Note: Series and parallel tunable resonators based on a nematic liquid crystal cell as variable capacitance

Abstract

In this work, tunable series and parallel resonators based on a nematic liquid crystal cell as variable capacitance are proposed and characterized. Tunable resonance frequencies in the range of kHz have been obtained for the combination of the inductance and the liquid crystal cell (capacitance) used in the proposed circuits. From an electrical point of view, a LC cell behaves as. a voltage dependent capacitor. 4 • 5 Using the impedance spectroscopy technique, an electrical equivalent circuit (EEC) of the LC device can be obtained and capacitances in the order of nanofarads (nF) and picofarads (pF) have been reported in practical devices . 4 • 5 On the other hand, circuits including inductive and capacitive elements show the property of resonance, which can be employed to create very effective bandpass and band-stop filters .In this work, tunable series and parallel resonators based on a nematic liquid crystal (NLC) cell behaving as a variable capacitance have been implemented and experimentally characterized.The study was carried out with a monopixel NLC cell. The structure of a NLC cell basically consists of two parallel transparent plates or substrates with a conductive transparent layer (electrodes). Inner surfaces are also conditioned by specific alignment process to achieve a homogenous molecular alignment of the LC material. The glass plates are arranged so the molecules adjacent to the top electrode are oriented at a right angle to those at the bottom . The sample has a thickness of 1.65 µ,m and an effective area of 60 mm 2 . A commercial NLC Kl5 was used as tunable dielectric material. The NLC cell was placed in a Linkam LTS-E350 programmable hotstage to guarantee a stable temperature of 25 °C during the measurement process . Tunable series and parallel resonators were implemented, using NLC sample as variable capacitance. Impedance magnitude and phase were taken with a Solartron 1260 and voltage dependent resonance frequency was measured in both cases. a) Author to whom correspondence should be addressed. Electronic mail:jctzafra@ing.uc3m.es. b)Electronic mail : cmarcos@ing.uc3m.es. c) Electronic mail : jmpena @ing.uc3m.es . d)Electronic mail : isaper@ing.uc3m.es. e)Electroni c mail : vurruchi @ing.uc3m.es.Ln order to experimentally determine the capacitance dependence on the applied voltage, complex impedance measurements were carried out. A bias voltage from 1 V to 3.5 V was applied to the NLC cell. Variation on impedance magnitude and phase are displayed in Figure I.An EEC of the sample was deduced using these complex impedance measurements through a method previously reported. 3 From the experimental measurements of impedance depicted in Figure I it can be deduced that the NLC sample has an EEC consisting of an ideal capacitor (CLc) with series (Rs) and parallel (Rp) resistors (Figure 2). Component values of the EEC as a function of the applied voltage are shown in Table I.Tunable resonators proposed in this study are designed to operate in the frequency range from 1 kHz to 20 kHz, where complex im...

Temperature-Phase Converter Based on a LC Cell as a Variable Capacitance

“…In addition, the tunable properties of LC can also be used for non-optical applications. For instance, we showed in previous works the use of LC cells to implement electronic devices such as sinusoidal oscillators [ 11 ], phase-locked loops (PLLs) [ 12 ] or tunable series-parallel resonators [ 13 ].…”