Radio-Frequency Current Through the Feet of a Grounded Human

Radio-frequency currents through the feet of grounded human subjects have been measured, from 0.5 to 10 MHz, using a TEM-cell exposure system. The RF current is directly proportional to both frequency f and electric field, with an average constant of proportionality of 344 ± 14 (4 data) ?/(V· m-1·MHz). For the maximum exposures recommended by ANSI standard C95.1?1982, the ankle specific absorption rates in some cases significantly exceed the ANSI limit of 8 W/kg. When the effects of footwear and near-field exposures are taken into account, ankle absorption rates will likely not exceed 8 W/kg. These current measurements permit the extension of the f2 law for human whole-body absorption rates from the HF band down to 60 Hz.     

基于锗硅BiCMOS工艺的射频带隙基准电流源设计

射频电路偏置电流源对噪声的要求越来越高,引进的噪声不容忽视,带隙基准源在保证温漂系数的同时要求有较低的噪声。设计了一款不采用运放结构简单的射频带隙基准源电路,电源电压3.3 V,基准电压V_(REF)为3.14 V,采用PNP双极管和电阻,利用缓冲器负载实现输出DC点和输出摆幅不变,改善了温度系数并且降低了噪声。基于0.18μm SiGeBiCMOS工艺的仿真结果表明,在-55~125℃温度范围内,温漂系数为9.613×10~(-6)/℃;7.5 GHz频率下,100 kHz处噪声为6.164 nV/sqrt(Hz),总输出噪声低至2.08×10~(-6) V。     

A Super-Resolution Framework for 3-D High-Resolution and High-Contrast Imaging Using 2-D Multislice MRI

A novel super-resolution reconstruction (SRR) framework in magnetic resonance imaging (MRI) is proposed. Its purpose is to produce images of both high resolution and high contrast desirable for image-guided minimally invasive brain surgery. The input data are multiple 2-D multislice inversion recovery MRI scans acquired at orientations with regular angular spacing rotated around a common frequency encoding axis. The output is a 3-D volume of isotropic high resolution. The inversion process resembles a localized projection reconstruction problem. Iterative algorithms for reconstruction are based on the projection onto convex sets (POCS) formalism. Results demonstrate resolution enhancement in simulated phantom studies, and ex vivo and in vivo human brain scans, carried out on clinical scanners. A comparison with previously published SRR methods shows favorable characteristics in the proposed approach.     

Radio-Frequency Interference Among Linear-FM Radars

The problem of radio-frequency interference (RFI) between radars using linear-FM pulses is examined. For a very large class of cases, the RFI is the same as if the FM were removed; for the cases not in that category, it is shown that the peak response of an unweighted receiver can be closely approximated by a hand calculation. The effects of cosine-on-a-pedestal weighting, in either the frequency or the . time domain, are then considered. Exact expressions for the RFI are developed, and it is shown that, if one is satisfied with knowing the peak of the response, the effect of the weighting can be well approximated by some quite simple expressions.     

高电流密度热阴极的进展

概述了近年来高电流密度扩散型热阴极的进展。     

Radio-Frequency Interference at Orbital Altitudes

The experimental portion of the Aerospace Corporation's program for the determination of RFI at orbital altitudes is described. The need for this effort is shown, and potential sources of RFI are outlined. The methods planned are described in detail, and actual hardware for use on the first experimental flight is discussed briefly. Hardware includes antennas, preselectors, and crystal video logarithmic receivers. It is expected that results of the first experiment will be available late in 1965.     

A Study of the Interaction Between Implanted Pacemakers and the Radio-Frequency Field Produced by Magnetic Resonance Imaging Apparatus

Specific absorption rate (SAR) and temperature increases produced inside a thorax model by an MRI apparatus equipped with a birdcage antenna operating at 64 MHz have been studied both experimentally and numerically. Considering a pacemaker (PM) equipped with a unipolar catheter inserted inside the thorax model, peak SARs averaged over 1 mg between 240 and 6400 W/kg, depending on the catheter section and length, on its position inside the phantom, and on field polarization have been obtained close to the catheter tip. On the other hand, the average SAR in the whole thorax is not influenced by the presence of the PM. Temperature increments from 0.6degC to 15degC have been obtained for 6-min MRI investigations with the lowest values when the radio-frequency (RF) magnetic field is linearly polarized along a direction perpendicular to the implant plane.     

A Bio-Inspired Active Radio-Frequency Silicon Cochlea

Fast wideband spectrum analysis is expensive in power and hardware resources. We show that the spectrum-analysis architecture used by the biological cochlea is extremely efficient: analysis time, power and hardware usage all scale linearly with $N$ , the number of output frequency bins, versus $Nlog(N)$ for the Fast Fourier Transform. We also demonstrate two on-chip radio frequency (RF) spectrum analyzers inspired by the cochlea. They use exponentially-tapered transmission lines or filter cascades to model cochlear operation: Inductors map to fluid mass, capacitors to membrane stiffness and active elements (transistors) to active outer hair cell feedback mechanisms. Our RF cochlea chips, implemented in a 0.13 $mu$m CMOS process, are 3 mm$,times,$ 1.5 mm in size, have 50 exponentially-spaced output channels, have 70 dB of dynamic range, consume $≪, $300 mW of power and analyze the radio spectrum from 600 MHz to 8 GHz. Our work, which delivers insight into the efficiency of analog computation in the ear, may be useful in the front ends of ultra-wideband radio systems for fast, power-efficient spectral decomposition and analysis. Our novel rational cochlear transfer functions with zeros also enable improved audio silicon cochlea designs with sharper rolloff slopes and lower group delay than prior all-pole versions.      

