An S-Band Reflection-Type Phase Shifter - A Design Example Using Ferroelectrics
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An S-Band Reflection-Type Phase Shifter - A Design Example Using Ferroelectrics Dongsu Kim1, Yoonsu Choi1, Mark G. Allen1, J. Stevenson Kenney1, David W. Stollberg2, and David Kiesling2 1 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A. 2 MicroCoating Technologies, Inc. Chamblee, GA 30341, U.S.A. ABSTRACT One of the challenges faced in using ferroelectrics in high frequency devices is how to effectively use the material in a circuit design. A compact reflection-type phase shifter fabricated on sapphire substrates coated with ferroelectric barium strontium titanate (BST) thin-films has been built which shows the promise of using BST thin films in the design of tunable microwave devices. The phase shifter, fabricated as one monolithic assembly, consists of a 3dB coupler, meandered line inductors and tunable interdigital capacitors. A continuously variable phase shift range of more than 100° using the branch-line coupler was obtained at a center frequency of 2.95 GHz, and more than 90° phase shift over 200 MHz bandwidth with a bias voltage range from 0 V to 175 V. The phase shifter using the Lange coupler has over 700 MHz bandwidth centered at 2.2 GHz with a phase shift of more than 90° and an insertion loss less than 2 dB and return loss of greater than 14 dB, over a bias voltage range from 0 V to 160 V. The loss of the BST phase shifter presented in this work is on the order of other commercially available RF front-end components, such as bandpass filters and RF switches. This holds promise for the practical realization of smart antenna systems in cellular handsets and wireless LAN cards. INTRODUCTION With the recent increase in market demand for mobile communications devices, there has been added pressure to minimize the size of microwave components, such as filters, couplers and phase shifters. It has been known that the high dielectric constant and electric field dependence of ferroelectric materials, such as strontium titanate (SrTiO3) and barium strontium titanate (BaxSr1-xTiO3), allow for miniature tunable microwave components [1]-[6]. In addition, ferroelectric-based phase shifters have several advantages over pin diode and ferrite phase shifters: monolithic integration, fast speed of tuning and much lower control-line power dissipation [5]. The phase shifter presented in this paper is a fully integrated monolithic device. All components are fabricated directly on the BST coated substrate. The design was targeted for S-Band operation to demonstrate the ability of this technology to achieve good low frequency phase shifter performance in a small volume. Coplanar waveguide (CPW) structures are used to confine electric fields near the substrate surface, keeping a large percentage of the electric field in the BST, hence maintaining a high effective dielectric constant and a large tunability. A thick copper (Cu) metallization process, adopted from MEMS techniques, is used to minimize conductor losses in the distributed elements [7]. Air bridge crossovers are
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