMS Switchable Interdigital Coplanar Filter," IEEE Transactions on Microwave Theory and Techniques, vol. 51, no. 1, pp. 320–324, 2003.

[8] C. S. Tsai, "Wideband Tunable Microwave Devices Using Ferromagnetic Film-gallium Arsenide Material Structures," Jou. Magn. Magn. Mater., vol. 209, no. 1–3, pp. 10–14, 2000.

[9] X. Zhang, C. Chen, M. Li, L. Zhou and B. Liu, "A Compact Tunable Dual-band Bandpass Filter Using Varactor-loaded Step-impedance Resonators," PIERS Proceedings, vol. 1, pp. 2642–2645, Prague, Czech Republic, 2015.

[10] L. Smadi, H. Sharif and Y. S. Faouri, "Dual-Band Tunable Microwave Bandpass Filter Using Stepped Impedance Technique," Proc. of IEEE Jordan Int. Jt. Conf. Electr. Eng. Inf. Technol., pp. 827–830, Apr. 2019.

[11] X. Y. Zhang, L. Gao, Y. Cao, X.-L. Zhao and Y. Ding, "Independently-tuned Dual-band Filter Using Varactor-loaded Resonators," Prog. Electromagn. Res. C, vol. 42, no. May, pp. 55–66, 2013.

[12] A. Golaszewski, M. Zukocinski and A. Abramowicz, "Design of Varactor Tuned Bandpass Filter," Proc. of the 14th Conf. Microw. Tech. Com., pp. 86–89, 2015.

[13] Y. Guan and Q. Guo, "A Varactor-tuned Bandpass Filter Based on Lowpass Filter and Shorted Stubs," Microw. Opt. Technol. Lett., vol. 56, no. 4, pp. 879–883, 2014.

[14] B. Kim and S. Yun, "Varactor-Tuned Combline Bandpass Filter," IEEE Trans. Microw. Theory Tech., vol. 52, no. 4, pp. 1279–1283, 2004.

[15] Y. Shang, W. Feng and W. Che, "Wideband Reconfigurable Bandpass Filter Using Coupled Lines Loaded with Varactor Loaded Stubs," Int. J. RF Microw. Comput. Eng., vol. 28, no. 2, 2018.

[16] S. Arain, P. Vryonides, M. A. B. Abbasi, A. Quddious, M. A. Antoniades and S. Nikolaou, "Reconfigurable Bandwidth Bandpass Filter with Enhanced Out-of-Band Rejection Using π-Section- loaded Ring Resonator," IEEE Microw. Wirel. Components Lett., vol. 28, no. 1, pp. 28–30, 2018.

[17] H. Sharif, L. Smadi and Y. S. Faouri, "Stub Resonator Tunable Bandpass and Lowpass Filters Using Shunt Stub Resonators," Proc. of IEEE Jordan Int. Jt. Conf. Electr. Eng. Inf. Technol., pp. 442–445, Apr. 2019.

[18] D. M. Pozar, Microwave Engineering, 4th Edition, New York: Wiley, 2012.

[19] Skyworks Solutions, "SMV123x Series : Hyperabrupt Junction Tuning Varactors,"[Online], Available at:, 2012.

[20] W. D. Reeve, "Filter Measurements with a Vector Network Analyzer," pp. 1–16,[Online], Available at:, 2016.