Patrick Scheele

Broad-band microwave characterization of liquid crystals using a temperature-controlled coaxial transmission line

Liquid crystals (LCs) promise to be suitable passive tunable material for microwave devices with excellent features concerning tunability and losses. In order to optimize the synthesis of LCs and the design of tunable microwave devices based on them, LCs have to be characterized at microwaves. For a microwave analysis between 360 MHz-23 GHz, use was made of a broad-band characterization method with a two-port coaxial line, temperature controlled within -7/spl deg/C and 115/spl deg/C. Two LCs are investigated: K15, as a quasi-standard, and MDA-03-2838, as a novel mixture with increased dielectric anisotropy and reduced microwave losses tan/spl delta/.

Tunable passive phase shifter for microwave applications using highly anisotropic liquid crystals

A tunable broadband inverted microstrip line phase shifter filled with Liquid Crystals (LCs) is investigated between 1.125 GHz and 35 GHz at room temperature. The effective dielectric anisotropy is tuned by a DC-voltage of up to 30 V. In addition to standard LCs like K15 (5CB), a novel highly anisotropic LC mixture is characterized by a resonator method at 8.5 GHz, showing a very high dielectric anisotropy /spl Delta/n of 0.32 for the novel mixture compared to 0.13 for K15. These LCs are filled into two inverted microstrip line phase shifter devices with different polyimide films and heights. With a physical length of 50 mm, the insertion losses are about 4 dB for the novel mixture compared to 6 dB for K15 at 24 GHz. A differential phase shift of 360/spl deg/ can be achieved at 30 GHz with the novel mixture. The figure-of-merit of the phase shifter exceeds 110/spl deg//dB for the novel mixture compared to 21/spl deg//dB for K15 at 24 GHz. To our knowledge, this is the best value above 20 GHz at room temperature demonstrated for a tunable phase shifter based on nonlinear dielectrics up to now. This substantial progress opens up totally new low-cost LC applications beyond optics.

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 Al/sub 2/O/sub 3/-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/spl deg//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/spl deg//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.

Modeling and Applications of Ferroelectric-Thick Film Devices With Resistive Electrodes for Linearity Improvement and Tuning-Voltage Reduction

Low-cost planar high-Q ferroelectric-thick film varactors (Qap90) are realized and component architectures using resistive electrodes for dc bias are investigated. A basic model for planar capacitors with resistive electrodes in the gap is developed and verified by finite-difference time-domain simulations and measurements of interdigital capacitors with high-resistivity indium-tin-oxide bias electrodes in the gap. An optimized high-Q capacitor design based on a series connection of ferroelectric varactors with resistive bias decoupling is presented. The approach allows the increase of device linearity and the reduction of tuning voltages. Based on this technology, a continuously tunable high-power impedance-matching network for 1.875 GHz with tuning voltages below 60 V was developed, realized, and characterized by small- and large-signal measurements with up to 33-dBm input power. The device requires no external dc-block or RF decoupling and features separated RF and dc contacts. The output IP3 of up to 47.8 dBm verifies the excellent device linearity

Passive Phase Shifter for W-Band Applications using Liquid Crystals

In this paper a ridged waveguide phase shifter at W-band frequencies is presented. A taper matches the ridged waveguide phase shifter to the rectangular waveguide feed. Three different liquid crystals (LC) are used: a quasi-standard LC K15, a highly anisotropic mixture MDA-03-2844, and a dual-frequency switching LC MDA-05-1132, all synthesized by Merck KgaA. The return loss of the phase shifter device is always below -10dB in the whole W-band. The maximum achievable figure-of-merit of these materials is 78deg/dB at 108 GHz

LH phase shifter using ferroelectric varactors

A tunable left handed (LH) transmission line used as continuously tunable phase shifter is presented. Using varactors based on barium-strontium-titanate (BST) thick films, a figure of merit FoM = 29deg /dB has been measured at 2.8 GHz. The bandwidth of the realized circuit using 3 unit cells reached 10%. The agreement of the presented result from simulation and measurement is good, the differences can be explained by tolerances of the used components. Additionally the simulations of a 180deg phase shifter based on the same components for the unit cell is presented. Simulated results of this phase shifter shows a bandwidth of 27% and a FoMmax = 31.5deg /dB

