Microwave measurement and modelling of capacitors with tunable dielectric constants

Materials with tunable dielectric constants find application in a wide variety of microwave devices, such as tunable filters and phase shifters. A parallel plate capacitor containing a ferroelectric material, for example, can replace a diode varactor in such an application. Due to the very large (100-1000) relative dielectric constant of these materials such a capacitor has a non-negligible electrical length that must be accounted for in its microwave model. This report discusses various approaches to modelling these capacitors and compares the models to experimental data. A model is developed that best describes the experimental data and a curve fit is used to extract device parameters. The voltage-dependent capacitance, one such parameter, is measured over the frequency range 45 MHz to 50 GHz and is compared to low frequency measurements.