$Ka$- and U-Band Tunable Bandpass Filters Using Ferroelectric Capacitors

This paper presents the realization of two second-order quasi-elliptic ferroelectric tunable bandpass filters for millimeter-wave applications in$Ka$- andU-band, respectively. These filters include a coplanar-waveguide topology implemented on epitaxial barium–strontium–titanate-on-sapphire technology. The$Ka$-band filter has a frequency tuning of 2.4 GHz (33.6–36 GHz) at low biasing levels(< 30 V), a stable 3-dB bandwidth of about 9%, a loss of 6.2–4 dB, and a compact size compared to state-of-the-art tunable$Ka$-band filters using similar technology (footprint area reduced by 35%). The proposed 50-GHz quasi-elliptic tunable filter, realized for the first time in this technology, tunes from 48.1 up to 52.1 GHz (8.3%) within the same low bias voltage range. It has a 3-dB bandwidth of 9.5%–8.7% and insertion loss of 7.6–4.9 dB. Both filters show excellent return loss, better than 20 dB.