X-band and Ka-band Tunable Devices Using Low-loss BST Ferroelectric Capacitors

This paper presents tunable filters and phase shifters using low-loss ferroelectric BST (Barium Strontium Titanate) capacitors. Optimization of the BST varactors has resulted in phase shifters and tunable filters with practically usable performances. Two X-band filters, A and B, exhibit insertion loss of 7 dB - 4.1 dB and 14.9% tuning, and 7.7 dB - 4.7 dB and 19.5% tuning, respectively, when a bias voltage of 0 V to 30 V is applied. A sharp roll-off with excellent out-of-band rejection was achieved on both filters. A Ka-band bandpass filter has a frequency tuning of 17% at a bias voltage of 30 V and a good insertion loss between 6.5 dB and 2.5 dB, comparable with the ones using MEMS technologies at these frequencies. A 35 GHz phase shifter showed an insertion loss of 7 dB with no bias, which improved to below 4 dB when biased at 30 V. The figure of merit is 55 deg/dB. All circuits present good performances in X-band and Ka-band. The measured results of these microwave tunable ferroelectric devices are presented.