Hairpin bandpass filter with tunable center frequency and tunable bandwidth based on screen printed ferroelectric varactors

This paper presents a twofold tunable planar hairpin filter to simultaneously control center frequency and bandwidth. Tunability is achieved by using functional thick film layers of the ferroelectric material Barium-Strontium-Titanate (BST). The center frequency of the filter is adjusted by varactors which are loading the hairpin resonators. Coupling varactors between the hairpin resonators enable the control of the bandwidth. The proposed filter structure is designed for a center frequency range from 650 MHz to 920 MHz and a bandwidth between 25 MHz and 85 MHz. This covers the specifications of the lower GSM bands. The functionality of the design is experimentally validated and confirmed by simulation results.