Collector optimization for tradeoff between breakdown voltage and cut-off frequency in SiGe HBT

As is well known, there exists a tradeoff between the breakdown voltage BVcEO and the cut-off frequency fT for a standard heterojunction bipolar transistor （HBT）. In this paper, this tradeoff is alleviated by collector doping engineering in the SiGe HBT by utilizing a novel composite of P＋ and N- doping layers inside the collector-base （CB） space-charge region （SCR）. Compared with the single N-type collector, the introduction of the thin P＋ layers provides a reverse electric field weakening the electric field near the CB metallurgical junction without changing the field direction, and the thin N layer further effectively lowers the electric field near the CB metallurgical junction. As a result, the electron temperature near the CB metallurgical junction is lowered, consequently suppressing the impact ionization, thus BVcEO is improved with a slight degradation in fT. The results show that the product of fTXBVcEo is improved from 309.51 GHz.V to 326.35 GHz.V.

Effect of Cut-Off Frequency of Butterworth Filter on Detectability and Contrast of Hot and Cold Regions in Tc-99m SPECT

In SPECT, noise is one of the major limitations that degrade image quality. To suppress the noisy signals in an image, digital filters are most commonly applied. However, in SPECT image reconstruction, selection of an appropriate filter and its functions has always remained a difficult task. In this work an attempt was made to investigate the effects of varying cut-off frequencies and in keeping the order of Butterworth filter constant on detectability and contrast of hot and cold re-gions images. A new insert simulating hot and cold regions which provides similar views in a reconstructed image was placed in the phantom’s cylindrical source tank and imaged. Tc-99m radionuclide was distributed uniformly in the phantom. SPECT data were collected in a 20% energy window centered at 140 keV by a Philips ADAC Forte dual head gamma camera mounted with a LEHR collimator. Images were generated by using the filtered backprojection technique. A Butterworth filter of order 5 with cut-off frequencies 0.35 and 0.45 cycles·cm-1 was applied. Images were examined in terms of hot and cold regions, detectability and contrast. Results show that the hot and cold regions’ detectability and contrast vary with the change of cut-off frequency. With a 0.45 cycles·cm-1 cut-off frequency, a significant enhancement in contrast of cold regions was achieved as compared to a 0.35 cycles·cm-1 cut-off frequency. Furthermore, the detectability of hot and cold regions improved with the use of a 0.45 cycles·cm-1 cut-off frequency. In conclusion, image quality of hot and cold regions affected in a different way with a change of cut-off frequency. Thus, care should be taken in selecting the filter cut-off frequency prior to reconstruction of images;particularly, when both types of regions are expected in the reconstructed image.

Feasibility of applying the lower cut-off frequency for the density radial coverage extension in EAST reflectometry measurement

The extraordinary mode(X-mode)lower cut-off frequency is proposed for use in the reflectometry diagnostic on ITER for the electron density profile measurement,which is a trade-off between extreme plasma parameters and the accessible probing frequency.In contemporary experiments,the lower cutoff frequency can be identified at the probing frequency below the electron cyclotron frequency(f_(ce)) under certain plasma conditions.We provide here,for the first time,the experimental validation of the use of the lower cut-off frequency for the density profiles via the reflectometry measurement on EAST.The corresponding group delay of the lower cut-off frequency evolves continuously with the upper one,revealing a reasonable radial coverage extension of reflectometry measurement toward the plasma core.It is concluded that the lower cut-off frequency can be used as a supplement to the upper one in the density profile inversion process,which is of particular interest in the high magnetic field and/or density discharge to extend the radial coverage of reflectometry measurement.

An Experimental Method to Determine the Cut-Off Frequency of an Acoustical Free Field in a Non-Ideal Environment

The availability of ideal conditions like anechoic chamber to characterize some sound parameters, like sound intensity and sound power necessities the determination of free field and cut off frequency measurements. In this article, full experiment was executed at Wayne State University (Detroit-Michigan), to determine the cut off frequency in all directions;the obtained results showed that the free field can be determined for a specified space. So other tests can take place in this space avoiding regions where reflections and consequently noise can be found. Upon these results tests related to noise abatement in vehicles can be done in such environment.

Wildfires in Botswana and Their Frequency of Occurrence

Fires play an essential part in the maintenance of the environment, but amplified fire activity often leads to adverse effects in the environment such as destruction of property and loss of life. Botswana has experienced wildfires that are caused by humans intentionally and unintentionally. Some of these wildfires grow into mega fires such as the 2008 wildfires. Data of wildfires reported in Botswana from the Department of Forestry and Range Resources and the frequency of occurrence was studied and analyzed. It shows the period of 2006-2017, Ghanzi and Ngamiland districts were mostly affected by wildfires. These districts have protected land such as the wildlife parks which may lead to the outbreak of natural wildfires that burns unnoticed due to minimal movements of people. The wildfires reported in each district show an increase over the years and this may affect the smooth running of operations.

