Radiometry in the Submillimeter Region Using the Interferometric Modulator

Radiometry in the submillimeter and far infrared regions involves problems of a type not encountered in the centimeter region which require solutions using techniques different from those used in centimeter-wavelength radiometry. The nonlinear variation of the magnitude of the black-body radiation spectral density with temperature and wavelength, the limitation of antenna beamwidth by factors connected with the size of the noncoherent detector and the antenna focal length (rather than by diffraction effects and the antenna aperture) and the heavy absorption of submillimeter radiation by atmospheric water vapor are typical of the problems normally not encountered in centimeter radiometry. The unavailability of microwave techniques (i.e., waveguides, coherent receivers, etc.) makes necessary the use of quasi-optical techniques in this wavelength region. The interferometric modulator, which has already been used in far infrared spectrometers, is proposed in this paper as the major component of a practical submillimeter radiometer. Its use as the wave-number-selection device in a radiometer is analyzed and estimates are obtained for the sensitivity of this submillimeter radiometer. It is estimated that a 0.2/spl deg/ minimum detectable temperature differential is achievable with this radiometer. Also discussed are the effects of atmospheric water vapor absorption and the sensitivity of a number of different types of radiation detectors suitable for use in the submillimeter-wavelength region.     

利用DS1620温度传感器构成温度监控系统

为了满足某些环境需要温度测量范围广、温度调节方便等要求,以DS1620温度传感器和单片机为基础,设计了一套温度监控系统。该系统不仅能够根据环境温度的改变自动调节温度,而且由于DS1620芯片的测量范围广,温度调节方便,使该系统在温度控制、温度测量方面更加精确与方便。     

Brightness Temperature Map Reconstruction from Dual-Polarimetric Visibilities in Synthetic Aperture Imaging Radiometry

Synthetic aperture imaging radiometers are powerful sensors for high-resolution observations of the Earth at low microwave frequencies. Within this context, the European Space Agency is currently developing the soil moisture and ocean salinity (SMOS) mission devoted to the monitoring of SMOS at global scale from L-band spaceborne radiometric observations obtained with a 2-D interferometer. This paper is concerned with the reconstruction of radiometric brightness temperature maps from interferometric measurements. More exactly, it extends the concept of ldquoband-limited resolving matrixrdquo to the case of the processing of dual-polarimetric data.     

Ultrahigh-field MRI whole-slice and localized RF field excitations using the same RF transmit array

In this paper, a multiport driving mechanism is numerically implemented at ultra high-field (UHF) magnetic resonance imaging (MRI) to provide 1) homogenous whole-slice (axial, sagittal, or coronal) and 2) highly localized radio frequency (RF) field excitation within the same slices, all with the same RF transmit array (here chosen to be a standard transverse electromagnetic (TEM) resonator/coil). The method is numerically tested using a full-wave model of a TEM coil loaded with a high-resolution/18-tissue/anatomically detailed human head mesh. The proposed approach is solely based on electromagnetic and phased array antenna theories. The results demonstrate that both homogenous whole-slice as well as localized RF excitation can be achieved within any slice of the head at 7 T (298 MHz for proton imaging).     

用VB6.0开发粮库温度监测系统的监控软件

以一个实际投入运行的粮库温度监测系统为例,介绍监控软件的总体功能设计和实现方法,并对其关键技术如微机通信、数据库管理、数据显示以及系统设置等进行了详尽的阐述。     

Absolute Measurement Fiber—optic Sensors in Large Structural Monitoring

The security of civil engineering is an important task due to the economic,social and evironmental significance.Compared with conventional sensors.the optical fiber sensors have their unique characteristics.Beijing durable,stable and insensitive to external perturbations,they are particular interesting for the long-term monitoring of civil structures.Focus is on absolute measurement optical fiber sensors,which are emerging from the monitoring large structural,including SOFO system,F-P optical fiber sensors,and fiber Bragg grating sensors.The principle,characteristic and application of these three kinds of optical fiber sensors are described together with their future prospects.     

利用GETTIM函数求全局极小值

利用FORTRAN语言提供的GETTIM函数，在规定的范围内，产生一组组随机值，作为优化的初始值，求全局极小值，明显地提高了优化结果的可靠度。     

基于射频技术的轮胎气压监测系统的设计

本文采用Motorola的压力/温度传感器MPXY8020A,在改进Motorola TPMS解决方案的基础上,对TPMS的电路以及软件进行优化设计,该系统可通过键盘设定轮胎标准压力值,测定每个轮胎内部的实际温度和压力,确定故障轮胎并实时通过声光提醒司机采取措施,防止事故发生.     

