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...
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