In microwave liquid crystal devices, with regard to controlling the response time, the conventional method controls the rise time with the strength of the bias field, while the decay time is controlled by mechanical constraining forces produced by rubbing the polyimide films formed at both boundary surfaces of the liquid crystal layer, and the decay time increases in proportion to the square of the layer thickness. Therefore, with a liquid crystal layer thickness of approximately 100µm, the decay time ends up being approximately 1000 times greater than the rise time of the devices using plain liquid crystals.

In this paper, with the aim of controlling the decay time by a bias field in the same way as the rise time, thereby reducing the decay time, we propose a high-speed variable phase shifter where a coplanar waveguide with a floating electrode (CPW-FE) is filled with a plain liquid crystal without rubbing in an effort to control the orientation of the liquid crystal molecules by changing the direction of the bias field. Fig. (a) shows the case where the bias field is applied between the CPW center conductor and ground plane, and Fig. (b) shows the case where it is applied between the FE and CPW center conductor, the ground plane. In Fig. (a), the longitudinal axes of the liquid crystal molecules are aligned in the direction of the bias field, which more or less matches the direction of the high-frequency electric field in the dominant mode of the CPW device. We will use the effective permittivity of the liquid crystal when the longitudinal axes of the liquid crystal molecules match the direction of the high-frequency electric field in this way. Meanwhile, in Fig. (b), the direction of the bias field is perpendicular to the direction of the high-frequency electric field of the CPW device. We will use the effective permittivity when the longitudinal axes of the liquid crystal molecules are perpendicular to the direction of the high-frequency electric field. By switching between the bias voltage application methods shown in Fig. (a) and (b), it is possible to use bias field in two directions to control the liquid crystal molecules so that they are aligned parallel with or perpendicular to the high-frequency electric field of the CPW device.

In the 18GHz band variable phase shifter we developed, we obtained a rise time of 75 ms and a decay time of 200 ms with a 500µm wide CPW center conductor, a gap of 20µm between the center conductor and ground plane, and a liquid crystal layer thickness of 50µm. Under these conditions, the birefringence of the liquid crystal was 0.23.

Fig. Liquid crystal molecules'alignment in a CPW-FE device.