A Thin-Film BST Varactor Model for Linear and Nonlinear Circuit Simulations for Mobile Communication Systems

A simple model of thin film BST varactors for the use in commercial circuit simulators is presented. The model implies the varactor's nonlinear capacitance as well as the frequency and voltage dependency of the losses. For the use of BST based components in mobile communication systems low tuning voltages are required. This leads unfortunately to high nonlinear distortion which is not acceptable. Another severe problem are the moderate to low quality factors and therefore high losses of the ferroelectric capacitors. The presented model allows estimating the losses and the distortion over a wide frequency range at every bias state with harmonic balance and envelope simulations. Further more it is now possible to predict the behavior of a designed circuit but also to deduce the requirements for BST varactors in order to achieve the demanded design goals. The model consists of a parallel circuit of equation based elements which represents the nonlinear capacitance and conductance. A constant series inductance and a constant resistance model the contacts. The required parameters can easily be extracted through measurements. To demonstrate the validity of the presented model a BST based SP2T switch at 1800 MHz was designed. An insertion loss of 1.4 dB at an isolation of 21 dB and a TOI of 35 dBm were achieved. The simulation and measurement results agree very good over a wide frequency range.