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

毫米波范围内的未来通信系统需要可重新配置和可调谐的RF组件。一个关键组件是具有可调中心频率和带宽的RF滤波器，为新服务和频带提供必要的灵活性。最有前途的是液晶（LC）带通滤波器，因为专门合成的LC和相应的基于LC的元件表现出准无功率调谐、相对低的介电损耗、高线性度和功率处理能力。因此，滤波器已经实现为具有非常高的品质因数，但是与非常有限的调谐范围相关联。如果通过使用更大的LC谐振腔来增加调谐范围，则由于损耗增加，品质因数将降低。在本项目中，将研究LC滤波器的新概念，该概念可以同时显著增加调谐范围和大幅降低插入损耗。代替使用具有可调通带和上述相关性的经典带通滤波器，使用具有两个独立可调阻带的多带阻滤波器，该两个独立可调阻带与不可调低通和高通滤波器级联，每个滤波器具有非常高的品质因数。由于这个概念，LC填充的谐振器在多带阻滤波器的阻带中的插入损耗不影响所得到的整体带通特性的通带。通带内的插入损耗仅由固定低通和高通滤波器的损耗决定。因此，谐振器可以被优化用于独立于其损耗的最大调谐范围，从而实现中心频率的相当大的调谐范围和带宽的极端可调谐性，这与材料的可调谐性相比要高得多。为了实现这种可变多带阻滤波器和一个固定的低通和高通滤波器的可调谐LC滤波器，适当的滤波器设计方法将被导出，实施和实验验证的实验室规模的示威者在Ka波段的金属空心波导拓扑结构。首先，适当的LC填充的波导谐振器将被调查，这是一个新的电极配置调谐。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，同时对不可调谐低通和高通滤波器进行了优化，使其在通带内的插入损耗尽可能低，从而得到带通特性。最后，将深入研究这些具有可调中心频率和带宽的波导演示器，以验证所提出的概念（概念验证），并评估毫米波范围内可调LC滤波器的能力和限制。

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.