Compact Liquid Crystal Based Tunable Band-Stop Filter with an Ultra-Wide Stopband by Using Wave Interference Technique

Longzhu Cai,Huan Xu,Daping Chu

doi:10.1155/2017/9670965

Longzhu Cai, Huan Xu + Show 1 more

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https://doi.org/10.1155/2017/9670965

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Abstract

A wave interference filtering section that consists of three stubs of different lengths, each with an individual stopband of its own central frequency, is reported here for the design of band-stop filters (BSFs) with ultra-wide and sharp stopbands as well as large attenuation characteristics. The superposition of the individual stopbands provides the coverage over an ultra-wide frequency range. Equations and guidelines are presented for the application of a new wave interference technique to adjust the rejection level and width of its stopband. Based on that, an electrically tunable ultra-wide stopband BSF using a liquid crystal (LC) material for ultra-wideband (UWB) applications is designed. Careful treatment of the bent stubs, including impedance matching of the main microstrip line and bent stubs together with that of the SMA connectors and impedance adaptors, was carried out for the compactness and minimum insertion and reflection losses. The experimental results of the fabricated device agree very well with that of the simulation. The centre rejection frequency as measured can be tuned between 4.434 and 4.814 GHz when a biased voltage of 0–20 Vrms is used. The 3 dB and 25 dB stopband bandwidths were 4.86 GHz and 2.51 GHz, respectively, which are larger than that of other recently reported LC based tunable BSFs.

Highlights

Band-stop filters (BSFs) with wide stopbands and compact structures are desirable in wireless communications because of their ability to suppress unwanted wideband noise and spurious signals [1]
In early 2002, the USA Federal Communications Commission (FCC) released the unlicensed frequency spectrum of ultra-wideband (UWB) 3.1–10.6 GHz for commercial use, which has led to the further development of wide stopband BSFs [2]
We demonstrate here an liquid crystal (LC) based tunable BSF with an inverted microstrip line (ML) structure and an ultra-wide and sharp stopband with large attenuation

Summary

Introduction

Band-stop filters (BSFs) with wide stopbands and compact structures are desirable in wireless communications because of their ability to suppress unwanted wideband noise and spurious signals [1]. In early 2002, the USA Federal Communications Commission (FCC) released the unlicensed frequency spectrum of ultra-wideband (UWB) 3.1–10.6 GHz for commercial use, which has led to the further development of wide stopband BSFs [2]. Due to increasing levels of complexity and demands for advanced communication systems, UWB technology has drawn more and more attention [4, 5]. The advantages of this technology consist of high resolution for ranging and geolocation and good resistance to small scale fading and narrowband interference [3, 6, 7]. In 2006, UWB transmission in Japan was allowed within the 3.1–4.8 and 6–10 GHz bands [3]

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