Tunable Lumped Element Filters With BST Thin-Film Varactors

Abstract

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 -15 dB 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.

Tunable band-stop filter based on single RF MEMS capacitive shunt switch with meander arm inductance

“…However, SAW filters are limited in frequency (<3 GHz). A survey of the recent literature reveals that apart from the traditional active [2] and distributed RC passive [3] notch filters, novel tunable band-stop filter topologies using photonic [4] or electromagnetic band gap (EBG) structures [5], quarter wavelength open stubs, radial stubs or slotted ground structures [6][7][8], complementary split ring resonators (CSRR) utilising metamaterials [9], lumped LC circuits [10,11], multilayer ferromagnetic resonance [12] and bulk acoustic wave (BAW) resonators [13] are chosen as alternatives to address similar problems. In general, all of the above approaches are quite complex systems, requiring large silicon areas (>6 mm in length) and exhibiting moderate performances.…”

“…Ferroelectric materials (such as BST, PZT or PST) have recently been used as varactors in tunable microwave circuits [6,11]. However, they are not usually integrated with the filter fabrication process and the insertion losses are relatively high.…”