Carsten Weil

Investigation of barium strontium titanate thick films for tunable phase shifters

Abstract The influence of the microstructure of Ba0.6Sr0.4TiO3 (BST) bulk ceramics and thick films on the dielectric properties have been studied. Thick films have been prepared by screen printing technique on Al2O3 substrates. The powder has been prepared by using the common mixed oxide technique. In comparison to dense bulk ceramics, the permittivity of thick films is approximately 10 times less. The effect of temperature on the permittivity and the tunability (change of the dielectric constant with applied voltage) has also been investigated at low frequency (1 kHz). At microwave frequencies, BST thick films have been characterized by measuring coplanar waveguides (CPW) at room temperature and utilizing an enhanced quasi-static CPW model for multilayer dielectric substrates.

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.

Tunable inverted-microstrip phase shifter device using nematic liquid crystals

This paper introduces an alternative low-cost planar integrated, tunable liquid crystal phase shifter device for microwave applications, using the dielectric anisotropy of a nematic liquid crystal in conjunction with inverted-microstrip technology and a DC control voltage. With a standard nematic liquid crystal, a differential phase shift of 53/spl deg/ was achieved at 18 GHz for a physical line length of 49 mm, indicating a figure-of-merit of up to 12/spl deg//dB with a control voltage of only 40 V and a very low power consumption of 0.1 mW. On this basis, the phase shifter performance can be considerably improved by miniaturizing the device and by optimizing liquid crystals in the microwave region.

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.

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.

Ferroelectric thick film ceramics for tunable microwave coplanar phase shifters

This paper introduces the processing and characterization of ferroelectric Barium-Strontium Titanate (BST) Ba^Sn-xTiOa thick film ceramics on AI2O3-microwave substrates. Applying a static interdigital capacitor model, the dielectric material properties of several different BST-thick films were determined by measurements of interdigital capacitors at 100 kHz and 1 MHz, and at room temperature of about 20 °C. Based upon different BSTthick films, microwave phase shifters in CoPlanar Waveguide (CPW) technology, qualified for cost-effective planar phased array antennas, were realized and measured at room temperature. The relative permittivity and the dielectric loss tangent of the BST films were assessed at K-band frequencies, utilizing an enhanced quasi-static CPW model for multilayer dielectric substrates. Übersicht Dieser Beitrag stellt die Herstellung und Charakterisierung von ferroelektrischen Barium-Strontium-Titanat (BST) BaxSri_JiO3 Dickschicht-Keramiken auf AI2O3-Mikrowellensubstraten vor. Unter Verwendung eines statischen Modells für Interdigitalkapazitäten wurde eine Materialcharakterisierung durch die Messung von Interdigitalkondensatoren bei Raumtemperatur sowie bei 100 kHz und 1 MHz vorgenommen. Auf der Basis des DickschichtBST-Materialkonzeptes wurde ein Mikrowellen-Phasenschieber in Koplanarleitungstechnik (CPW) realisiert und bei Raumtemperatur vermessen, der für den kostengünstigen Einsatz in planaren phasengesteuerten Antennen geeignet ist. Die relative Permittivität und der dielektrische Verlustfaktor des BST-Films im K-Band wurden mit Hilfe eines erweiterten quasi-statischen CPW-Modells für mehrschichtige dielektrische Substrate abgeschätzt. Für die Dokumentation Ferroetektrika / BST / Dickschicht-Keramik / Interdigitalkapazität / Koplanarteitung / Phasenschieber

A Comparative Study of Paraelectrics and Novel Liquid Crystals for Tunable Microwave Devices

Abstract Two kind of tunable dielectrics are investigated and compared with respect to their microwave dielectric properties and their application potential in tunable microwave phase shifters: (a) ParaBectric (PE) thick films and (b) nematic Liquid Crystals (LC), both designed for room temperature operation. In particular, the frequency dispersion of the dielectric properties and tunability of PE BaxSn. JiQjand BaZryTii-/D3-thick films have been extracted from broadband measurements of CoPlanar Waveguides (CPW) structures over a wide frequency range up to 26.5 GHz, using an introduced enhanced quasi-TEM CPW model. A substantial progress is reported in the design, characterization and application of novel LC mixtures for microwaves, leading to a tunable phase-shifter figure-of-merit of 110°/dB at 24 GHz, that is the highest value ever reported for tunable microwave dielectrics above 10 GHz at room temperature. This opens up totally new low-cost LC applications beyond optics, particularly in the millimeter wave range.