Analysis and compensation of low frequency characteristics of sensors for vibration testing

In view of the influence and harm of low frequency vibration environment on the structure of spaceflight products,a low frequency dynamic study method for piezoelectric sensor based on the dynamic system of sinusoidal pressure is proposed.This method uses a sinusoidal pressure dynamic system with two-way dual channel import and export synchronization technology to study the low frequency characteristics of a piezoelectric sensor of PCB company,and its lower cut-off frequency is 0.26 Hz.It is also studied that when the frequency of the measured vibration or shock signal is 1-200 kHz,the error range of signal positive pressure action time is 4.87%-0.03%.The dynamic compensation for the low frequency of the vibration sensor is carried out,and the compensation effect is good.

Radio-frequency transistors from millimeter-scale graphene domains

Graphene is a new promising candidate for application in radio-frequency （RF） electronics due to its excellent elec- tronic properties such as ultrahigh carrier mobility, large threshold current density, and high saturation velocity. Recently, much progress has been made in the graphene-based RF field-effect transistors （RF-FETs）. Here we present for the first time the high-performance top-gated RF transistors using millimeter-scale single graphene domain on a SiO2/Si substrate through a conventional microfabrication process. A maximum cut-off frequency of 178 GHz and a peak maximum os- cillation frequency of 35 GHz are achieved in the graphene-domain-based FET with a gate length of 50 nm and 150 nm, respectively. This work shows that the millimeter-scale single graphene domain has great potential applications in RF devices and circuits.

The Time-frequency Characteristic of a Large Volume Airgun Source Wavelet and Its Influencing Factors

Through analyzing the near-field hydrophone records of the airgun experiment in the Jiemian reservoir,Fujian,we study the time-frequency characteristic of airgun source wavelet and the influence of gun depth and firing pressure,and explain the process of bubble oscillation based on the Johnson( 1994) bubble model. The data analysis shows that:( 1) Airgun wavelet is composed of primary pulse and bubble pulse. The primary pulse,which is of large amplitude,short duration and wide frequency band,is usually used in shallow exploration. The bubble pulse,which is concentrated in the low-frequency range,is usually used in deep exploration with deep vertical penetration and far horizontal propagation.( 2) The variation of primary pulse amplitude with gun depth is very small,bubble pulse amplitude and the dominant frequency increase,and peak-bubble ratio and bubble period decrease. When the gun depth is 10 m,primary pulse amplitude and peakbubble ratio are maximum,which is suitable for shallow exploration; when gun depth is25 m,bubble pulse amplitude is large, and peak-bubble ratio is minimum, which is suitable for deep exploration.( 3) The primary pulse amplitude,bubble pulse amplitude,peak-bubble ratio,and bubble period increase and the dominant frequency decreases with increased firing pressure.

MHz cut-off frequency and permeability mechanism of iron-based soft magnetic composites

The lack of soft magnetic composites with high power density in MHz frequency range has become an obstacle in the efficient operation of the electrical and electronic equipments.Here,a promising method to increase the cut-off frequency of iron-based soft magnetic composites to hundreds of MHz is reported.The cut-off frequency is increased from 10 MHz to 1 GHz by modulating the height of the ring,the distribution of particles,and the particle size.The mechanism of cut-off frequency and permeability is the coherent rotation of domain modulated by inhomogeneous field due to the eddy current effect.An empirical formula for the cut-off frequency in a magnetic ring composed of iron-based particles is established from experimental data.This work provides an effective approach to fabricate soft magnetic composites with a cut-off frequency in hundreds of MHz.

温胆汤加减联合倍他司汀治疗低频下降型突发性耳聋痰火郁结证临床观察

目的探讨温胆汤加减联合倍他司汀治疗低频下降型突发性耳聋(SD)痰火郁结证的临床疗效,以及对患者纯音听阈水平及血液流变学、血管内皮功能、免疫功能指标的影响。方法回顾性分析医院2019年1月至2020年12月收治的低频下降型SD痰火郁结证患者80例,按治疗方式的不同分为对照组(盐酸倍他司汀,38例)和观察组(温胆汤加减联合盐酸倍他司汀,42例),均连续治疗10d。结果观察组总有效率为92.86%,显著高于对照组的68.42%(P<0.05)。治疗后,两组患者的中医证候积分、纯音听阈水平均显著降低,且观察组显著低于对照组(P<0.05);两组患者的血液流变学指标全血黏度(低切、中切、高切)、血细胞比容、血浆黏度、血小板聚集率均显著降低,且观察组显著低于对照组(P<0.05);两组患者的超氧化物歧化酶水平均显著升高,过氧化脂质、一氧化氮水平均显著降低,且观察组均显著优于对照组(P<0.05);两组患者的免疫球蛋白A(IgA),IgG,IgM水平均显著升高,且观察组均显著高于对照组(P<0.05)。观察组和对照组患者不良反应发生率相当(7.14%比5.26%,P>0.05)。结论温胆汤加减联合倍他司汀治疗低频下降型SD痰火郁结证的临床疗效显著,可有效改善患者的纯音听阈水平,优化血管内皮功能、血液流变学、免疫功能,且安全性良好。

