Note: Phase-locked loop with a voltage controlled oscillator based on a liquid crystal cell as variable capacitance

Abstract

A phase-locked loop is demonstrated using a twisted-nematic liquid crystal cell as a capacitance that can be varied as a function of applied voltage. The system is formed by a phase detector, a low-pass filter, as well as a voltage controlled oscillator including such variable capacitance. A theoretical study is proposed and experimentally validated. Capture and locked ranges of hundreds of kHz have been obtained for the configuration used in this circuit. An application as frequency demodulator using a practical implementation of this circuit has been demonstrated. A phase-locked loop (PLL) is a negative-feedback circuit that allows synchronizing the output signal of an oscillator with a reference signal in frequency and phase. 1PLLs can be constructed from subcircuits or purchased as medium scale integration devices. There are basically three classes of PLLs: the linear or analog PLL, the digital PLL, and the all-digital PLL. A PLL generally consists of three basic blocks:2 phase comparator, a low-pass filter, and a voltage-controlled oscillator or as more commonly known, a VCO as shown in schematic form in Figure 1.Liquid crystals (LCs) are composed of moderate size organic molecules, which tend to be elongated. Because of their elongated shape, under appropriate conditions the molecules can exhibit orientational order, such that all the axes line up in a particular direction. A direct consequence of this ordering is the anisotropy of mechanical, electric, magnetic, and optical properties.3 In order to build practical devices, liquid crystals are sandwiched between two electrodes showing from an electrical point of view a plane capacitor behavior with a nonideal dielectric. Liquid crystals are very sensitive to an electric field, 4 and it is precisely this property the most traditionally used for their application to optical device technology. In this way, applications such as displays, optical modulators, variable optical attenuators, spatial light modulators, and optical multiplexers have been widely reported. 5-8However, only a few applications for non-optical applications have been described until now and mainly related to microwave region. 9-12As it is well known, the capacitance value for a plane capacitor is given by the following expression:where ε LC is the effective dielectric permittivity of the LC material, which depends on the applied rms voltage across to a) Authors to whom correspondence should be addressed. Electronic addresses: cmarcos@ing.uc3m. es, jmpena@ing.uc3m.es, jctzafra@ing. uc3m.es, isaper@ing.uc3m.es, and vurruchi@ing.uc3m.es. the device, S is the effective area of the electrodes, and d is the thickness of the device.In this work, we study the static and dynamic operation of a PLL structure implemented based on a VCO made of a LC nematic cell. Lock and capture ranges have been measured for this circuit, and its right operation as frequency demodulator in the range from 135 kHz to 230 kHz has also been demonstrated.Various circuits can be used to build a phase detector. The one w...

A novel tunable microwave filter integrated power dividers using liquid crystal

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“…The electric field causes a change in the dielectric constant of the wavelength changes, with respect to a specific wavelength of the resonator. If wavelength changes, the resonant frequency changes [8][9][10][11][12].…”

Temperature-Frequency Converter Using a Liquid Crystal Cell as a Sensing Element

“…However, in the last decade new LC-based applications are focusing the interest of the research community [1,2]. New LC materials with ad-hoc anisotropic properties have been synthesized and innovative non-optic applications such as passive tunable radio frequency (RF) and microwave devices have been fabricated and reported [3–6]. …”

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

“…1 Recently, however, this trend has shifted, and new non-optical applications for LC materials have been reported.~· 3 From an electrical point of view, a LC cell behaves as. a voltage dependent capacitor.…”

“…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.…”