Table of Contents
International Journal of Microwave Science and Technology
Volume 2011, Article ID 468074, 9 pages
Research Article

Fully Distributed Tunable Bandpass Filter Based on Thin-Film Slow-Wave Structure

1INRS—Centre Énergie Materiaux Télécommunications, 1650 boulevard Lionel-Boulet, Varennes, QC, Canada J3X 1S2
2Poly-Grames Research Center École Polytechnique de Montréal, 2500 Chemin Polytechnique, Montréal, QC, Canada H3T 1J4

Received 13 April 2011; Accepted 15 June 2011

Academic Editor: Carlos Collado

Copyright © 2011 Sébastien L. Delprat et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


This paper presents simulation and measurement results of fully distributed tunable coplanar bandpass filters (BPFs) with center frequencies around 6 GHz that make use of ferroelectric Barium Strontium Titanate (BaxSr1−xTiO3 or BST-x) thin film as tunable material. The two experimental bandpass filters tested are based on a novel frequency-agile structure composed of cascaded half wavelength slow-wave resonators (2 poles) and three coupled interdigital capacitors (IDCs) optimized for bias voltage application. Devices with gap dimensions of 10 and 3 μm are designed and fabricated with a two-step process on polycrystalline Ba0.5Sr0.5TiO3 thin films deposited on alumina substrate. A frequency tunability of 9% is obtained for the 10 μm gap structure at ±30 V with 7 dB insertion loss (the BST dielectric tunability being 26% with 0.04 loss tangent for this gap size). When the structure gap is reduced to 3 μm the center frequency shifts with a constant 9 dB insertion loss from 6.95 GHz at 0 V to 9.05 GHz at ±30 V, thus yielding a filter tunability of 30% (the BST dielectric tunability being 60% with 0.04 loss tangent for this gap size), a performance comparable to some extent to localized or lumped element BPFs operating at microwave frequency (>2 GHz).