Fast Large-Tip-Angle Multidimensional and Parallel RF Pulse Design in MRI

Large-tip-angle multidimensional radio-frequency (RF) pulse design is a difficult problem, due to the nonlinear response of magnetization to applied RF at large tip-angles. In parallel excitation, multidimensional RF pulse design is further complicated by the possibility for transmit field patterns to change between subjects, requiring pulses to be designed rapidly while a subject lies in the scanner. To accelerate pulse design, we introduce a fast version of the optimal control method for large-tip-angle parallel excitation. The new method is based on a novel approach to analytically linearizing the Bloch equation about a large-tip-angle RF pulse, which results in an approximate linear model for the perturbations created by adding a small-tip-angle pulse to a large-tip-angle pulse. The linear model can be evaluated rapidly using nonuniform fast Fourier transforms, and we apply it iteratively to produce a sequence of pulse updates that improve excitation accuracy. We achieve drastic reductions in design time and memory requirements compared to conventional optimal control, while producing pulses of similar accuracy. The new method can also compensate for nonidealities such as main field inhomogeneties.     

冷链温度监测

冷链在控制温度敏感性产品、保证食品等物品质量和减少物流损耗方面有重要的作用,但是对冷链进行监测却很难实现,本文将介绍一种准确有效、快捷低廉的冷链监测方式,即使用RFID技术对冷链进行全过程监测。     

Direct Monitoring of RF Overstress in High-Power Transistors and Amplifiers

Light emission from power transistors at a compression level in the range of 2–3 dB has been imaged using a microscope-mounted camera. Results show that the emitted light intensity distribution across the transistor is highly nonuniform and depends on the load impedance, direct current, and RF conditions. The light intensity correlates with a negative gate current, which is a result of the RF-induced impact ionization in the transistors. The nonuniformity in the light intensity is attributed to the RF-induced voltage overstress in the transistors. The observed light emission may be used as a direct and contactless monitor of the RF-induced overstress in transistors and power amplifiers.      

Temperature measurement within myocardium during in vitro RF catheter ablation

While most commercial ablation units and research systems can provide catheter tip temperature during ablation, they do not provide information about the temperature change inside the myocardium, which determines the lesion size. We present the details of a flow simulation and temperature measurement system, which allows the monitoring of the temperature change inside the myocardium during in vitro radio frequency (RF) cardiac catheter ablation at different blood flow rates to which the catheter site may be exposed. We set up a circulation system that simulated different blood flow rates of 0 to 5 L/min at 37 degrees C. We continuously measured the temperature at the catheter tip using the built-in thermistor and inside the myocardium using a three-thermocouple probe. The system provides a means for further study of the temperature inside myocardium during RF catheter ablation under different flow conditions and at different penetration depths.     

Interpretation of Absolute Laser Reflectance During Optical Monitoring of Polycrystalline GaAs Deposition on Quartz Using Metalorganic Chemical Vapor Deposition

Gallium arsenide (GaAs) was deposited by metalorganic chemical vapor deposition in a horizontal quartz reactor tube using trimethylgallium and arsine at 400°C to 500°C. Nucleation time and deposition rate were monitored using in situ laser reflectometry. This allowed differentiation between film and parasitic growth, which was not possible with other optical techniques. An absolute reflectance model was developed using measurements prior to GaAs deposition, and then employed to calculate values for GaAs on quartz. Detected reflectance intensities during experimental GaAs deposition were low compared with the model due to three-dimensional island growth, causing scattering of the incident laser radiation.     

Polynomial modeling and reduction of RF body coil spatial inhomogeneity in MRI

The usefulness of statistical clustering algorithms developed for automatic segmentation of lesions and organs in magnetic resonance imaging (MRI) intensity data sets suffers from spatial nonstationarities introduced into the data sets by the acquisition instrumentation. The major intensity inhomogeneity in MRI is caused by variations in the B1-field of the radio frequency (RF) coil. A three-step method was developed to model and then reduce the effect. Using a least squares formulation, the inhomogeneity is modeled as a maximum variation order two polynomial. In the log domain the polynomial model is subtracted from the actual patient data set resulting in a compensated data set. The compensated data set is exponentiated and rescaled. Statistical comparisons indicate volumes of significant corruption undergo a large reduction in the inhomogeneity, whereas volumes of minimal corruption are not significantly changed. Acting as a preprocessor, the proposed technique can enhance the role of statistical segmentation algorithms in body MRI data sets.     

