Ferrite Properties and Technology Issues for Improved Microwave Systems
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Ferrite Properties and Technology Issues for Improved Microwave Systems Gerald F. Dionne and Daniel E. Oates Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood Street, Lexington, MA 02420, U.S.A. ABSTRACT Microwave device engineers continually seek materials advances to improve performance of magnetic components at reduced size and cost. Wherever possible, microstrip or stripline device configurations are adopted in preference to bulky waveguide structures. In radar and communications applications, the nonreciprocal propagation properties of ferrites are essential for realizing phase shifters, circulators, isolators, and power limiters. The introduction of superconductor circuits has led to the development of very low-loss phase shifters and circulators. Recent demonstrations of tuning reciprocal rf permeability by varying the state of magnetization at very low magnetic fields has led to the development of high-speed, high-Q tunable filters. In this paper, design issues of four classes of microwave device are reviewed from the standpoint of their ferrite material requirements: (1) low-loss microstrip phase shifters (2) microstrip tunable resonators, (3) self-biased microstrip circulators with normal or in-plane uniaxial anisotropy, and (4) high-power quasi-optical circulators. INTRODUCTION For many decades, microwave device engineers relied on the performance of bulk ceramic ferrites where nonreciprocal effects on propagation were needed. Wherever practical, bulky and costly waveguide systems have begun to be replaced by planar microstrip and stripline components that introduce the possibility of hybrid integration with semiconductor circuits [1]. Monolithic integration with semiconductors, such as Si and GaAs, involves the deposition of thick-film ferrites of spinel, garnet or hexagonal crystal structures [2]. For this purpose, emphases have been placed on the preparation of ferrites by film deposition processes, sometimes combining with layered structures of nonmagnetic materials. Despite the focus on these more modern materials science methods, however, the need for ferrites to satisfy the requirements of exotic applications have preserved an interest in the more conventional ceramic technologies. In this paper, some examples of the present and future device needs will be reviewed from the standpoint of their ferrite materials challenges. MEANDERLINE PHASE SHIFTER AND CIRCULATOR Following the discovery of high-Tc cuprate superconductors, nonreciprocal switchable phase shifters employing a meanderline superconducting circuit in physical contact with magnetized ferrite in a closed magnetic circuit were developed [3,4]. The successful demonstration of the device sketched in figure 1 was highlighted by the reduction of insertion losses from about 3 dB to 0.1 dB. As indicated by this design, a materials compatibility challenge was recognized once the superconductor was changed from elemental metal Nb (Tc ≈ 9 K) to the crystallographically
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Figure 1. Sketch of X-band microstrip meander
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