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.

High-order Bragg forward scattering and frequency shift of low-frequency underwater acoustic field by moving rough sea surface

Acoustic scattering modulation caused by an undulating sea surface on the space-time dimension seriously affects underwater detection and target recognition.Herein,underwater acoustic scattering modulation from a moving rough sea surface is studied based on integral equation and parabolic equation.And with the principles of grating and constructive interference,the mechanism of this acoustic scattering modulation is explained.The periodicity of the interference of moving rough sea surface will lead to the interference of the scattering field at a series of discrete angles,which will form comb-like and frequency-shift characteristics on the intensity and the frequency spectrum of the acoustic scattering field,respectively,which is a high-order Bragg scattering phenomenon.Unlike the conventional Doppler effect,the frequency shifts of the Bragg scattering phenomenon are multiples of the undulating sea surface frequency and are independent of the incident sound wave frequency.Therefore,even if a low-frequency underwater acoustic field is incident,it will produce obvious frequency shifts.Moreover,under the action of ideal sinusoidal waves,swells,fully grown wind waves,unsteady wind waves,or mixed waves,different moving rough sea surfaces create different acoustic scattering processes and possess different frequency shift characteristics.For the swell wave,which tends to be a single harmonic wave,the moving rough sea surface produces more obvious high-order scattering and frequency shifts.The same phenomena are observed on the sea surface under fully grown wind waves,however,the frequency shift slightly offsets the multiple peak frequencies of the wind wave spectrum.Comparing with the swell and fully-grown wind waves,the acoustic scattering and frequency shift are not obvious for the sea surface under unsteady wind waves.

On the Downshift of Wave Frequency for Bragg Resonance

For surface gravity waves propagating over a horizontal bottom that consists of a patch of sinusoidal ripples,strong wave reflection occurs under the Bragg resonance condition.The critical wave frequency,at which the peak reflection coefficient is obtained,has been observed in both physical experiments and direct numerical simulations to be downshifted from the well-known theoretical prediction.It has long been speculated that the downshift may be attributed to higher-order rippled bottom and free-surface boundary effects,but the intrinsic mechanism remains unclear.By a regular perturbation analysis,we derive the theoretical solution of frequency downshift due to third-order nonlinear effects of both bottom and free-surface boundaries.It is found that the bottom nonlinearity plays the dominant role in frequency downshift while the free-surface nonlinearity actually causes frequency upshift.The frequency downshift/upshift has a quadratic dependence in the bottom/free-surface steepness.Polychromatic bottom leads to a larger frequency downshift relative to the monochromatic bottom.In addition,direct numerical simulations based on the high-order spectral method are conducted to validate the present theory.The theoretical solution of frequency downshift compares well with the numerical simulations and available experimental data.

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.

Wave height estimation using the singular peaks in the sea echoes of high frequency radar

The popular methods to estimate wave height with high-frequency（HF） radar depend on the integration over the second-order spectral region and thus may come under from even not strong external interference. To improve the accuracy and increase the valid detection range of the wave height measurement, particularly by the smallaperture radar, it is turned to singular peaks which often exceed the power of other frequency components. The power of three kinds of singular peaks, i.e., those around ±1,±√2 and ±1√2 times the Bragg frequency, are retrieved from a one-month-long radar data set collected by an ocean state monitoring and analyzing radar,model S（OSMAR-S）, and in situ buoy records are used to make some comparisons. The power response to a wave height is found to be described with a new model quite well, by which obvious improvement on the wave height estimation is achieved. With the buoy measurements as reference, a correlation coefficient is increased to 0.90 and a root mean square error（RMSE） is decreased to 0.35 m at the range of 7.5 km compared with the results by the second-order method. The further analysis of the fitting performance across range suggests that the peak has the best fit and maintains a good performance as far as 40 km. The correlation coefficient is 0.78 and the RMSE is 0.62 m at 40 km. These results show the effectiveness of the new empirical method, which opens a new way for the wave height estimation with the HF radar.

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.

