High Temperature Superconducting and Tunable Ferroelectric Microwave Devices
Doctoral thesis, 1998
This thesis addresses some applications of High Temperature Superconductors (HTS) and ferroelectric materials in tunable microwave devices and circuits. The work is divided into three main areas: i) modeling of multilayered structures including ferroelectric layers, ii) realization and characterization of tunable coplanar waveguides (CPW), capacitors and resonators based on ferroelectrics, and iii) an investigation of microstrip HTS K-band filters. The motivation for the work has been the increased demands on advanced microwave communication systems requiring miniaturized, low loss, and tunable microwave components.
Closed form analytical models for multilayered CPW and coplanar strip (CPS) transmission lines, interdigital and straight gap planar capacitors are presented. The models are easy to implement in general purpose CAD-programs and may be used for synthesis and optimization of microwave devices in the quasi-static and quasi-TEM approximations. In this work the models are used for the design of CPWs lumped interdigital, and straight gap planar capacitors that include thin ferroelectric films. In addition the models are used for extraction of ferroelectric thin film parameters from measurements. The models show good agreement when compared with FEM simulations for a CPW. The models also predict an enhanced current crowding, dependent on geometry and dielectric constant of the ferroelectric film, at the conductor edges for a CPW on a substrate with a thin ferroelectric film.
Tunable parallel-plate resonators based on bulk single crystal SrTiO3 and KTaO3 have been developed and studied. The resonators show a tuning of the resonant frequency exceeding 50 % from an applied bias electric field. The losses however increased monotonically with the applied electric field, and in the case of KTaO3, exhibited strong hysteresis. Tunability and losses for thin film ferroelectric planar capacitors and phase-shifters based on CPWs have been investigated for different geometries and ferroelectric thin film materials. A phase shift of 130 degrees per dB loss was obtained at 20 K and 20 GHz along a 3 mm long CPW.
Narrow-band 18 GHz Chebyshev filters made of end- or side-coupled HTS microstrip line sections have also been developed. The influence of package related effects, such as substrate or package modes, on the filter performance is studied. K-band YBa2Cu3O7-.delta. (YBCO) based filters are shown to have better performance than identical Cu filters below 80 K with an insertion loss of 0.9 dB at 18 GHz and 77 K.
This thesis also presents the development and operation of a cryogenic microwave probe station, for characterization of microwave devices operating between 20 K and room temperature and at frequencies up to 40 GHz.
tunable microwave circuits
cryogenic probe station
high temperature superconductors