Two-Pole $X$ -Band-Tunable Ferroelectric Filters With Tunable Center Frequency, Fractional Bandwidth, and Return Loss

Two two-pole $X$ -band-tunable ferroelectric filters with constant fractional bandwidths and constant return losses are presented in this paper. The center frequency, the fractional bandwidth, and the return loss of the filters are tuned simultaneously through high-$Q$ barium–strontium–titanate capacitors with fast switching speed. The constant fractional bandwidth is achieved while tuning the center frequency and keeping the constant return loss. A center frequency of 10 GHz and a frequency tuning range of 740 MHz (7.4%) are achieved with a low dc bias voltage of 30 V. The first tunable filter with constant fractional bandwidth has an equal-ripple bandwidth of 8.45%. The second filter with a constant fractional bandwidth and a constant return loss exhibits an equal-ripple bandwidth of 8% and a return loss of 17 dB. The measured insertion loss is between 2.7–2.2 dB for both filters, with a dc bias range of 0–30 V.