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

• Applicant: Professor Dr.-Ing. Rolf Jakoby
• Subject Area: Communication Technology and Networks, High-Frequency Technology and Photonic Systems, Signal Processing and Machine Learning for Information Technology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 504169447

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.

• DFG Programme: Research Grants