Thin film Barium-Strontium-Titanate varactor-tuned single spiral band-stop filter for X-band applications

A novel thin film Barium-Strontium-Titanate(BST) varactor-tuned single spiral notch filter is presented. The thin film BST varactor device operation is based on the nonlinear dielectric tunability of the BST thin film sandwiched between two metal layers in a conductor-backed coplanar waveguide (CBCPW) transmission line configuration. The considered spiral-structured tunable inductors of different sizes in the bottom metal layer are designed for integration into the notch filters. A schematic model is created to extract the electrical parameters. The s-parameters measured from fabricated devices have a reasonable agreement with the results extracted from the schematic model. The fabricated notch filter can reach 20.52dB rejection while the insertion loss at pass-band is less than 5dB with a tunable center frequency from 10.83GHz to 12.96GHz as the dc bias increases from 0V to 4V.