A Novel Microwave Tunable Band-Pass Filter Integrated Power Divider Based on Liquid Crystal

This paper proposes a novel microwave continuous adjustable band-pass filter integrated power divider based on nematic liquid crystals (LCs). The proposed power divider uses liquid crystal (LC) as the dielectric material. It can realize phase shift by changing the dielectric anisotropy, when biasing the high anisotropy nematic liquid crystal. It is mainly used inmicrowave frequencies. It has a large number of advantages compared to conventional filter integrated power divider, such as low loss, multifunction integration, continuous adjustable, miniaturization, low processing costs, low operating voltage, high phase shift, and convenient manufacture. Therefore, it has shown great potential for application.


Introduction
With the rapid development of modern wireless communication technology, it gives a higher request to the volume, performance, and integration of RF and microwave devices.And filter and power divider are two important passive components in the system.They sometimes take the form of cascade in the circuit.Therefore, with integration of two functions in one device, it is helpful to reduce connection loss and manufacturing cost and miniaturize the circuit.The current adjustable techniques mainly include diode switch, ferrite, ferroelectric substance, and RF MEMS.Yttrium iron garnet (YIG) has many advantages, but slow tuning speed and large volume are two of the biggest problems to limit its application [1].Most indices of barium strontium titanate (BST) are in the middle level.But it has a poor linearity [2].RF MEMS has better performances, but it has lesser tuning ratio and higher cost [3].The tuning speed of diode is fast.But it has a poor linearity as well [4].It is clear that the traditional microwave continuous adjustable filter and power divider fail to meet the developing requirements of technology.So it is the inevitable trend of development that the high performance microwave continuous adjustable band-pass filter integrated power divider is designed by new material, new technology, and new method [5].
Liquid crystal belongs to organic compounds.Its interior has long molecular structure.It is especially affected by the external environment.Especially in different concentration and temperature, it presents the orderly flow between liquid and solid.Thus, it has both the fluidity of liquid and the anisotropy of crystal.Liquid crystal is a fluid, whose configuration of director distribution or molecules is regular.Mainly, its optical properties, thermal properties, electrical properties, magnetic properties, and mechanical properties all present the crystal anisotropy.And these properties are closely related to liquid crystal molecules alignment.Also, liquid crystal is as liquid as common liquid.Although liquid crystal merges the main properties of crystal and liquid, it presents special magnetic and electric properties different from the ordinary crystal and liquid.Therefore, these properties of liquid crystal are widely used in different fields, particularly in the field of electromagnetic instruments.A lot of devices can be made of liquid crystal, such as liquid crystal sensitive print head, electronic aperture, phase diffraction grating, and spatial modulator.So, the applied research of liquid crystal has become a research focus in the new interdiscipline [6].
In this paper, a novel microwave continuous adjustable band-pass filter integrated power divider, based on nematic liquid crystals with better performance for microelectromechanical system application, has been presented, and an idea structure of the novel liquid crystal circuit is shown in Figure 1.

Filter Integrated Power Dividers Design
As shown in Figure 2, this paper mainly uses folded inverted microstrip line structure.The inverted microstrip line has many advantages, such as simple structure, low dielectric loss, easy processing, and convenience in connecting with other microwave devices.In this design, the electric field in inverted microstrip line makes the higher efficiency on liquid crystal relative to the ordinary microstrip line.
The mentioned power divider is designed by ROGERS5880 with 1-layer substrate.The substrate has a thickness of 0.508 mm with a relative permittivity of 2.2 and loss tangent of 0.0009.And its dimension is 31 × 42 mm 2 .This material has high dielectric consistency, low dielectric loss, and certain hardness.And it is mainly for resonators for power distribution of band-pass filter integrated power divider and input/output feed lines.Resonators for power distribution of band-pass filter integrated power divider

Electrode
Liquid crystals 1 mm thickness of slot on the upper surface of the metal substrate is used for carrying liquid crystals; that is, the thickness of LC is 0.1 mm.We use anisotropic conductive film (ACF) to connect the proposed ROGERS5880 substrate and the metal substrate for package stacking.Then we trickled the nematic liquid crystals into the slot by filling plug, using capillary attraction [7][8][9].As shown in Figure 3, when the nematic liquid crystals work without bias voltage ( = 0), the long axes of the liquid crystal molecules are parallel to each other and parallel to the ROGERS5880 substrate.At this point, the measured dielectric constant is  ⊥ .When applying an electric field  between the metallic ground plane and the inverted microstrip line with bias voltage ( =  max ), the orientation of liquid crystal molecules will gradually turn to the direction of the electric field.When the bias voltage is high enough and achieves the saturation voltage of liquid crystals, the polarization of the nematic liquid crystals reaches saturation.At this point, the measured dielectric constant is  // .And the dielectric constant stops changing over bias voltage  max .Thus, it realizes to modulate dielectric constant.For a certain physical size of the resonator, its electric length is also modulated by the external voltage.By this way, the resonant frequency of the proposed power divider changes accordingly.And finally it implements continuous tuning for band-pass [10,11].These optimization works are managed by using commercial 3D electromagnetic software HFSS.

Results and Discussion
To verify the model, a novel adjustable filter integrated power divider is simulated, fabricated, and tested at room temperature 25 ∘ C. The photograph of the fabricated device is shown in Figures 4 and 5.As the front content argues, Figure 6    shows that we trickled the nematic liquid crystals into the slot by filling plug, using capillary attraction.And Figure 7 shows the test environment.The peak voltage provided by function/arbitrary waveform generator, ranges from 0 to 20 V.And the corresponding waveform used for driving the LC material is 1 kHz square signal.

Figure 3 :
Figure 3: Working principle diagram of the nematic liquid crystals.

Figure 5 :
Figure 5: Photograph of the proposed novel filter integrated power divider.