Concept of frequency-agile multi-bandstop filters for low-loss liquid crystal filters with large tuning ranges of the center frequency and bandwidth

未来毫米波范围的通信系统需要可重新配置和可调谐的射频组件。一个关键部件是中心频率和带宽可调的RF滤波器，为新服务和频段提供必要的灵活性。最有希望的是液晶(LC)带通滤波器，因为专门合成的LC和相应的基于LC的元件具有准无功率调谐、相对较低的介质损耗、高线性度和功率处理能力。因此，实现的滤波器具有非常高的品质因数，但与非常有限的调谐范围相关。如果使用更大的LC腔来增大调谐范围，则会由于损耗的增加而降低品质因数。在本项目中，我们将研究LC滤波器的新概念，它可以同时显著地增加调谐范围和显著降低插入损耗。不是使用具有可调通带和上述相关性的传统带通滤波器，而是使用具有两个独立可调阻带的多带阻抗滤波器，其中两个阻带与不可调低通和高通滤波器级联，每个阻带具有非常高的质量因数。由于这一概念，在多带阻抗滤波器的阻带中填充LC的谐振器的插入损耗不影响由此产生的总体带通特性的通带。通带内的插入损耗仅由固定的低通和高通滤波器的损耗决定。因此，谐振器可以进行优化，以获得与其损耗无关的最大调谐范围，从而实现相当大的中心频率调谐范围和极高的带宽可调谐性，这比材料的可调性高得多。为了实现这种具有可变多带阻滤波器和固定低通和高通滤波器的LC可调谐滤波器，我们将推导出合适的滤波器设计方法，并在Ka波段金属中空波导拓扑结构中加以实现和实验验证。首先，将研究合适的填充LC的波导谐振器，它由一种新的电极结构来调谐。然后，对可调谐LC带阻滤波器进行了综合，并在此基础上设计了具有最大调谐范围和可调谐性的多带阻滤波器，同时对不可调谐的低通和高通滤波器进行了优化，使所得到的带通特性的通带内插入损耗尽可能低。最后，将深入研究这些中心频率和带宽可调的波导演示器，以验证所提出的概念(概念验证)，并评估可调谐LC滤波器在毫米波范围内的能力和极限。

Future communication systems in the millimeter wave range require RF components, which are reconfigurable and tunable. A key component is the RF filter with tunable center frequency and bandwidth, providing the necessary flexibility for new services and frequency bands. Most promising are Liquid Crystal (LC) bandpass filters, since specifically synthesized LCs and accordingly LC-based components exhibit quasi-powerless tuning, relatively low dielectric losses, high linearity and power handling capability. Thus, filters have been realized with a very high-quality factor, but associated with a very limited tuning range. If the tuning range is increased by using a larger LC cavity, the quality factor will be reduced due to increasing losses.In this project, a new concept for LC filters will be investigated, which enables simultaneously, a significantly increased tuning range and drastically decreased insertion losses. Instead of using a classical bandpass filter with a tunable passband and the above correlation, use is made of a multi-bandstop filter with two independently tunable stopbands concatenated with a non-tunable low-pass and high-pass filter, each with very high-quality factor. Due to this concept, the insertion losses of the LC-filled resonators in the stopbands of the multi-bandstop filter does not affect the passband of the resulting overall bandpass characteristic. The insertion losses within the passband are determined by the losses of the fixed low-pass and high-pass filters only. Therefore, the resonators can be optimized for maximum tuning range independent of its losses, enabling quite large tuning range for the center frequency and extreme tunability of the bandwidth, which is much higher as the material’s tunability. For to realize this tunable LC filter with variable multi-bandstop filter and a fixed low-pass and high-pass filter, appropriate filter-design methods will be derived, implemented and experimentally verified by lab-scale demonstrators in metallic hollow waveguide topology at Ka-band. First, appropriate LC-filled waveguide resonators will be investigated, which are tuned by a novel electrode configuration. Then, a synthesis will be carried out for tunable LC-bandstop filter, and based on it, the design of the multi-bandstop filter with maximum tuning range and tunability, while the non-tunable low-pass and a high-pass filters will be optimized in terms of lowest insertion losses possible for the passband of the resulting bandpass characteristic. Finally, these waveguide demonstrators with tunable center frequency and bandwidth will be investigated in depth to validate the proposed concept (Proof-of-Concept) and to evaluate the capability and limits of tunable LC filters in the millimeter wave range.