Bifunctional co-design of liquid crystal phase shifter and band-stop filter

In this paper, a new co-design approach for a bifunctional band-stop filtering tunable phase shifter is proposed. The bifunctional microwave component consists of a tunable liquid crystal phase shifter (LCPS) and a band-stop filter. It can simultaneously achieve tunable phase shift and self-embedded band-stop filtering functions. Particularly, this band-stop filtering LCPS is designed by a LC loaded coplanar waveguide (CPW), which is magnetically coupled with a series of split-ring resonators (SRRs). In this case, when the LC's permittivity is tuned by the external bias magnetic field, both the continuously tunable phase shift of the LC-CPW and the constant band-stop filtering function stemmed from the SRRs are achieved at the same time. The equivalent circuit model analyses and the transmission parameter characterization are conducted to comprehensively interpret the mechanism of the bifunctional microwave component. Then, a prototype sample is designed, fabricated, and tested to verify the proposed co-design method. According to the measured results, even though the LC material with large loss tangent is used, the proposed band-stop filtering tunable LCPS achieves tunable phase shifts with the FoM of more than 7.9 degrees dB(-1) in the passband (from 4.0 to 6.1 GHz and from 6.9 to 10.0 GHz) and a constant stopband from 6.2 to 6.8 GHz. In addition, the band-stop filtering LCPS loaded with better LC material (Merck GT-23001) with lower loss tangent is investigated based on numerical simulations. Results show that, in this case, the FoM of the band-stop filtering LCPS can reach up to the same level as that of the previously reported LCPS without the filtering function, which means that the high phase shift performance is maintained while the additional filtering function is embedded in the bifunctional component.