Nonlinear dielectrics for tunable microwave components

Agile materials and technologies based on nonlinear dielectrics like ferroelectrics or liquid crystals offer a line of passive tunable microwave components such as varactors, filters and phase shifters, suitable as key components in phased-array antennas e.g. for automotive radar sensors and in future reconfigurable (frequency-agile) RF-frontends, e.g. in mobile communication systems with multiband operation. The trend towards these commercial microwave applications involves a demand for cheaply integrated, compact devices with both, high tunability and low insertion loss. Therefore, distinct research interests have been focused on agile materials for tunable microwave components as promising alternatives to active semiconductor devices or MEMS varactors. Starting with some results of ferroelectric thin-film devices from literature, the focus of this paper will be on our research of tunable microwave components based on (1) ferroelectric thick-films on Al₂O₃-substrate and (2) liquid-crystals. Up to now, only very few approaches used an anisotropy at microwaves, e.g. for phase shifting purposes. However, with recently developed, novel highly-anisotropic microwave LCs, a figure-of-merit of above 110°/dB at 24 GHz has been achieved for a inverted-microstrip line phase shifter with comparatively low control voltages less than 30 V. At the same frequency, this exceeds by far the figure-of-merit of 30 to 50°/dB of BST-coplanar waveguide phase shifters, however, much lower tuning speed. This substantial progress opens up totally new low-cost LC applications beyond optics.