Tunable Lumped Element Filters With BST Thin-Film Varactors

Tunable lumped element filter topologies are discussed based on ferroelectric thin-film varactors. The design methodology for the proposed filter topologies relies on already known techniques and is enhanced with the tuning functionality by introducing the Barium-Strontium-Titanate (BST) varactors. Prototype circuits of a lowpass and a bandstop filter were fabricated and the measurement results showed good agreement to the corresponding calculated data. The lowpass prototype achieves a continuous tuning range from 1.5 GHz to 1.9 GHz. The highest measured passband insertion loss is about 4 dB and the input matching is better than -15dB for all bias states. The implemented bandstop filter allows a continuous tuning of the notch frequency from 1.5 GHz to 2.1 GHz with a minimum attenuation of 17 dB.