共查询到20条相似文献,搜索用时 0 毫秒
1.
《Microwave Theory and Techniques》1963,11(6):513-522
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. 相似文献
2.
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。 相似文献
3.
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. 相似文献
4.
Ultrahigh-field MRI whole-slice and localized RF field excitations using the same RF transmit array 总被引:1,自引:0,他引:1
Ibrahim TS 《IEEE transactions on medical imaging》2006,25(10):1341-1347
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). 相似文献
5.
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。 相似文献
6.
WANGZhao-ying WUXing TIANHe-bin LIShi-chen 《半导体光子学与技术》2003,9(2):102-106
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. 相似文献
7.
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。 相似文献
8.
9.
Grissom W.A. Dan Xu Kerr A.B. Fessler J.A. Noll D.C. 《IEEE transactions on medical imaging》2009,28(10):1548-1559
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. 相似文献
10.
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。 相似文献
11.
《Electron Device Letters, IEEE》2007,28(5):357-359
12.
Cao H Vorperian VR Tsai JZ Tungjitkusolmun S Woo EJ Webster JG 《IEEE transactions on bio-medical engineering》2000,47(11):1518-1524
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. 相似文献
13.
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. 相似文献
14.
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. 相似文献
15.
16.
《Electron Devices, IEEE Transactions on》2009,56(1):13-19
17.
Buccella C. De Santis V. Feliziani M. 《Electromagnetic Compatibility, IEEE Transactions on》2007,49(4):825-833
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. 相似文献
18.
Eric Anterrieu 《Geoscience and Remote Sensing, IEEE Transactions on》2007,45(3):592-601
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 相似文献
19.
Temperature study of the dielectric polarization effects of capacitive RF MEMS switches 总被引:2,自引:0,他引:2
Papaioannou G. Exarchos M.-N. Theonas V. Guoan Wang Papapolymerou J. 《Microwave Theory and Techniques》2005,53(11):3467-3473
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. 相似文献
20.
Jin-Hyun Hwang P. Markondeya Raj Chong Yoon Hyung-Mi Jung Rao Tummala 《Microelectronic Engineering》2008,85(3):553-558
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. 相似文献