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Characterization of Barium Strontium Titanate at G-Band
Keywords: Barium Strontium Titanate , Thick film, mm-wave frequency
New data are presented on the behaviour of BST (Barium Strontium Titanate) at millimetre-wave frequencies. BST is a ferroelectric material whose permittivity is known to change as a function of DC electric field. The tunable property of BST has been exploited in many applications, such as tunable filters, phase shifters and antenna beam-steering circuits. By incorporating BST in microstrip circuits in a distributed manner, the circuits can offer all of the advantages of planar microstrip circuits, plus a tunabale capability. At lower microwave frequencies, these types of circuit have mainly been fabricated using thin-film technology There is a common misassumption that the material is simply too lossy for useful application at these higher frequencies even though there has been no published data on the behaviour of the material at these frequencies. As the trend is moving towards using higher millimeter-wave frequencies for communication systems, there is need for accurate characterization of the BST materials at higher frequencies. In this paper we show that losses in BST can be comparable to those in LTCC at frequencies above 100GHz, with loss tangents less than 10-3, and that consequently these losses do not detract from the use of the material in practical, tunable devices. The work also compares the performance between thick film BST paste and the normally used thin film BST. A free-space technique was used to characterize both thin-film and thick-film BST at frequencies between 150GHz and 155GHz. The main useful outcome of the work was in showing through practical measurement that moderate losses in BST mean this material can provide a useful tuning element at high millimetre-wave frequencies. Clearly using BST in a thick-film format has advantages, both in terms of lower cost, and in the opportunity to form multilayer structures.
Nurul Osman, Ph.D. Student
University of Surrey
Guildford, Surrey GU2 7XH,
UK


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