毫米波多内导体布喇格结构电磁特性研究

基于多模耦合理论,利用三维电磁仿真软件CST对毫米波双内导体和三内导体布喇格结构电磁特性分别进行了比较研究。结果表明:双内导体布喇格结构相比同轴布喇格结构可以抑制竞争模式,反射率稳定并接近1,对称性结构电磁特性优于非对称性结构;随着内导体距离同轴轴心距离的增大,双内导体布喇格结构频率响应带宽变宽,且当内导体距离同轴轴心距离较远时,带宽变宽趋势明显,而反射率值趋于稳定;对于三内导体布喇格结构,随着内导体距离同轴轴心距离的增大,频率响应带宽变窄,且当内导体距离同轴轴心距离较近时,带宽变宽趋势明显。因此,可根据实际需要恰当选择毫米波多内导体布喇格结构参数,拓宽其作为反射器或者滤波器的性能,提高模式的选择性及模式的纯度,改善布喇格结构的性能。

高速精确的光纤光栅信号解调方案设计

为提升光纤光栅的应用效果,为此,设计高速精确的光纤光栅信号解调方案。依据LPFG的光纤光栅信号解调原理,设计两个LPFG双边缘滤波器处理两路反射光,并且利用光电探测器探测光功率结果,以此解调光栅的中心波长;采用光纤干涉仪检测解调频率,对其进行正反扫描,依据光传播时延引起的解调误差和解调频率扫描方向之间的关联,完成解调结果补偿。测试结果显示:该方案具有较好可行性,光谱发生不同程度重叠时,光纤光栅信号的解调结果标准差均小于1.85 pm;光栅波长的解调误差均在-5~5 pm之间;有效降低解调误差,提升光纤光栅解调精度,可靠获取信号中的信息。

基于等强度悬臂梁的光纤Bragg光栅低频加速度传感器

由于粘贴于等强度悬臂梁的光纤Bragg光栅受振动质量块驱动而产生应变,一种光纤Bragg光栅低频加速度传感器被研制和定义为基座运动引起的单自由度系统的受迫振动模型。模型计算表明该加速度传感器的固有频率fn为16.96 Hz.固定于激振器的光栅加速度传感器受正弦激励。加入硅油后,光栅加速度传感器的频响范围从原来的3-11Hz增大为3-22 Hz.试验结果表明：当激励振动频率接近系统固有振动频率时,阻尼系统决定了传感器的工作范围。然而,由于悬臂梁的粗端固定在基座上,传感器的频响受限于悬臂梁的固有频率。

光纤Bragg光栅水听器特性及实验研究

论述了光纤Bragg光栅(FBG)水听器探头基元-(FBG)的传感特性,分析了FBG的耦合系数、反射率、反射带宽和栅长对光纤Bragg光栅水听器传感特性的影响·通过改进光纤Bragg光栅水听器探头封装结构,增加了其压力敏感系数·并将实验结果与标准水听器(压电型)比较,标定出光纤Bragg光栅水听器的声压灵敏度;对传感信号进行电路解调,得出了解调结果,结果显示与原始声波信号基本一致·试验表明,在1kHz^25kHz的声波检测范围,光纤Bragg光栅水听器响应平坦度好,信号输出稳定,证明文中采取的改进措施是有效的·

铰链互补结构的宽频段高灵敏度光纤光栅振动传感器

针对振动传感器工作频段与灵敏度相互制约的问题,设计了一种基于铰链互补结构的宽频段高灵敏度光纤光栅振动传感器。将两个“工作区”与谐振峰互补的振动传感单元进行组合,在保证高灵敏度测量的同时,扩展了传感器的工作频率范围。首先建立了传感单元的结构模型,通过力矩平衡方程,得出了影响传感器灵敏度和固有频率的因素,然后使用Ansys与SolidWorks联合仿真,优化了传感器结构参数,最后制作了传感器实物,搭建封装和测试系统完成封装和特性测试。实验结果表明:两个传感单元的谐振峰位于650Hz和925Hz,分别与传感单元2和传感单元1的“工作区”互补,将工作频率范围扩展到0~1500Hz,灵敏度优于410.4pm/g,测量分辨率为0.16mg。文中所设计的传感器具有灵敏度高、工作频段宽等优点,为微振动监测提供新的技术手段。

光纤Bragg光栅用于高频应变测试的研究(英文)

