Transistor RC Oscillator with Single-Element Tuning

RC oscillators based on two active phase shifters of the all-pass type prove to be tunable over a 10-to-1 frequency range by varying a single resistance. Harmonic distortion can be less than 1 percent over this range. Analysis is made of the variation of Q with tuning and the dependence of phase shift on detailed circuit parameters. Oscillators of this type can be used in telemetry to generate a subcarrier frequency which is a measure of transducer resistance. With temperature compensation, oscillator frequency stability is sufficient to permit the measurement of temperature to ±0.1°C over the range -5°C to 40°C using a thermistor sensor.     

Thermistor Bead Matching for Temperature Compensated RF Power Thermistor Mounts

The temperature compensated thermistor-power meter of the dual bridge design has for many years been the most widely used instrument for microwave power measurement. During these many years very little if anything has been written with regard to the manner or criteria for matching the thermistor beads. This paper attempts to fill that void. The procedure to match thermistor beads for less than 2 ?W/°C drift is to be accomplished by pairing the dR/dt characteristic of each bead and adjusting the dP/dR characteristic for a match. dR/dt of the beads may be determined from resistance data at 90°C and 110°C, and the expression dR/dt = 9.4497 [ln Ra-1.11648 ln Rb + 0.61749] which is derived for 200 ? operating resistance thermistors. The thermistor beads are then paired by selecting such that dR1/dt and dR2/dt are within 1 percent of each other. The conditions necessary for matched dP/dR of the thermistor beads are developed and procedures indicated. The paired beads are installed in the thermistor mount and the constant dP/dR of each bead adjusted for equality by moving the heat sink of one of the thermistors until a balance of the slave bridge and master bridge is obtained simultaneously.     

Conversion of Fluid Thermal Parameters into Frequency and Time by Means of Thermistors

A versatile direct converter of fluid thermal parameters into frequency and time is described. The circuit is based on a complementary multivibrator, the frequency of which is controlled by the temperature-sensitive current generator. Temperature-to-time (?-T) and temperature-to-frequency (?-f) conversion is obtained by simple rearranging of passive circuit elements. The linearizing conditions and sensitivity for various probe configurations are given. In battery telemetering systems, the circuit is economical in power supply consumption due to the very small mark-to-space ratio of output pulses. The sensitivity as well as the transducer Rmax/Rmin ratio are considerably increased in comparison with other direct thermistor converters. With the positive temperature coefficient (PTC) thermistor probe, the frequency change can be about three decades in the temperature range of 30°C, which represents the highest obtained sensitivity of ?-T/f converters. The circuit is also successfully applied with self-heated miniature bead thermistors as a sensor when it represents the linearized air speed-to-time converter within the range of 0-3.6 m/s.     

Linearizing the Self-Heated Thermistor Thermometer

A numerical method of linearizing a dc thermistor thermometer in which the power dissipated by the thermistor is sufficient to heat itself to a temperature significantly above ambient is developed and tested. It is shown that if the dissipation constant of the thermistor remains fixed, then the thermometer is able to detect temperature fluctuations considerably smaller than the internal temperature rise above ambient of the thermistor itself.     

On the Maintenance of Saturated Standard Cells at 3°C through the Use of Thermoelectric Cooling

Saturated standard cells at temperatures between 20°C and 40°C exhibit a high negative-voltage coefficient with respect to temperature and have required temperature stabilization within ±0.01°C over an extended period to achieve a voltage stability of ±1 ?V. Examination of standard cell data in the vicinity of 0°C, on the other hand, indicated higher voltages and a much lower temperature coefficient. In fact, the cell voltage reaches a maximum and consequently the voltage coefficient is zero at approximately 3°C, and is near zero over a range of several degrees. A cell maintenance temperature of 3°C, therefore, appears the most appropriate operating environment for a saturated cell in order to obtain maximum voltage stability. Heretofore, a convenient, high-stability, maintenance-free temperature bath operating near 3°C has not been available for laboratory use. The utilization of thermoelectricity to cool (Peltier effect) is currently available, and this method has been employed to provide a stable low-temperature ambient for saturated cells. This paper describes a practical air chamber-controller combination with thermoelectric heat pumping designed to test the behavior of saturated cells near 3°C. Details of the thermal and electrical characteristics of the unit will be discussed. Results show a ±0.2°C control is adequate to provide a primary voltage reference with ±1 ?V stability at 3.1°C. Advantages and precautions in maintaining cells at low temperatures will also be presented.     