TDLAS在森林可燃物热解气体多组分同步测量中的应用分析

森林可燃物热解的研究,对森林火灾预警和控制具有重要意义。森林可燃物热解过程中主要生成CO、 CO_(2)和CH_(4)等含碳气体,这些气体的不断释放容易引发森林火灾并加剧温室效应,快速准确检测这三种组分的浓度有利于火灾早期预警和大气环境保护。通过频分复用结合可调谐二极管激光吸收光谱(TDLAS)技术准确测量了六种山林树种样品热解气体中CO、 CO_(2)和CH_(4)三种组分的浓度,证明了频分复用-TDLAS技术对森林可燃物热解气体多组分同步测量的适用性。首先,介绍了频分复用-TDLAS技术的基本原理,确定了三种组分互不干扰、且谱线强度适宜的吸收谱线。其次,为了准确反演不同的浓度,对二次谐波(2f)信号和二次谐波/直流(2f/DC)信号的特性进行了研究。通过Simulink仿真的方式,比较了利用2f信号和2f/DC信号反演不同浓度准确性的差异,结果表明2f/DC信号具有更大的线性区间,适用于森林可燃物热解气体中不同浓度组分的测量。最后,利用两支中心波长分别为1 580.0和1 653.7 nm的分布式反馈(DFB)激光器搭建了CO、 CO_(2)和CH_(4)同步测量的实验装置。在900℃管式炉中对六种树种样品进行热解以及热解气体制备,利用Herriott吸收池测量了三种组分的2f/DC信号,通过标准气体建立标定模型获得了热解气体中三种组分的绝对浓度。结果显示,三种组分的2f/DC信号峰值与浓度满足良好的线性关系,线性度大于0.995,且在焦炭气化反应和Boundouard反应的作用下,六种树种样品热解气体中CO浓度明显高于CO_(2)和CH_(4)。通过分析枫香叶样品的光谱,表明在2 s测量时间内,该光谱系统对于CO、 CO_(2)和CH_(4)的最低检测限低于0.008%,灵敏度优于0.005%,满足森林火灾预警的需求。该研究为森林可燃物热解气体多组分同步测量以及森林火灾预警提供了方法参考。

外电场作用下改进的Leaky Integrate-And-Fire模型的峰放电频率适应性研究

峰放电频率适应性是神经元在信息处理过程中重要的动力学特性之一.当神经系统受到外电场作用时,会对其动力学行为以及神经电信息的产生、传导产生影响.我们基于Leaky integrate-and-fire(LIF)神经元模型,建立了外电场作用下改进的LIF神经元模型.采用随时间演化的膜电位曲线和峰放电频率曲线,以及随外电场变化的起始峰放电频率曲线和稳态峰放电频率曲线,研究不同强度、频率外电场作用下改进的LIF模型的适应性变化.此外,还利用相邻峰峰间期(ISI)之间的相关性进一步阐明外电场对神经元适应性的影响.

基于改进FIRE滤波技术和Prony算法的低频振荡模式辨识研究

提出一种改进FIRE(Fuzzy Inference Ruled by Else-action)滤波和改进Prony法相结合的低频振荡模式辨识方法.该方法用改进后具有检测层和调整层的FIRE滤波技术对数据进行快速预处理,再用改进Prony法对滤波后的信号进行分析得到电力系统的低频振荡模式.将该方法分别用于分析试验信号和IEEE 4机系统振荡信号,并同带有传统模糊滤波器的Prony分析结果比较可以看出,改进后的方法可将低频振荡的主导模式在一定的噪声环境下比较准确和快速的辨识出来,且辨识的阶数更接近于实际阶数,验证了其可靠性和实用性.