提出了一种传感部分采用光纤Bragg光栅(Fiber Bragg grating,FBG)传感器的高频应变测试新方法。设计并构建了基于非平衡马赫-曾德(Mach-Zehnder)干涉仪的高频解调系统。完整的测试系统由FBG传感器、高频应变产生装置和解调系统三部分组成。所产生的应变信号的频率可以通过专门的实验装置来控制,这是一种广泛用于测试材料机械性能的变形分离式霍布金逊压杆(Split Hopkinson PressureBar,SHPB)实验装置,应变持续的时间根据与撞击弹性等径的冲锤长度计算。借助高频振动信号研究了测试系统可测信号的频率范围。给出了振动和冲击实验结果,并进行了讨论。所设计的系统可以实时测试动态应变的幅值,实验结果表明,该系统对频率为8kHz以下的信号具有好的响应。

光纤Bragg光栅测量汽轮机叶片频率的试验研究

利用光纤Bragg光栅(FBG)来测量汽轮机叶片的模态频率.在无约束条件下,FBG测得叶片4阶模态频率分别为1005.00、2048.00、2297.25和2786.00 Hz.当叶片受约束时,FBG测得叶片3阶模态频率分别为921.50、1482.00和2454.75 Hz.为分析FBG测量结果的可靠性,采用加速度传感器(AT)进行对比测量,在无约束条件下其结果分别为1005.00、2048.00、2297.00和2786.00 Hz;在有约束条件下,其结果分别为917.00、1480.75和2454.25 Hz.结果表明,FBG和AT对叶片模态频率的测量结果十分吻合.通过研究FBG的解调和波分复用方式,实现两个FBG对叶片进行同时测量.因此FBG可组成传感网络,进行多点测量,用于汽轮机叶栅频率的测量.通过实现进入FBG和出射的光路非接触耦合方式,FBG可用于旋转部件的测量和叶片的动频测量.

基于拍频解调边孔光纤光栅传感系统研究

针对边孔光纤光栅传感器在井下高温高压环镜波长扫描分辨率低的问题,对传统拍频解调的传感光路进行改进,设计了一种适用于井下环镜的基于激光拍频的边孔光纤光栅压力传感系统,可以在井下进行长距离传感,有效的提高了其分辨率。并且结合自注入锁定技术,与传统拍频解调光路把激光器作为传感器不同,将传感器和光纤激光器分离,便于应用于井下环境。系统使用线性啁啾光纤布拉格光栅作为色散补偿单元和激光器一侧的反射镜,将光纤光栅由于压力引起的波长变化转变为谐振腔的腔长变化,提升了系统的检测灵敏度。解释了所用方法的结构和测量原理,分析和计算了测量的结果和精度。实验结果表明,该系统具有较高的灵敏度和测量精度,压力灵敏度为0.12553 MHz/MPa。所提方法具有解调系统简单,稳定性好,传感距离可调等优点。

光纤Bragg光栅在分离式Hopkinson压杆实验中的应用

光纤Bragg光栅用于高频应变的测试是一个重要研究课题．文中介绍了其高频解调原理,探讨了光纤光栅传感器用于分离式霍布金逊压杆(SHPB)实验装置中应变测试的相关问题；推导了弹性杆中传播的应力波σ与传感光纤光栅中光波相位的变化量Δ(?)之间的关系式,进而建立了含有光纤光栅传感变化量的弹性杆应力波计算公式；构建了基于光纤Bragg光栅的动态应变测试系统,对中心波长分别为1550nm和1545nm的裸光纤光栅传感器以及应变片,两种传感器粘贴在被撞击弹性杆(输入杆)表面测试应变的情形进行了比较研究,给出了实验结果．研究表明,光纤Bragg光栅在高频动态应变测量中具有很好的应用潜力和前景,测试方法和结论对于光纤Bragg光栅在高频动态应变的实际测试中具有积极意义．

THz波在空芯Bragg光纤中的传输特性

使用可调节边界条件傅里叶分解法,分析了蜘蛛网包层空芯布喇格光纤在THz波段的色散关系和损耗系数,计算了三种波形的THz脉冲在该光纤中传输时的频谱变化,得到了输出脉冲波形.分析计算结果表明,蜘蛛网包层空芯布喇格光纤存在与纤芯半径和传输长度有关的特征频率,其色散和损耗对于低于该特征频率的THz波频谱影响较大,相当于一个低频段有相移的高通滤波器,选择合适的纤芯半径,可以在短距离内有效地传输THz波脉冲.