A Log-Antilog Analog Control Circuit for Constant-Power Warm-Thermistor Sensors— Application to Plant Water Status Measurement

Warm-thermistor sensors are useful for making measurements of flow and the thermal properties of materials. This paper describes a log-antilog analog control circuit capable of monitoring a thermistor's internal temperature in both ambient temperature sensing mode and self-heating mode, while maintaining infinite precision, wide dynamic range and fast sensor response. A single self-referencing thermistor is used to avoid the problems associated with having to match a thermistor pair in order to obtain an over-temperature signal. Instead, thermistor power levels are switched over a range of 1:1000, allowing measurement of both ambient temperature and heated thermistor temperature during the rise and fall of the heat pulse by a single thermistor at the center of the heat field. This provides a sensitive measurement of the thermal diffusivity and advective properties of the surrounding medium. In this paper, such a sensor is used to investigate the thermal properties of a grapevine cane under conditions of irrigation stress.      

热敏电阻在液体火箭发动机试验中的应用

阐述了目前应用于低温推进剂液体火箭发动机试验的热敏电阻温度传感器的特点，介绍了热敏电阻温度传感器在液体火箭发动机试验中的应用方案，包括信号变换器设计、传感器校准数据处理方法、传感器自热效应分析以及测量系统组成，分析了影响测量不确定度的因素，给出了应用热敏电阻温度传感器的测量不确定度小于0．3K。     

用于滚筒流道温度测量的热敏电阻的实验研究

滚筒流道温度的准确计量是沥青拌和站自动控制的关键技术,以热敏电阻为检测元件,基于嵌入式控制核心开发了滚筒流道温度实时测量系统。构建了热敏电阻非线性温度补偿模型,基于粒子群算法对热敏电阻的温度特性进行了非线性补偿,消除了测量温度的非线性误差。分别针对红外、热电阻和热敏电阻对滚筒流道温度进行了对比实验研究。现场实验结果表明,基于热敏电阻的温度测量系统能减小粉尘和热惯性的影响,满足滚筒流道温度实时智能控制的需求,提高沥青拌和料的性能。     

High-stability erbium-doped photonic crystal fiber source

A single-pass backward configuration superfluorescent fiber source (SFS) based on erbium-doped photonic crystal fiber (EDPCF) with a high mean wavelength stability was proposed. The EDPCF was used to improve the intrinsic temperature dependence of the SFS. Using the optimal EDPCF length of 24.2 m and pump power of 204 mW, a 20.7 ppm mean wavelength stability of a prototype SFS was demonstrated with increased temperature from -40 °C to 60 °C. The mean wavelength had an ultra stability of 10.3 ppm with increased temperature from -20 °C to 60 °C.     

ZrO2 thin-film-based sapphire fiber temperature sensor

A submicrometer-thick zirconium dioxide film was deposited on the tip of a polished C-plane sapphire fiber to fabricate a temperature sensor that can work to an extended temperature range. Zirconium dioxide was selected as the thin film material to fabricate the temperature sensor because it has relatively close thermal expansion to that of sapphire, but more importantly it does not react appreciably with sapphire up to 1800 °C. In order to study the properties of the deposited thin film, ZrO2 was also deposited on C-plane sapphire substrates and characterized by x-ray diffraction for phase analysis as well as by atomic force microscopy for analysis of surface morphology. Using low-coherence optical interferometry, the fabricated thin-film-based sapphire fiber sensor was tested in the lab up to 1200 °C and calibrated from 200° to 1000 °C. The temperature resolution is determined to be 5.8 °C when using an Ocean Optics USB4000 spectrometer to detect the reflection spectra from the ZrO2 thin-film temperature sensor.     

