Substrate-integrated coaxial-cavity filter with tunable center frequency and reconfigurable bandwidth

This paper presents a substrate-integrated coaxial-cavity bandpass filter with both continuous center frequency tuning and continuous bandwidth (BW) tuning. A surface mount varactor connects the center conductors of the coaxial resonators. Tuning this varactor's capacitance changes the inter-resonator coupling between the resonators and tunes the BW. Another surface mount varactor on the coplanar waveguide feed line adjusts the external coupling at different BWs. Lumped varactors are also used to capacitively load the coaxial transmission line to tune the center frequency. Measured results show a BW range of 20–100 MHz from 0.8 GHz to 1.13 GHz and a BW range of 34–40 MHz from 0.55 GHz to 1.13 GHz with return loss better than 10 dB. Peak insertion loss of 1.07 dB is measured at 1.13 GHz with 100 MHz BW. Due to dispersive inter-resonator coupling, a transmission zero occurs above the passband that sharpens skirt selectivity. Fabrication is compatible with standard PCB technology with single-layer surface-mount lumped elements as tuners.