Bandwidth and Center Frequency Reconfigurable Waveguide Filter Based on Liquid Crystal Technology

Abstract

This paper presents for the first time a fully electronically reconfigurable waveguide filter tunable in bandwidth and center frequency based on liquid crystal (LC) technology. A continuously reconfigurable two pole bandpass filter is designed and characterized in the Ka-band at $30 \,\mathrm{GHz}$. To be able to tune both center frequency and bandwidth independently, the resonators and coupling structures are filled with LC as tunable material. Hence, the filter’s center frequency and coupling strengths can be tuned and, furthermore, tuning with constant filter characteristic is possible. To tune the LC, a novel two-layer electrode design for waveguide structures is presented, which is simple to integrate and provides a high tuning efficiency with low insertion loss. By applying different bias configurations, the LC’s effective permittivity can be varied, and therefore, also the resonators’ electrical lengths. The presented two pole filter can adapt its center frequency from $29.8 \,\mathrm{GHz}\,{\rm to}\, 30.7 \,\mathrm{GHz}$ with a maximum $3 \,{\rm dB}$ bandwidth variation from $660 \,\mathrm{MHz}\,{\rm to}\, 870 \,\mathrm{MHz}$. The measurements are carried out with bias voltages up to $\pm$$250 \,\mathrm{V}$.

Keywords

• Microwave filter
• liquid crystals
• millimeter wave communication
• tunable circuits and devices
• K-band