In this paper, a wideband band-pass frequency tunable filter using liquid crystal is proposed for millimeter-wave applications, which achieves the band-pass characteristics by introducing four transmission zeros. The main filter structure is the ring resonator loaded with two quarter-wavelength open stubs using inverted microstrip structure. The ring resonator contacts directly with the liquid crystal layer to control the bias voltage to adjust the dielectric constant of the liquid crystal material. Experimental results show that the central frequency reconstruction range varies from 27.7GHz to 24.6GHz, and the corresponding relative bandwidth changes from 48.17% to 49.82%. The group delay within the pass-band is less than 0.45ns before and after the filter reconstruction, which indicates its applicability in K-band filter.
