This paper presents simulation and measurement results of fully distributed tunable coplanar bandpass filters (BPFs) with center frequencies around 6？GHz that make use of ferroelectric Barium Strontium Titanate (BaxSr1？xTiO3 or BST-x) thin film as tunable material. The two experimental bandpass filters tested are based on a novel frequency-agile structure composed of cascaded half wavelength slow-wave resonators (2 poles) and three coupled interdigital capacitors (IDCs) optimized for bias voltage application. Devices with gap dimensions of 10 and 3？μm are designed and fabricated with a two-step process on polycrystalline Ba0.5Sr0.5TiO3 thin films deposited on alumina substrate. A frequency tunability of 9% is obtained for the 10？μm gap structure at ±30？V with 7？dB insertion loss (the BST dielectric tunability being 26% with 0.04 loss tangent for this gap size). When the structure gap is reduced to 3？μm the center frequency shifts with a constant 9？dB insertion loss from 6.95？GHz at 0？V to 9.05？GHz at ±30？V, thus yielding a filter tunability of 30% (the BST dielectric tunability being 60% with 0.04 loss tangent for this gap size), a performance comparable to some extent to localized or lumped element BPFs operating at microwave frequency (>2？GHz). 1. Introduction The next generation of wireless networks such as reconfigurable and cognitive radio systems will require low-cost and highly integrated tunable microwave components that must handle room-temperature operations for multibands with wide tuning range, low power consumption, and small size. In particular, there is an increasing need for frequency-agile bandpass filters (BPFs) to replace large and costly filter banks used in the design of multiband microwave receivers [1]. In this context, significant efforts have been made since the last decade on the development of frequency-agile structures operating at microwave frequencies (above 2？GHz). Compared to semiconductor diodes [2] and RF microelectromechanical systems (MEMS) [3], ferroelectric-based devices present many advantages such as high power handling, continuous tuning, nanosecond switching speeds [4], and operation in a large frequency range with ease of fabrication and operation. Generally, the tuning is obtained by applying a bias voltage on the ferroelectric material and ferroelectric devices can be fully coplanar, very compact, and do not require hermetic packaging [5, 6]. Ferroelectric materials, in particular BaxSr1？xTiO3 (BST-x with x~0.5), present almost continuous dielectric constant tuning in the presence of an electric field and can
References
[1]
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.
[2]
S. R. Chandler, I. C. Hunter, and J. G. Gardiner, “Active varactor tunable bandpass filter,” IEEE Microwave and Guided Wave Letters, vol. 3, no. 3, pp. 70–71, 1993.
[3]
J. Brank, J. Yao, M. Eberly, A. Malczewski, K. Varian, and C. Goldsmith, “RF MEMS-based tunable filters,” International Journal of RF and Microwave Computer-Aided Engineering, vol. 11, no. 5, pp. 276–284, 2001.
[4]
A. B. Kozyrev, V. N. Osadchy, D. M. Kosmin, A. V. Tumarkin, T. Kaydanova, and D. Ginley, “Time tuning of ferroelectric film varactors under pulse voltages,” Applied Physics Letters, vol. 91, no. 2, Article ID 022905, 2007.
[5]
E. Marsan, J. Gauthier, M. Chaker, and K. Wu, “Tunable microwave device: status and perspective,” in Proceedings of the 3rd International IEEE Northeast Workshop on Circuits and Systems Conference, (NEWCAS '05), pp. 279–282, Quebec, Canada, June 2005.
[6]
M. J. Lancaster, J. Powell, and A. Porch, “Thin-film ferroelectric microwave devices,” Superconductor Science and Technology, vol. 11, no. 11, pp. 1323–1334, 1998.
[7]
A. Tombak, J.-P. Maria, F. T. Ayguavives, et al., “Voltage-controlled RF filters employing thin-film barium-strontium-titanate tunable capacitors,” IEEE Transactions on Microwave Theory and Techniques, vol. 51, no. 2, pp. 462–467, 2003.
[8]
M. T. Nguyen, W. D. Yan, and E. P. Horne, “Broadband tunable filters using high Q passive tunable ICs,” in Proceedings of the IEEE MTT-S International Microwave Symposium Digest, pp. 951–954, Atlanta, Ga, USA, June 2008.
[9]
G. Sanderson, A. H. Cardona, T. C. Watson, et al., “Tunable if filter using thin-film BST varactors,” in Proceedings of the IEEE MTT-S International Microwave Symposium, pp. 679–682, Honolulu, Hawaii, USA, June 2007.
[10]
J. Nath, D. Ghosh, J. P. Maria, et al., “An electronically tunable microstrip bandpass filter using thin-film Barium-Strontium-Titanate (BST) varactors,” IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 9, pp. 2707–2712, 2005.
[11]
S. Courreges, Y. Li, Z. Zhao, K. Choi, A. Hunt, and J. Papapolymerou, “A low loss X-band quasi-elliptic ferroelectric tunable filter,” IEEE Microwave and Wireless Components Letters, vol. 19, no. 4, pp. 203–205, 2009.
[12]
S. Courreges, Y. Li, Z. Zhao, K. Choi, A. T. Hunt, and J. Papapolymerou, “Two-pole X-band-tunable ferroelectric filters with tunable center frequency, fractional bandwidth, and return loss,” IEEE Transactions on Microwave Theory and Techniques, vol. 57, no. 12, Article ID 5339095, pp. 2872–2881, 2009.