Simultaneous strain and temperature measurement using a compact photonic crystal fiber inter-modal interferometer and a fiber Bragg grating

An all-fiber sensor scheme for simultaneous strain and temperature measurement is presented. The sensing head is formed by serially connecting a polarization maintaining photonic-crystal-fiber-based inter-modal interferometer (IMI) with a fiber Bragg grating (FBG). The IMI, exhibiting an opposite strain response as compared to that of the FBG, is highly sensitive to strain, while it is insensitive to temperature. This has potential for improving the strain and temperature measurement resolutions. A sensor resolution of ±8.3 με in strain and ±2 °C in temperature are experimentally achieved within a strain range of 0-957.6 με and a temperature range of 24 °C-64 °C, respectively.     

A digital anemometer

A digital anemometer has been developed for air flow measurements in home and office environments. The principle is based on hot-wire anemometry. The probe consists of a cold thermistor for flow temperature detection and a hot thermistor for flow rate detection. The latter is self-heated by a thermal bridge. Its unbalance voltage due to the air flow is compared with the reference response transformed into the time domain by direct digital synthesizing. This pulse-width modulation provides the linear digital representation of the flow rate under measurement. The unbalance component due to flow temperature is compensated by scaling the reference response depending on temperature detected by a cold thermistor. These linearization and compensation techniques make accurate measurements possible with a simple configuration. Performances of a prototype anemometer built using a one-chip 4-bit microcomputer are also presented to demonstrate the validity of these techniques     

A widely linear temperature to frequency converter using athermistor in a pulse generator

A modification of an existing technique for thermistor linearization is presented. A temperature-measuring circuit using a thermistor in a specially designed pulse generator is used to obtain a highly linear relation between absolute temperature and frequency. The use of a general-purpose operational amplifier and two switching transistors in the design of the pulse generator provides a temperature scale that is linear over a wide range of values on a high degree of sensitivity. The result shows that an extremely high degree of linearity is achieved by this method     

Fiber-optic temperature sensor based on interaction of temperature-dependent refractive index and absorption of germanium film

The interaction of a large temperature-dependent refractive index and a temperature-dependent absorption of semiconductor materials at 1550 nm can be used to build a very sensitive, film coated fiber-optic temperature probe. We developed a sensor model for the optical fiber-germanium film sensor. A temperature sensitivity of reflectivity change of 0.0012/°C, corresponding to 0.1°C considering a moderate signal processing system, over 100°C within the temperature regime of -20°C to 120°C, has been demonstrated by experimental tests of the novel sensor. The potential sensitivity and further applications of the sensor are discussed.     

Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications

This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K(2) and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.     

Discrimination between strain and temperature by cascading single-mode thin-core diameter fibers

A simple fiber-optic sensor capable of discrimination between temperature and strain is proposed and experimentally demonstrated. The sensor head is formed by cascading two sections of single-mode thin-core diameter fibers (TCFs) that act as two different inter-modal interferometers (IMIs). Due to the different sensitivity responses of the two IMIs to strain and temperature, it is possible to discriminate temperature and strain by monitoring the resonant wavelength shifts. The experimental results indicate that the measured strain and temperature resolutions are 37.41 με and 0.732 °C within a strain range of 0-1333.3 με and a temperature range from 26.9 °C to 61.7 °C. The sensing sensitivities of strain and temperature are -1.03 pm/με and 30.74 pm/°C, respectively. The proposed sensor features the advantages of easy fabrication, low cost and high sensitivity, and it exhibits great potential in dual-parameter measurement.     

