Improving the Pass-Band Return Loss in Liquid Crystal Dual-Mode Bandpass Filters by Microstrip Patch Reshaping.

Javier Torrecilla,Daniel Segovia,Noureddine Bennis,Virginia Urruchi,José Sánchez-Pena,Alejandro García

doi:10.3390/ma7064524

Abstract

In this paper, the design and experimental characterization of a tunable microstrip bandpass filter based on liquid crystal technology are presented. A reshaped microstrip dual-mode filter structure has been used in order to improve the device performance. Specifically, the aim is to increase the pass-band return loss of the filter by narrowing the filter bandwidth. Simulations confirm the improvement of using this new structure, achieving a pass-band return loss increase of 1.5 dB at least. Because of the anisotropic properties of LC molecules, a filter central frequency shift from 4.688 GHz to 5.045 GHz, which means a relative tuning range of 7.3%, is measured when an external AC voltage from 0 Vrms to 15 Vrms is applied to the device.

Highlights

During the last two decades, liquid crystals (LC) have become a promising approach for the design of tunable devices at microwave frequencies
The results of the patch dimensions for the topology presented in Figure 1a is solved for a filter central frequency nearby 7.5 GHz considering empty the LC cavity
LC molecules are aligned parallel to the microstrip line by rubbing a thin film of polyimide which acts as an alignment layer

Summary

Introduction

During the last two decades, liquid crystals (LC) have become a promising approach for the design of tunable devices at microwave frequencies. The LC permittivity can be varied between two extreme values, εr⊥ and εr‖, as a function of the applied voltage This is the reason which permits LC-based devices to be voltage-controlled. Some examples of tunable devices based on LC technology at microwave frequencies in the bibliography are capacitors [1], antennas [2], filters [3], etc. The advantages of using LC in size, cost or power consumption [4] compared to other studied technologies for designing tunable microwave devices, make these materials very suitable for this purpose. A tunable LC-based band-pass filter with microstrip geometry is presented. A reshaping of a dual-mode square patch geometry reported in previous works has been proposed as the microstrip filter This new patch structure is expected to improve the filter performance, increasing the pass-band return loss. The design, manufacture and experimental results of the device are detailed in this paper

Liquid Crystal Bandpass Filter Design

Manufacturing of the Device

Experimental Results and Discussion

Conclusions