基于液晶材料的毫米波滤波器宽带可调技术研究

The dynamic frequency selective characteristics in wideband frequency are required in special field such as millimeter wave radar, guidance and communication system, etc. The tunable filter not only can realize frequency hopping and frequency selective, but also can effectively reduce the size of the system. Through giving the power on the up and down electrode of liquid crystal and changing the voltage value, it changes the liquid crystal vector, which is to change the effective dielectric constant of the liquid crystal substrate, then the center frequency of tunable filter can realize dynamic selective frequency rapidly. The traditional liquid crystal filter is implemented by nematic crystal materials with coplanar waveguide structure, thus causing high insertion loss and relatively narrow frequency tuning range. In this project, it is planned that the mixed crystal materials with larger anisotropic is used to increase the variation of the effective dielectric constant of the liquid crystal, which combined with the broadband impedance matching technology to widen the tuning range of the filter; it uses the inverted microstrip line structure with high modulation efficiency, which can realize a larger tunable range, faster tuning speed and lower loss;Based on the equivalent circuit theory, the liquid crystal material model is researched, which influences the wideband tunable characteristics of millimeter-wave tunable filter. The research in this project can supplement the theory of fast design on the tunable filter. Finally, the millimeter-wave filter with wideband tunable center frequency is developed and completed.

特殊领域毫米波雷达、制导和通信等系统要求在宽频带范围内实现动态选频，采用可调滤波器不但可以实现动态选频，还可有效降低系统的尺寸。通过在液晶材料上下表面电极加电并改变电压值使得液晶指向矢发生变化即改变液晶基片的有效介电常数，以此调谐滤波器中心频率实现快速动态选频。传统的液晶调谐滤波器通常采用向列相单晶材料结合共面波导等电路结构，滤波器插入损耗较大、调谐范围较窄。本课题拟采用各向异性较大的混晶材料以增大液晶材料有效介电常数的可变化量，并结合宽带匹配技术增大滤波器的调谐范围；采用对液晶材料调制效率更高的倒置微带线结构实现快速宽带调谐、低损耗设计；再基于等效电路理论，研究液晶材料在毫米波频段的物理特性模型及影响液晶毫米波宽带可调滤波器性能的关键参量等效模型，为快速设计液晶材料宽带可调滤波器提供理论补充。最终完成基于该类材料中心频率宽带可调的毫米波滤波器研制。

随着电子技术的快速发展，目前通信、雷达、遥测遥感、电子对抗等军民用电子等系统要求在宽频带范围内实现动态选频，因此采用可调滤波器不但可以实现动态选频，还可有效降低系统的尺寸。针对毫米波可重构滤波器，本项目从液晶材料毫米波频段特性、毫米波可调滤波器设计与实现、可调滤波器建模三个方面，开展了适用于毫米波系统的新型高性能毫米波宽带可重构滤波器。具体内容包括：.1.首先在理论上分析推导了液晶材料介电各向异性，并利用微带线测试法得到了在毫米波低频段的电场调控下液晶的介电各向异性。实测结果表明，使用1KHz的低频方波信号，外加偏置电压的变化范围为：0V-20V，被测器件的工作频率范围为：23GHz-38GHz。测得液晶材料的等效相对介电常数变化范围为2.38-3.2，导电损耗角正切变化范围0.023-0.037。.2.其次采用倒置微带结构，利用金属过孔垂直过渡，解决了宽带可调平稳过渡以及施加偏置电压而不影响信号传输的问题，并采用梯形阻抗梯度线进行宽带阻抗匹配，设计了毫米波双模谐振结构频率可重构滤波器，实现滤波器的小型化。滤波器中心频率可重构滤波器具有两个极点，约3.1％的带宽(1.1GHz)，并且在液晶等效介电常数变化范围是2.5-4.5时，中心频率可调范围为34.4 GHz-35.3 GHz。.3.最后液晶材料毫米波可调滤波器的建模分析。采用矢量法对双频液晶指向矢进行求解，并采用数值解法对电场以及液晶分子场进行建模，利用MATLAB进行仿真模拟。建立了影响器件关键性能的模型，研究了各种影响因素与器件尺寸结构之间的关系，并分析了影响因素对宽带可调特性。