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Concepts for and Synthesis of Liquid Crystal-Tunable Ka-band Filters with Independently Tunable Center Frequency and Bandwidth

Subject Area Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 424862129
 
Final Report Year 2024

Final Report Abstract

Within the framework of this project three different microwave filters based on liquid crystal (LC) were simulated, manufactured and measured. First, a waveguide filter at 30 GHz was designed and manufactured. The second-order filter uses additional resonators to enable tunability of the bandwidth. Therefore, each cavity contains a Rexolite container filled with LC. Thin electrodes are attached above and below the cavities, which can generate an electric field that influences the orientation of the rod-like liquid crystal molecules. Herewith, the effective permittivity of the LCs changes. By selecting an orthogonal or parallel external electric field, the electric length of the resonator and thus the resonant frequency can be varied. The 3dB bandwidth of the filter is continuously controllable from 655 to 872 MHz, which corresponds to a tunability of 24 %. The center frequency can be varied by 3 %. The modular design, which is necessary due to the required electrodes, causes relatively high insertion losses of 3.5 to 4.2 dB, depending on the selected transmission state. Therefore, another filter with identical specifications was designed and measured using “groove gap waveguide” (GGW) technology. The advantage is that the side walls consist of several rows of pins that do not have to be in contact with the cover. This simplifies the implementation of the electrodes in particular. Furthermore, a novel hybrid control concept was developed and submitted as a patent application. The LC cavities can be influenced by a superimposed electric and magnetic field with relatively low implementation effort for the control system with a high degree of controllability. Two permanent magnets generate an orthogonal magnetic field, while the electrode, which is still positioned above the Rexolite cavity filled with LC, generates a parallel electric field. This makes it possible to continuously align the liquid crystal molecules between the orthogonal and parallel states by varying the electric field strength. This improved control reduces the voltage required in the electrodes to Vpp,max = 100 V. The center frequency of the filter can again be continuously tuned by 3 %. The bandwidth tunability is approx. 6.5 %. The measures described enabled the insertion losses of the demonstrator to be significantly reduced to between 1.65 and 1.95 dB. This corresponds to an unloaded quality factor of Qu ≈ 300. Finally, another GGW filter with hybrid control was designed. In this filter, the filter order was increased to three, an oversized cavity was used to create a transmission zero (TZ) and a higher-order mode was used in the passband. The TE301 mode and the TE102 mode resonate at different frequencies in the same oversized cavity, creating this TZ. It is used to increase the slope on one side of the passband. In direct comparison to the previous demonstrator, the filter has a similar tunability, but offers a steeper slope due to the higher filter order and the TZ.

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