Imaging Electric Properties of Biological Tissues by RF Field Mapping in MRI

The electric properties (EPs) of biological tissue, i.e., the electric conductivity and permittivity, can provide important information in the diagnosis of various diseases. The EPs also play an important role in specific absorption rate calculation, a major concern in high-field MRI, as well as in nonmedical areas such as wireless telecommunications. The high-field MRI system is accompanied by significant wave propagation effects, and the RF radiation is dependent on the EPs of biological tissue. On the basis of the measurement of the active transverse magnetic component of the applied RF field (known as ${rm B}_{1}$-mapping technique), we propose a dual-excitation algorithm, which uses two sets of measured ${rm B}_{1}$ data to noninvasively reconstruct the EPs of biological tissues. The finite-element method was utilized in 3-D modeling and ${rm B}_{1}$ field calculation. A series of computer simulations were conducted to evaluate the feasibility and performance of the proposed method on a 3-D head model within a TEM coil and a birdcage coil. Using a TEM coil, when noise free, the reconstructed EP distribution of tissues in the brain has relative errors of 12%–28% and correlated coefficients of greater than 0.91. Compared with other ${rm B}_{1}$-mapping-based reconstruction algorithms, our approach provides superior performance without the need for iterative computations. The present simulation results suggest that good reconstruction of EPs from B1 mapping can be achieved.      

Analysis of DC–RF Dispersion in AlGaN/GaN HFETs Using RF Waveform Engineering

This paper describes how dc–radio-frequency (RF) dispersion manifests itself in AlGaN/GaN heterojunction field-effect transistors when the devices are driven into different RF load impedances. The localized nature of the dispersion in the $I$– $V$ plane, which is confined to the “knee” region, is observed in both RF waveform and pulsed $I$–$V$ measurements. The effect is fully reproduced using 2-D physical modeling. The difference in dispersive behaviors has been attributed to the geometry of a trap-induced virtual-gate region and the resulting carrier velocity saturation being overcome by punchthrough effects under high electric fields.      

Prediction of Temperature Increase in Human Eyes Due to RF Sources

A numerical study is proposed to investigate the effects of different RF sources on the specific absorption rate (SAR) and maximum temperature increase in the human eye at different frequencies. In particular, a new model of the human head is presented and compared with an anatomical model of the visible human. The high resolution (0.5 mm) of the proposed model allows to consider more eye tissues than previous studies distinguishing the sclera from the retina and choroid. New values of blood perfusion and metabolic rate of these tissues are derived. A plane-wave field is considered as far-field exposure, while realistic models of mobile phone and dipole antennas are used as primary sources for near-field exposure. The obtained results show that the distributions of the SAR and temperature increase depend on the frequency, position, and kind of sources. Finally, attention is paid to the maximum temperature increase in the lens for the SAR values prescribed by the Commission on Non-Ionizing Radiation Protection. To this aim, a scaling approach is proposed, and significant values of temperature increase are found (about C for general public exposure and about 1.5 degC for occupational exposure) for the most critical cases of near-field exposures.     

On the Reduction of the Reconstruction Bias in Synthetic Aperture Imaging Radiometry

Synthetic aperture imaging radiometers (SAIRs) are powerful instruments for high-resolution observation of planetary surfaces at low microwave frequencies. This paper is concerned with the reconstruction of radiometric brightness temperature maps from SAIR interferometric measurements. Even in the absence of modeling errors and radiometric noise, a systematic error, or bias, has been observed in the reconstructed maps. The origin of this bias is analyzed and an efficient solution is proposed for reducing it. The core reconstruction procedure is not changed, and no additional measurements are needed. Throughout the scientific rationale, particular emphasis is laid on numerical simulations carried out for the Soil Moisture and Ocean Salinity space mission, a project led by the European Space Agency and devoted to the remote sensing of soil moisture and ocean salinity from a low-orbit platform     

Temperature study of the dielectric polarization effects of capacitive RF MEMS switches

This paper investigates both theoretically and experimentally the dielectric charging effects of capacitive RF microelectromechanical system switches with silicon nitride as dielectric layer. Dielectric charging caused by charge injection under voltage stress was observed. The amphoteric nature of traps and its effect on the switch operation were confirmed under both positive and negative control voltages. It has been confirmed that charging is a complicated process, which can be better described through the stretched exponential relaxation. This mechanism is thermally activated with an activation energy being calculated from the temperature dependence of the capacitance transient response. The charging mechanism, which is responsible for the pull-out voltage and the device failure, is also responsible for the temperature-induced shift of the capacitance minimum bias.     

Temperature dependence of the dielectric properties of polymer composite based RF capacitors

The temperature dependence of the dielectric properties of high Q polymer-based composites was discussed in terms of microstructural differences with various filler contents. Liquid coatable high Q polymer-based composites with 3-4× improvement in dielectric constant and precisely controlled temperature coefficient of capacitance (TCC) were developed with Ta₂O₅ as the inorganic filler. The base polymer showed a negative TCC of −250 ppm/°C from room temperature to 125 °C. Measurement of TCC with samples containing Ta₂O₅ greater than 30 vol.% showed TCC of ±50 ppm/°C with moderate capacitance density, whereas no detectable improvement in the TCC was observed in samples having 20 vol.% Ta₂O₅. The electron micrographs of this low filler content sample gave a clear indication of clustering, i.e. particle agglomeration. The homogeneity of the particle distribution was more pronounced in the sample with 40 vol.% Ta₂O₅.