Thermistor Multivibrator as the Temperature-to-Frequency Converter and as a Bridge for Temperature Measurement

The symmetrical multivibrator, one timing resistor of which is replaced by a network consisting of a thermistor and passive resistors, is analyzed. The circuit represents at the same time a linear temperature-to-frequency (T-f) converter and a multivibrator bridge (MB), both having the same linearity range at any thermistor network configuration. When the balancing state is reached at an inflection point, the sensitivities of the converter and the bridge are the same. The MB has higher sensitivity than the Wheatstone bridge (WB) and can be balanced independently of the inflection point in the wide range. The converter and bridge performances with practical thermistor transducers are analyzed theoretically and numerically.     

Thermal and stress studies of normal incidence Mo/B4C multilayers for a 6.7 nm wavelength

Wavelength, reflectance, and stress stability of Mo/B(4)C multilayers were studied as a function of postdeposition annealing up to 900 °C. These multilayers are of interest as normal incidence coatings for wavelengths above the boron K-absorption edge. Mo/B(4)C multilayers deposited at low sputtering pressure have high compressive stress. Zero stress can be achieved at 360 °C-370 °C, but annealing at <200 °C is sufficient to reduce stress by ～40%. This stress relaxation is accompanied with a multilayer period expansion of ～0.02 nm and a <0.5% decrease in normal incidence reflectivity. The multilayer period remains stable up to ～600 °C, while intrinsic stress changes from compressive to tensile. A four-layer model with amorphous molybdenum and boron carbide layers separated by amorphous layers of molybdenum borides (Mo(x)B(y)) is presented. These interlayers are present already in the as-deposited state and continue to grow with increasing temperature. Their presence lowers the optical contrast and the achievable reflectivity. However, they also increase multilayer thermal stability. At temperatures >600 °C, a noticeable decrease in reflectivity associated with the phase transition from amorphous to crystalline molybdenum boride is observed. This is accompanied with an increase in interface and surface roughness and a change in stress as a function of temperature.     

Digitally Controlled Absolute Voltage Division

In this paper, a principle for absolute voltage division is presented. The division ratio of a voltage divider on this principle does not depend on the values of its elements but depends exclusively on the configuration of the divider network. Hence, calibration is not necessary and not even possible. Absolute voltage division is obtained by cyclically shifting the network elements along all positions in the divider network. Each position is maintained for an equally long time interval. The average output voltage of such a dynamic divider is identical to that of a static divider with the same network configuration but composed of elements which all have the same value. To verify the principle in practice, a digitally controlled resistive voltage divider has been built. It has been realized with easily available electronic components, such as carbon resistors with ± 5-percent tolerance, junction field-effect transistors as electronic switches and digital integrated circuits for the generation of the switch drive signals. The inaccuracy of this divider is less than 5 ×10-6 and the temperature coefficient of the division ratio is less than 5 × 10-8/°C from 0°C to 40°C. It is expected that the performance, of this prototype can be improved.     

A Novel Approach to Modeling and Simulation of NTC Thick-Film Segmented Thermistors for Sensor Applications

This paper describes the design, modeling, simulation, and fabrication of thick-film segmented thermistors. These thermistors were printed on alumina using negative temperature coefficient 3K3 paste, composed of nanometer powder. Their room temperature resistance was measured versus the number of segments and electrode surface value for the fixed layer thickness and electrode spacing. After that, very large thermistors were printed to serve as both the powerful self-heaters and the heat loss sensors in the thermistor volume air flow meter and anemometer. For an application in AC bridges, impedance Z(f) and insertion loss S21 [dB] of the same largest segmented thermistor were measured using network analyzer HP8752A. Impedance modeling was performed using simple equivalent electrical circuit with circuit parameters estimated by fitting procedure (traditional approach), as well as using a commercial electromagnetic simulation program microwave office (MWO, novel approach). This was followed by the modeling of electrical current distribution over a number of segments done within the MWO. The results obtained from simulations and measurements were mutually compared.