基于贝叶斯网络分析的煤化工消防系统检修频率研究

因煤气化厂房内的消防系统维护检修不到位,造成系统无法发挥作用甚至瘫痪的事件屡屡发生。本文基于贝叶斯网络的风险评估方法,通过对煤气化厂房火灾的发生原因进行分析,开展了合成气泄漏及事故后果的风险评估,并结合风险评估结果对消防系统的检修频率进行探究,提出了一种与煤气化厂房固有风险相适应的消防系统检修维护周期评估算法。该算法可较高程度地实现事故预防并减轻灾害损失。

汽轮机高位布置燃煤发电系统调频动态特性模拟研究

汽轮机高位布置技术可大幅缩短四大管道的长度,降低四大管道阻力,有望在降低发电煤耗率的同时提升机组的响应能力。采用GSE软件建立了660MW汽轮机高位布置超超临界燃煤发电机组动态模型,研究了主汽阀、高加抽汽节流和低加抽汽节流参与一次调频时的调节性能,并与汽轮机常规布置机组进行比较。结果表明:汽轮机高位布置可减少主汽阀参与一次调频时的调节时间,负荷扰动分别为-0.3%P e、-0.4%P e和-0.5%P e时,高位布置机组的调节时间分别减少了2s、2.1s和2.6s,负荷扰动分别为+0.3%P e、+0.4%P e和+0.5%P e时调节时间减少了2.1s、3.5s和4.1s;但是汽轮机高位布置对抽汽节流参与调频时的调节时间影响较小。

基于快速动态矩阵控制的抗积分饱和方法

积分饱和在闭环控制系统中是一个不可忽视且非常常见的工业现象,它会导致闭环系统控制品质降低甚至引起闭环不稳定。传统采用二次规划(QP)的QDMC技术往往会消耗大量计算资源和计算时间,并且难以在嵌入式系统中实现。为此,提出一种基于动态矩阵控制算法(DMC)的抗积分饱和方法,该方法模仿QDMC在处理输入积分饱和时的控制逻辑,显著加快了计算速度,而且算法不需要进行二次规划,还可以进行嵌入式设计。模拟研究结果证实了所提算法的有效性。

基于SiP技术的宽带小型化锁相源设计

针对射频微波电路中频率源宽带、小型化、通用化的应用需求,采用基于高温共烧陶瓷(HTCC)的系统级封装(SiP)技术实现宽带锁相源的小型化,并搭配多功能电路单元实现放大、分频、倍频等电路功能。设计的锁相源尺寸仅15 mm×12 mm×3 mm,搭配不同的内部功能电路,其输出信号频率为0.1~40 GHz。测试结果表明其性能指标满足工程应用需求,对促进宽带锁相源的工程化应用具有重要作用。

天然气管道周边区域个人风险计算分析

为了分析输气管道周边区域风险水平,建立了个人风险计算模型。该模型包括管道失效频率计算和失效危害后果计算两部分内容。该模型将管道失效原因分为6类,采用修正基准频率的方法计算每类失效原因造成的管道失效频率;危害后果计算分析包括:泄漏速率计算、引燃概率计算分析、火灾计算分析和伤害程度,采用喷射火表征管道泄漏危害后果。在失效频率和泄漏后果计算分析的基础上,计算管道周边区域的个人风险,明确风险是否可接受。风险计算实例表明,破裂是构成风险的主要因素,建议强化对破裂诱发因素的管控。

无线无源多栅格结构式温压传感器设计与制备

针对高温、高压等恶劣环境下的温度和压力参数测量,基于微波散射原理和高温共烧陶瓷(HTCC)技术设计了一种无线无源多栅格结构式温压传感器,能够实现25~800℃内温度的测量和0~300 kPa内压力的测量。使用高频电磁仿真软件对传感器进行结构设计,使所测参数与传感器回波损耗(S11)具有线性关系,然后通过传感器制备与实验对传感器性能开展验证。测试结果表明,在测量范围内,无线无源多栅格结构式温压传感器具有良好的可靠性和线性度,温度平均灵敏度为423 kHz/℃,压力灵敏度为209 kHz/kPa,温度和压力的最大相对测量误差分别为3.1%和1.13%。

雷火灸联合低频脉冲治疗经产妇产后盆腔脏器脱垂临床观察

目的 探究雷火灸联合低频脉冲治疗经产妇产后盆腔脏器脱垂效果。方法 72例产后盆腔脏器脱垂患者随机分为对照组(36例,采取常规盆底康复治疗)和试验组(36例,于对照组基础给予雷火灸联合低频脉冲理疗),比较2组治疗前后盆底肌力、脱垂程度和性生活质量,并计算漏尿率。结果 治疗后,试验组阴道收缩肌电值、收缩持续时间及最大收缩力大于对照组,疲劳度小于对照组,脱垂改善程度优于对照组,性生活质量高于对照组,漏尿率低于对照组,差异均有统计学意义(P<0.05)。结论 雷火灸联合低频脉冲治疗经产妇产后盆腔脏器脱垂效果显著,可以增强患者盆底肌力,改善盆腔脏器脱垂程度,提高性生活质量,预防漏尿。