Fast and accurate tuning of a cross-coupled split-ring resonator filter

This work addresses the design and tuning of a cross-coupled split-ring microstrip resonator filter. The filter is designed for a center frequency of 1.65 GHz while having a bandwidth of 65 MHz. The selectivity of the filter is increased by introducing a cross coupling between the first and fourth resonator generating two transmission zeros. Tunability is achieved by using thin film barium strontium titanate (BST) varactors with which the center frequency of each resonator is tunable. Furthermore, they enable tuning of the coupling between resonator one and two and resonator three and four. A fast and accurate tuning of the proposed filter is achieved by using the Cauchy-method algorithm and an implementation of a powerful optimization technique. As a result, a more reliable algorithm is obtained for a better fitting between coupling matrix and measured s-parameters. The robustness of the algorithm is demonstrated by extracting the coupling matrix of a strong detuned filter tuning state.