Tunable microwave filters based on discrete ferroelectric and semiconductor varactors

This study presents a direct comparison between semiconductor and barium strontium titanate (BST) tuning technologies. Tapped line combline two-pole and four-pole bandpass filters were designed in a coplanar waveguide configuration. Discrete BST interdigital capacitors were fabricated and integrated into the filter circuits in a hybrid manner. The BST varactors performed as tuning elements in the filters. The same filter was also made using gallium arsenide (GaAs) varactors and the tuning and loss performance were compared with the BST filters. The centre frequency of the two-pole BST filter tuned 30.3% from 1.55 to 2.02 GHz with insertion loss ranging from 3.7 to 1.1 dB, at an electric field of 6.5 V/mu m. This filter has a figure of merit of 0.87 dB(-1). The two-pole GaAs filter showed a wide tuning range from 0.94 to 2.44 GHz at 14 V with insertion loss ranging from 4.2 to 0.8 dB. The results are discussed in detail, as it is important to clarify the comparison with relation to the quality of the materials, as well as the particular filter designs and centre frequencies involved.