$Ka$ - 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- and U-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.