Cross-coupled open-loop resonator bandpass filter with independently tunable center frequency and bandwidth

This paper presents a twofold tunable cross-coupled open-loop resonator bandpass filter to simultaneously and independently control the center frequency and the bandwidth. Frequency selectivity is increased by introducing two transmission zeros via cross coupled resonators. Tunability is achieved by using thin film barium strontium titanate (BST) varactors. The structure is applicable in future seamless and band less cognitive radio systems. The center frequency is adjusted by varactors which are loading the open-loop resonators, while coupling varactors between the resonators enable the control of bandwidth. The proposed filter structure is designed for a center frequency range from 1.5 GHz to 1.75 GHz with a constant bandwidth of 100 MHz and a tunable bandwidth between 70 MHz and 180 MHz at a fixed center frequency of 1.625 GHz. The functionality of the design is experimentally validated.