First Demonstration of a Periodically Loaded Line Phase Shifter Using BST Capacitors
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ABSTRACT Periodically loaded line phase shifter circuits using voltage tunable BaSrTiO 3 (BST) parallel plate capacitors have been demonstrated at X-band. The first such phase shifter circuit was capable of 100' of phase shift with an insertion loss of 7.6 dB at 10 GHz. Subsequently, the monolithic fabrication procedure was refined resulting in an improved phase shifter circuit with 200' of phase shift and an insertion loss of 6.2 dB at 10 GHz. In addition to promising loss performance (32°/dB) at 10 GHz, the circuits reported here have several desirable features such as moderate control voltages (20 V), room temperature operation, and compatibility with monolithic fabrication techniques. INTRODUCTION The cost of phase shifters is a major component of the cost of modem phased array antennas. Thus it is of paramount importance to reduce the cost of phase shifters to ensure widespread acceptance of phased arrays in military/civilian applications. Ferroelectric thin film based phase shifters promise to be low cost because of two factors- 1) the ferroelectric material can be deposited relatively inexpensively using RF sputtering/MOCVD 2) the films can be processed using low cost, high volume monolithic fabrication techniques. Apart from cost, the use of ferroelectric thin films in phase shifter circuits also has potential performance advantages such as low insertion loss, high power handling capability and low DC power requirements. Several groups [1-4] are investigating the possibility of implementing phase shifter circuits using barium strontium titanate (BST) which has an electric field tunable dielectric constant. In these circuits the ferroelectric material (BST) either forms the entire microwave substrate [1,2] on which the conductors are deposited (thick films/bulk crystals) or a fraction of the substrate with thin BST films sandwiched between the substrate and the conductors [3,4]. These circuits rely on the principle that the phase velocity of the waves propagating on these structures can be altered by changing the permittivity of the ferroelectric layer. This approach has several limitations including high conductor losses, inefficient use of the BST tunability and high control voltages. The approach used by us was to periodically load a coplanar wave guide transmission line with voltage tunable ferroelectric (BST) capacitors. The phase velocity of the periodically loaded line depends on the values of the BST capacitors and thus could be changed by applying bias to the BST capacitors. Parallel plate capacitors were employed here since they utilize the tunability of the BST film effectively and require much lower control voltages than interdigitated capacitors. Also, the use of discrete BST capacitors made it easy to control the amount of capacitive loading due to the ferroelectric film and thus allowed the structure to be optimized for good loss performance
[5].
37 Mat. Res. Soc. Symp. Proc. Vol. 603 © 2000 Materials Research Society
THEORY The schematic of the proposed phase shifter circuit is shown in f
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