Frequency agile RF/microwave circuits using BST varactors

Abstract

The research has focused on characterizing barium strontium titanate or BST film at RF/microwave frequencies, improving BST capacitor quality factors and designing frequency agile RF/microwave circuits.

A simple and fast measurement technique is developed to extract BST loss tangent and dielectric constant in a parallel plate capacitor. An error analysis is performed to indicate the measurement accuracy. In addition, the BST capacitor layout is optimized to achieve the best possible quality factor.

BST capacitor based tunable band-pass filters are designed. Two coupled half-wavelength microstrip resonators are used in the filter. BST capacitors are placed at the end of resonators, which change the filter performance. Analysis shows that with the increase in BST capacitance, the filter skirt sharpness increases, the circuit size decreases, and the tunability increases but the insertion loss increases. Different filter specifications are achieved using various BST capacitances. In addition, a new topology is developed to maintain the 3-dB bandwidth of filters at different biases. The bandwidth change decreases from 54% to only 4% using the new topology.

A tunable microstrip antenna is designed, fabricated and measured. Multiple varactors are used to load the rectangular microstrip antenna. A figure of merit is defined to find the optimum number of varactors. Good tunability of 25% and maximum gain of 7.8 dB are achieved within the tuning range. Almost uniform radiation patterns at different biases are also obtained. The measurement results prove that tunable microstrip antennas using multiple varactors loading can achieve better performance than those using single varactor loading.

A tunable high impedance surface is designed, fabricated and measured. The surface uses lumped elements to form a parallel resonant circuit. Unlike other lumped-element high impedance surfaces, the inductor in this surface is obtained from the grounded substrate, which shows much higher quality factor and requires less substrate height than those using vias to obtain inductance. Measured tunability of 62% is achieved.