[13]
J. Sigman, C. D. Nordquist, P. G. Clem, G. M. Kraus, and P. S. Finnegan, “Voltage-controlled Ku-band and X-band tunable combline filters using barium-strontium-titanate,” IEEE Microwave and Wireless Components Letters, vol. 18, no. 9, pp. 593–595, 2008.
[14]
J. Papapolymerou, C. Lugo, Z. Zhiyong, X. Wang, and A. Hunt, “A miniature low-loss slow-wave tunable ferroelectric BandPass filter from 11–14？GHz,” in Proceedings of the IEEE MTT-S International Microwave Symposium Digest, pp. 556–559, San Francisco, Calif, USA, June 2006.
[15]
V. N. Keis, A. B. Kozyrev, M. L. Khazov, J. Sok, and J. S. Lee, “20？GHz tunable filter based on ferroelectric (Ba,Sr)TiO3 film varactors,” Electronics Letters, vol. 34, no. 11, pp. 1107–1109, 1998.
[16]
S. Courreges, Y. Li, Z. Zhao, et al., “A Ka-band electronically tunable ferroelectric filter,” IEEE Microwave and Wireless Components Letters, vol. 19, no. 6, pp. 356–358, 2009.
[17]
H. T. Su, P. M. Suherman, T. J. Jackson, F. Huang, and M. J. Lancaster, “Novel tunable bandpass filter realized using barium-strontium-titanate thin films,” IEEE Transactions on Microwave Theory and Techniques, vol. 56, no. 11, Article ID 4657392, pp. 2468–2473, 2008.
[18]
V. Pleskachev and I. Vendik, “Tunable microwave filters based on ferroelectric capacitors,” in Proceedings of the 15th International Conference on Microwaves, Radar and Wireless Communications, (MIKON '04), vol. 3, pp. 1039–1043, May 2004.
[19]
G. Subramanyam, N. Mohsina, A. Al Zaman, et al., “Ferroelectric thin-film based electrically tunable Ku-band coplanar waveguide components,” in Proceedings of the IEEE-MTT-S International Microwave Symposium Digest, pp. 471–474, May 2001.
[20]
S. L. Delprat, C. Durand, J. H. Oh, M. Chaker, and K. Wu, “Correlation between the lattice parameter and the dielectric tunability in nonepitaxial Ba0.5Sr0.5 Ti O3 thin films,” Applied Physics Letters, vol. 91, no. 6, Article ID 063513, 3 pages, 2007.
[21]
N. Li, F. Xu, K. Wu, S. L. Delprat, and M. Chaker, “Slow-wave line filter design using full-wave circuit model of interdigital capacitor,” in Proceedings of the 35th European Microwave Conference, vol. 1, pp. 409–412, October 2005.
[22]
F. Xu, N. Li, K. Wu, S. Delprat, and M. Chaker, “Parameter extraction of interdigital slow-wave coplanar waveguide circuits using finite difference frequency domain algorithm,” International Journal of RF and Microwave Computer-Aided Engineering, vol. 18, no. 3, pp. 250–259, 2008.
[23]
A. Gorur, C. Karpuz, and M. Alkan, “Characteristics of periodically loaded CPW structures,” IEEE Microwave and Guided Wave Letters, vol. 8, no. 8, pp. 278–280, 1998.
[24]
J. Zhou, D. Hung, M. J. Lancaster, H. T. Su, and X. Xiong, “A novel superconducting CPW slow-wave bandpass filter,” Microwave and Optical Technology Letters, vol. 34, no. 4, pp. 255–259, 2002.
[25]
H. Yoon, K. J. Vinoy, and V. K. Varadan, “Design and development of micromachined bilateral interdigital coplanar waveguide RF phase shifter compatible with lateral double diffused metal oxide semiconductor voltage controller on silicon,” Smart Materials and Structures, vol. 12, no. 5, pp. 769–775, 2003.
[26]
E. Carlsson and S. Gevorgian, “Conformal mapping of the field and charge distributions in multilayered substrate CPWs,” IEEE Transactions on Microwave Theory and Techniques, vol. 47, no. 8, pp. 1544–1553, 1999.
[27]
M. Ouaddari, S. Delprat, F. Vidal, M. Chaker, and K. Wu, “Microwave characterization of ferroelectric thin-film materials,” IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 4, pp. 1390–1397, 2005.
[28]
P. M. Suherman, T. J. Jackson, Y. Y. Tse, et al., “Microwave properties of Ba0.5Sr0.5TiO3 thin film coplanar phase shifters,” Journal of Applied Physics, vol. 99, no. 10, Article ID 104101, 7 pages, 2006.
[29]
W. Chang, L. M. B. Alldredge, S. W. Kirchoefer, and J. M. Pond, “Device loss of (Ba,Sr)TiO3 film-based capacitors at 1–20？GHz,” Journal of Applied Physics, vol. 105, no. 3, Article ID 034115, 8 pages, 2009.
[30]
J. H. Oh, S. Delprat, M. Ismail, E. E. Djoumessi, M. Chaker, and K. Wu, “Improvement of Ba0.5Sr0.5TiO3 thin films microwave properties using codoping with Mg-W and Al-W,” Integrated Ferroelectrics, vol. 112, no. 1, pp. 24–32, 2009.
