In this work, a compact design of an electrically tunable notch filter, based on liquid crystal (LC) technology, has been designed, manufactured, and characterized. The proposal has been achieved through particular configuration schemes with low cost inverted-microstrip structures and conventional spurlines structures due to its ease of integration. Central frequency tunability has been induced by applying low ac voltages, thus involving low power consumption. For these devices, filter responses have been approached specifically at microwave C-band frequency allocated for many satellite communications applications. Also, it has taken advantage of new highly anisotropic nematic LC mixtures at those frequency ranges. Recently, liquid crystal (LC) technology has begun to be used in nonoptical applications due to its promising features in further electronic applications ranging from kilohertz to megahertz frequencies. Intrinsic anisotropy of some LC properties, which implies different properties depending of the direction in which they are measured, allows new advanced devices to be designed with tunable features, by using these materials.The use of LC to design tunable devices at microwave frequency bands is not a new conception; nevertheless there has been an increasing interest in improving their performance, particularly in the last decade. Liquid crystals were recognized as candidates for microwave dielectric substrates in the early 1990s. 1 Although first approaches to LC devices in waveguides led to bulky and large consumption designs (due to strong magnetic fields for switching LC molecules), most of the recent prototypes, such as tunable phase shifters, 2, 3 capacitors, 4 filters, 5,6 or antennas, 7 have reported practical functions involving electric fields for orienting LC at those frequencies.Filters are very valuable devices because they represent a powerful tool for frequency response processing. They are designed in order to select or to remove bands of frequency. A band-rejection filter or band-stop filter is a filter that attenuates a frequency band, while the other frequencies remain unchanged. A notch filter is a band-rejection filter with a narrow stopband, with a high quality factor, that is mainly used to remove spurious frequencies and to filter noise signals.In this work, the design of a notch filter for about 5 GHz rejection frequency (f 0 ), based on the electric field switching of a LC, has been proposed and its feasibility has been demonstrated. Filters working at this frequency, allocated in the C-band, are intended to be used in satellite telecommunication systems or, for example, to avoid the interference a) Author to whom correspondence should be addressed. Electronic mail:vurruchi@ing.uc3m.es.between the UWB (Ultra Wide-Band) and WLAN (Wireless Local Area Networks) systems. 8 UWB systems are particularly promising for short-range high-throughput wireless communications. Multiple solutions have been proposed for providing tunability in microwave devices, such as varactors based on sem...
ABSTRACT:In this article, the design, fabrication, and characteriza-tion of a tunable microwave notch filter based on liquid crystal (LC) using inverted-microstrip technology is presented. A spiral spurline structure is used because of its good performance as a single-resonator notch filter. Based on the LC dielectric anisotropy, a voltage-controlled rejection frequency of the filter is achieved, ranging from 3.40 to 3.75 GHz, which means that the tuning range relative to the central rejection frequency is about 10%. At the same time, this device exhibits negative group delay around the rejection frequency and the measured values throughout the tuning frequency range are presented.
A new temperature-frequency converter based on the variation of the dielectric permittivity of the Liquid Crystal (LC) material with temperature has been demonstrated. Unlike other temperature sensors based on liquid crystal processing optical signals for determining the temperature, this work presents a system that is able to sense temperature by using only electrical signals. The variation of the dielectric permittivity with temperature is used to modify the capacitance of a plain capacitor using a LC material as non-ideal dielectric. An electric oscillator with an output frequency depending on variable capacitance made of a twisted-nematic (TN) liquid crystal (LC) cell has been built. The output frequency is related to the temperature of LC cell through the equations associated to the oscillator circuit. The experimental results show excellent temperature sensitivity, with a variation of 0.40% of the initial frequency per degree Celsius in the temperature range from −6 °C to 110 °C.
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...
Abstract. We present a microcontrolled optoelectronic system to measure online the average velocity of a projectile impacting on aircraft and spacecraft structures. The projectile velocity can vary in the range from subsonic to supersonic. The implemented optical system is based on three optical barriers that are crossed by the projectiles before impacting on the structural elements. A simple optoelectronic prototype is constructed and tested. The flight times among the three optical barriers are obtained by the microcontroller system. The measured velocity accuracy in test conditions is better than 1%. A high-brightness liquid crystal display is used to show messages of the system configuration and also the measured projectile velocity. To ensure safety in the experimental impact tests, serial RS-232 communication can be used to monitor this information using a remote computer. The system is highly robust since it is able to measure projectile velocity even when an optical barrier is missed by failure in either the optical transmitter or the receptor.
In this work, a novel equivalent electric circuit for modeling liquid crystal microlenses is proposed. This model is focused on explaining a lens behavior at the micrometric scale, using its manufacturing parameters. It suggests an approach to predict the solution of the voltage gradient distribution across a microlens. An interesting feature of the model is that it provides an analytical solution for microlenses with modal and hole-patterned electrode schemes, by a simple software tool. The model flexibility allows lens designers to apply complex waveform signals with different harmonics. The voltage distribution has been tested. The simulated and measured voltage profiles are fairly in agreement. The design of new liquid crystal (LC) microlenses is involving currently important research resources. Prototypes based on this technology take advantage of the tunable electrooptical properties of these materials mainly for the focal length tunability. This distinctive feature together with a reduced weight and volume of the devices, compared to other technologies, makes them good candidates for a wide range of applications. Cell phone cameras, auto-focusing, 2D/3D switchable displays, tunable photonic devices, or devices in optical communication systems are potential applications for providing final added value. In order to simplify the structures and to decrease the device control voltage, many topologies have been researched. Furthermore, the capacity of manufacturing LC microlenses is limited at the micrometric scale with the constraint of lithographic resolution. Some approaches, such as Fresnel zone method, with arrangements of multiple electrodes, complicate the final design. Other strategies consist of creating a lens effect by shaping the electric field generated with a hole-patterned electrode structure. In holepatterned electrode structures, the phase gradient is generated by the fringe field as the diameter/thickness ratio is less than 2-3. 1 Fig. 1(a) shows the design of a cylindrical microlens following this approach; a top view of its hole-patterned electrode is depicted in the upper part. The spreading of the electric field lines, yielded from the teeth of the comb electrode to the center, creates a gradient of the LC director profile that simulates a lens response. Nevertheless, this approach does not work properly as the lens aperture increases in size, because of the electric field vanishes gradually without reaching the entire area of the hole. An option to mitigate this problem is to exploit the modal control technique. This method consists in adding a layer of high sheet resistance in areas which are free of the comb electrode. Fig. 1(b) illustrates a modal cylindrical microlens; in this arrangement, an electric field gradient is generated across the high resistivity layer. a) Author to whom correspondence should be addressed. Electronic mail:vurruchi@ing.uc3m.es. This inclusion can be a possible solution to avoid the use of several electrodes in lenses with wider apertures. On the contrary, mod...
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...
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