Tunable Dual Band Filter with BST Capacitors

A tunable double band band-stop filter based on BST is presented. This configuration has a coplanar microstrip loaded with stepped impedance resonators. Ferroelectric BST parallel plate capacitors were loaded at the end of the stepped impedance resonators. When a DC voltage is applied to the BST capacitors, the change in capacitance shifts the resonance frequency of the stepped impedance transformers. The filters were designed with notch frequencies at 1.5 GHz and 3.5 GHz without loading them with BST capacitors. The filter circuits were simulated with Agilent ADS and were fabricated on FR-4 substrates. By loading the resonators with BST capacitors with no DC applied voltage, the first and second notch frequency shifts significantly. The first notch frequency can be tuned from 570 MHz to 781 MHz and the second notch frequency from 2.16 GHz to 2.55 GHz by applying the bias voltage of 5volts. The performance of the BST tuned filters were compared with filters tuned with p-n junction varactors.