射频电路PCB设计

介绍了采用PROTEL99SE进行射频电路PCB设计的设计流程。为了保证电路的性能,在进行射频电路PCB设计时应考虑电磁兼容性。因而重点讨论了元器件的布局与布线原则来达到电磁兼容的目的。     

Radio-Frequency Inverters With Transmission-Line Input Networks

A soft-switching inverter topology (the Class Phi ) is presented which draws dc source current through a transmission line or a lumped-network approximation of a distributed line. By aligning the inverter switching frequency just below the line's lambda/4-wave resonance, the Class Phi topology enforces odd-and even-harmonic content in its drain voltage and input current, respectively. The symmetrizing action of the transmission-line dynamics results in natural square-wave operation of the switch, reducing the inverter stresses (relative to a Class E) for a given power throughput. The inverter waveforms and normalized power-output capability are analyzed in simple terms, and supported by measurements of an inverter built around a length of distributed line, and an inverter incorporating a lumped L-C ladder network. The latter implementation is constructed with air-core magnetics and inter-layer capacitances that are integrated into the thickness of a printed-circuit board. A comparison with a Class E inverter of similar size and ratings demonstrates the small passive-component values and manufacturing advantages afforded by the Class Phi topology.     

用于太赫兹真空器件的大电流密度阴极

主要介绍了太赫兹器件对电子源的需求,并对国际上各类适用于太赫兹器件的电子源进行了分析和对比.作者认为正在研发的热阴极和冷阴极(场发射阵列阴极)是未来太赫兹器件电子源的两个选择.目前,热阴极技术和场发射冷阴极也正在因为器件需求的推动而进一步发展,并已经取得了较好的成果.文章最后就北京真空电子技术研究所近年来的热阴极技术进展和场发射冷阵列阴极技术发展进行了简明介绍;所取得的技术成果表明,热阴极和场发射阵列阴极都具有广泛的太赫兹器件应用前景.     

Line-Integral Projection Reconstruction (LPR) with Slice Encoding Techniques: Multislice Regional Imaging in NMR Tomography

Line-integral projection reconstruction (LPR) in NMR imaging was found to be useful and has several advantages such as the imaging capability of objects having short T2 and compensation of phase fluctuations arising from the system instability. Although single slice LPR is found to be inefficient and poor in signal-to-noise ratio (SNR), the multislice encoded LPR method is of interest since it has a high SNR and also the capability of selected regional volume or multislice imaging. The latter, i.e., regional volume imaging capability, is a unique property of NMR imaging and offers a variety of imaging capabilities such as simultaneous multislice imaging of sagittal, transaxial, or coronal views. In this paper, we have investigated two basic forms of the multislice encoded imaging methods using LPR, i.e., Fourier and Hadamard-like encoding matrices. Applications of the methods to the experimented NMR imaging show good agreement with predicted behavior.     

Tin Whisker Growth Induced by High Electron Current Density

The effect of electric current on the tin whisker growth on Sn stripes was studied. The Sn stripes, 1 μm in thickness, were patterned on silicon wafers. The design of the Sn stripes allowed the simultaneous study of the effect of current crowding and current density. Current stressing was performed in ovens set at 30, 50, or 70°C, and the current density used ranged from 4.5 × 10⁴ A/cm² to 3.6 × 10⁵ A/cm². It was found that the stress induced by the electric current caused the formation of many Sn whiskers. A higher current density caused more Sn whiskers to form. Of the three temperatures studied, 50°C was the most favorable one for the formation of the Sn whiskers. In addition, the current-crowding effect also influenced whisker growth.     

Resistance Compression Networks for Radio-Frequency Power Conversion

A limitation of many high-frequency resonant inverter topologies is their high sensitivity to loading conditions. This paper introduces a new class of matching networks that greatly reduces the load sensitivity of resonant inverters and radio frequency (RF) power amplifiers. These networks, which we term resistance compression networks, serve to substantially decrease the variation in effective resistance seen by a tuned RF inverter as loading conditions change. We explore the operation, performance characteristics, and design of these networks, and present experimental results demonstrating their performance. Their combination with rectifiers to form RF-to-dc converters having narrow-range resistive input characteristics is also treated. The application of resistance compression in resonant power conversion is demonstrated in a dc-dc power converter operating at 100MHz     

Radio-Frequency System of the Cambridge Electron Accelerator

The requirements for the RF system of the Cambridge Electron Accelerator are investigated and the choice of the major parameters of the system is discussed. The strongly coupled waveguide cavity system is analyzed and the performance of the system with various types of imperfections is calculated.     

脉冲应力下电流密度指数因子的研究与分析

介绍了一种与传统ＭＴＦ法完全不同的测量电流密度因子ｎ的新型动态电流斜坡测试法。测试了４种不同样品。结果表明，ｎ值与材料有关，并符合ＢＬＡＣＫ方程。     

大电流密度下欧姆接触退化机理的研究

对大电流密度下欧姆接触的结构进行了改进,采用只对接触区域老化而其它区域非破坏性的结构特点,保证测量数据的真实有效性。工艺制备中采取多次SiO2铺垫多次光刻技术解决了引线电极断裂的可能。通过施加达到或超过105A/cm2电流密度,利用文中结构测得比接触电阻早期快速失效,且随电流密度增加退化加剧,对样品老化前后进行能谱分析得知,大电流密度下接触层中Al离子发生了扩散从而破坏了良好接触。     

射频遥控切换方式

介绍一种基于射频信号的遥控切换方式以及它的一些优点 ;对方案组成进行详细说明 ,简要分析其工作原理