Ferroelectric- and liquid crystal- tunable microwave phase shifters

This paper introduces two different low-cost, voltage-controlled tunable phase shifter devices based on (1) coplanar waveguide on a ferroelectric thick film ceramics and Al2O3-microwave substrate and (2) inverted-microstrip line with standard and novel liquid crystal (LC) in-between. For the first type, the differential phase shift can be controlled continuously and exceeds 360° at 38 GHz for a maximum dc voltage of 100 V, corresponding to an electrical field strength of 3.3 V/¿m. The best phase shifter figure-of-merit values, i.e. the quotient of differential phase shift and insertion loss, amounts to 15 to 28°/dB at 24 GHz for a maximum field strength of 10 V/¿m. This can be increased up to 50°/dB at 24 GHz for very high field strengths above 20 V/¿m. In contrast, with comparatively low control voltage of 40 V (0.18 V/¿m), the figure-of-merit of the LC type phase shifter amounts to 16°/dB at 20 GHz for a standard nematic LC K15 and 68°/dB for an advanced LC mixture.

Influence of Fe–F-co-doping on the dielectric properties of Ba0.6Sr0.4TiO3 thick-films

Abstract The influence of Fe acceptor and Fe – F acceptor – donor co-doping on the dielectric response of screen-printed Ba0.6 Sr0.4 TiO3 thick-films on alumina substrates has been investigated. The Ba0.6 Sr0.4 TiO3 powders were synthesized by freeze-drying of sols. Permittivity, dielectric loss, and tunability were investigated at kHz frequencies with a planar metal-insulator-metal capacitor structure, and at GHz frequencies up to 40 GHz using coplanar waveguide structures. Pure acceptor doping by Fe was found to have a distinct influence on permittivity and dielectric loss-factor at kHz-frequencies and at GHz frequencies due to an internal bias field and pairs of associated defects. Co-doping with F significantly suppresses the influence of the internal bias field and defect-associates at low and microwave frequencies. The commutation quality factor at 10 GHz and Eeff = 5.8 V μm-1 was increased by co-doping the thick-films with Fe and F. Such doped Ba0.6 Sr0.4 TiO3 films show a high potential for tunable microwave applications.

Passive tunable liquid crystal finline phase shifter for millimeter waves

In this paper, the investigation of a finline based phase shifter device filled with liquid crystal, fed by a standard rectangular Ka-band waveguide, is described. A broadband transition from waveguide to antipodal finline is investigated and optimized, leading to an insertion loss of below 1.7 dB of the back-to-back transition. An antipodal finline transmission line filled with liquid crystal between its overlapping conductors is used as a phase shifter. Two different liquid crystals are investigated: K15 as a quasi-standard and MDA-03-2838 as a novel mixture with a steeply increased dielectric anisotropy and reduced losses compared to K15. At 40 GHz, a phase shift of 75/spl deg/ arises in connection with 8dB insertion loss for K15 whereas the phase shift of MDA-03-2838 amounts to 303/spl deg/ and 4.8 dB insertion loss, all measured at room temperature. Additionally, the response times of the phase shifter device are investigated for both liquid crystals.

Optimization of uniplanar multilayer structures using nonlinear tunable dielectrics

For an optimization of the tunability of coplanar waveguides (CPW) different geometries have been studied on barium-strontium-titanate (BST) thick films. The presented measurement results show a strong dependency of tunability vs. geometry which can not be calculated using standard conformal mapping (CM) approaches. To calculate a cross section of a CPW, an FDTD (finite difference time domain) algorithm, expanded to field strength dependent relative permittivities has been implemented. A geometry efficiency factor /spl chi/ is introduced to review the effective tunability for different geometries.