Novel continuously tunable and miniaturized passive "slow-wave" phase shifters with fast response time for millimeter wave applications based on a combined Liquid Crystal (LC) and Nanowire-filled Membrane (NaM) technology

新型液晶（LC）技术是达姆施塔特工业大学（TU Darmstadt）在过去十年中创新性地应用于光学器件之外的技术，由于LC损耗随频率降低，因此它似乎是毫米波可调谐元件最有前途的方法。在30 GHz时，它具有非常低的介电损耗（tand < 0.006）和高达27%的连续可调谐性，从而在所有调谐状态下，空心波导或微带线移相器的最大差分相移与最高插入损耗之比（FoM）分别约为200°/dB或110° /dB。虽然平面技术具有比中空波导拓扑结构更高的系统固有损耗，但它们是低轮廓的，并且可以利用类似于成熟的LCD技术的自动化制造技术来容易地实现。这使得即使对于小批量生产也能够低成本地制造更大的阵列。由于LC是电介质，其调谐功耗极低。然而，LC移相器面临两个关键参数：对于360°相移来说太大的长度，比天线元件下方的受限空间大得多，以及太慢的响应时间（通常> 1分钟），其中对于移动的应用中的高波束转向速率需要至少小于30 ms。为了克服这些问题，本项目旨在开发一种新技术，以满足所有要求，即高FoM，低插入损耗，快速响应时间，低功耗，高线性度，低成本，可靠性，重量轻，紧凑（小型化）和扁平化（低调）同时，在全球范围内首次合并两项独立开发的创新技术：液晶（LC）和纳米线填充膜（NaM）技术。组合的LC-NaM技术通常能够实现可调谐微波器件，例如利用慢波效应小型化至少三倍的移相器。作为这一新技术的概念验证，可调谐无源慢波微带线移相器将首次设计，实现和研究，在60 GHz的WPAN应用的典范。目标是仅在纳米线顶部和信号电极之间实现几μm的有效LC层厚度，从而保持平面微带拓扑的高性能，在60 GHz下FoM至少为70°/dB。与此同时，与偏置电极之间距离为50至150 µm的传统可调谐LC微带线相比，该器件的响应时间将从1分钟以上大幅缩短至30 ms。因此，利用该技术，实现新一代小型化移相器是可行的，该移相器在毫米波下具有快速响应时间和高FoM，具有非常低的功耗，并且可以容易地集成到大型波束控制天线阵列的每个天线元件中。

Novel Liquid Crystal (LC) technology, which has been innovatively adapted beyond optics during last decade by TU Darmstadt, appears to be the most promising approach for tunable components at millimeter waves, since LC losses decrease with frequency. At 30 GHz, it features very low dielectric losses of tand < 0.006 and continuous tunability up to 27 %, resulting in a Figure-of-Merit (FoM) defined by the ratio of the maximum differential phase shift over the highest insertion loss in all tuning states of about 200°/dB or 110° /dB for hollow waveguide or microstrip line phase shifters, respectively. Although planar technologies own system-inherently higher losses than hollow-waveguide topologies, they are low-profile and can easily be realized, utilizing automated manufacturing techniques similar to well-established LCD technology. This enables a low-cost fabrication of larger arrays even for a low-volume production. Since LC is a dielectric, its power consumption for tuning is extremely low. However, LC phase shifters face two critical parameters: too large length for 360°-phase shift, much more than for the restricted space beneath the antenna elements, and too slow response time (typically > 1 min), where at least less than 30 ms is required for high beam-steering rate in mobile applications. To overcome these problems, the proposed project aims for a new technology, enabling to satisfy all the addressed requirements, i.e. high FoM, low insertion loss, fast response time, low-power consumption, high linearity, low-cost, reliability, light-weight, compact (miniaturized) and flat (low-profile), simultaneously, by merging for the first time worldwide two independently developed innovative technologies: Liquid Crystal (LC) and Nanowire-filled Membrane (NaM) Technologies. The combined LC-NaM technology enables in general tunable microwave devices such as phase shifters miniaturized by factor of at least three, utilizing the slow-wave effect. As a proof-of-concept of this new technology, tunable passive slow-wave microstrip line phase shifters will be designed, realized and investigated for the first time, exemplary for WPAN application at 60 GHz. The objective is to achieve effective LClayer thickness of few µm only between the top of the nanowires and signal electrode, thus, keeping high performances for the planar microstrip topology with FoM of at least 70°/dB at 60 GHz. At the same time, the device's response time will be reduced dramatically from more than 1 min down to 30 ms compared to conventional tunable LC-based microstrip lines with 50 to 150 µm distances between the biasing electrodes. Thus, with this technology, it is feasible to realize a new generation of miniaturized phase shifters, having a fast response time and high FoM at millimeter waves with very low power consumption, and which could easily be integrated into each antenna element of a large beam-steering antenna array.