Effect of random surface errors on radiation characteristics of the side-fed offset Cassegrain antenna

In this paper the average power pattern of the side-fed offset Cassegrain (SFOC) dual reflector antenna is analyzed, and the effect of the random surface error on radiation characteristics of the antenna is introduced. Here, the random surface error is defined as the error of the standard reflector in its normal direction and the errors in a small zone of the reflector are considered as equal. We also assume that the phase error on the aperture led by the random surface error obeys a Gaussian distribution with zero mean, under which the expression of the average power pattern is deduced. Finally, the data related to the radiation characteristics of the antenna are calculated and the corresponding curves are presented. The obtained results can be used for the user to determine the manufacturing accuracy of the reflector of the SFOC antennas. __________ Translated from Journal of Xidian University (Natural Science Edition), 2005, 32(6): 865–869 (in Chinese)     

A switch‐utilization‐improved switched‐inductor switched‐capacitor converter with adapting stage number

A novel closed‐loop switched‐inductor switched‐capacitor converter (SISCC) is proposed by using the pulse‐width‐modulation (PWM) compensation for the step‐up DC–DC conversion/regulation, and together by combining the adaptive‐stage‐number (ASN), control for the higher switch utilization and wider supply voltage range. The power part of SISCC is composed of two cascaded sub‐circuits, including (i) a serial‐parallel switched‐capacitor circuit with n_c pumping capacitors and (ii) a switched‐inductor booster with m_c resonant capacitors, so as to obtain the high step‐up gain of (n_c + 1) × m_c /(1 ? D) at most, where D is the duty cycle of PWM adopted to enhance output regulation as well as robustness to source/loading variation. Besides, the ASN control is presented with adapting the stage number n (n = 0, 1, 2, …, n_c) of pumping capacitors to obtain a flexible gain of (n + 1) × m_c /(1 ? D), and further in order to make the SISCC operating at a properly small duty cycle for improving switch utilization and/or supply voltage range. Some theoretical analysis and control design include formulation, steady‐state analysis, ASN‐based conversion ratio, efficiency, output ripple, stability, inductance and capacitance selection, and control design. Finally, the performance of this scheme is verified experimentally on an ASN‐based SISCC prototype, and all results are illustrated to show the efficacy of this scheme. Copyright © 2015 John Wiley & Sons, Ltd.     

Signal Decay Correction in 2D Ultra-Short Echo Time Imaging

Objectives: In ultra-short echo time (UTE) imaging, the transverse magnetization experiences significant decay during data acquisition, which gives rise to a loss of signal intensity and spatial resolution in conventionally reconstructed images. The present work proposes an iterative algorithm to correct these adverse effects. Materials and methods: The algorithm involves solving a large linear system of equations and requires a separate reference scan to map the spatially variant transverse relaxation time. It was implemented and applied to simulations and to experiments with custom-built resolution phantoms. The evaluation focused on its ability to improve the actual point-spread function (PSF) and on its influence upon the local signal-to-noise ratio (SNR). Results: The algorithm is demonstrated to virtually restore the ideal PSF of the acquisition. It proves to provide images with better signal intensity and spatial resolution but reduced SNR, if the transverse relaxation time is known with sufficient accuracy. Conclusions: The present work shows the basic feasibility of correcting signal decay effects in UTE imaging.     

基于GaN器件Buck电路死区功耗分析与优化

氮化镓功率器件以其优异的高速、高效特性而有望在电源转换领域取得广泛应用。在Buck开关电源应用中,系统采用GaN HEMT替换传统Si功率器件后,系统死区损耗成为阻碍系统效率提升的一个重要因素。针对GaN器件的电源转换系统死区功耗展开理论及仿真讨论,详细分析Si功率器件与GaN HEMT在buck型开关电源系统中不同的工作机制以及死区时间对系统功耗的影响。优化结果表明,输入电压为12 V、输出电压为1.2 V、开关频率为700 k Hz的GaN基电源转换系统,在死区时间Td1=20 ns、Td2=0 ns、负载电流为20 A的情况下系统转换效率可达到92%。     

椭球反射面聚焦天线的设计和仿真

本文根据聚焦要求,采用物理光学法分别设计了工作在Ka和Ku波段的椭球反射面天线,计算了正馈和偏馈两种方式下的反射面的焦场,并用电磁场仿真软件进行验证,结果表明理论计算与软件仿真结果一致。同时,对影响焦斑大小的因素进行了分析和比较,结果表明无遮挡的偏馈方式的汇聚效果稍优于正馈,口面越大聚焦效果越好。     

A gain/efficiency‐enhanced bidirectional switched‐capacitor DC‐DC converter

A closed‐loop gain/efficiency‐enhanced bidirectional switched‐capacitor converter (BSCC) is proposed by combining an adaptive‐conversion‐ratio (ACR) phase generator and pulse‐width‐modulation (PWM) controller for bidirectional step‐up/down DC‐DC conversion and regulation. For realizing gain‐enhanced, the power part consists of one m_c‐stage cell and one n_c‐stage cell in cascade between low‐voltage (LV) and high‐voltage (HV) sides to boost HV voltage into m_c × n_c times voltage of LV source at most, or convert LV voltage into 1/(m_c × n_c) times voltage of HV source at most. For realizing efficiency‐enhanced, the ACR idea with adapting stage number m, n is built in the phase generator to obtain a suitable step‐up/down gain: m × n or 1/(m × n) (m = 1, 2, …, m_c, n = 1, 2, …, n_c). Further, the output regulation and robustness to source/loading variation can be enhanced by PWM on the LV/HV sides. Some theoretical analysis and control design are included as: modeling, steady‐state analysis, conversion ratio, efficiency, capacitance selection, and control design. Finally, the performance of this scheme is verified experimentally on a BSCC prototype, and all results are illustrated to show the efficacy of this scheme. Copyright © 2012 John Wiley & Sons, Ltd.     

一种固定指向多波束天线安装测量技术研究

某固定指向多波束天线由主反射体和馈源系统组成,天线主反射体固定且主波束指向唯一,馈源系统可以沿倾斜轨道运动以匹配不同的卫星。与全动或限动式天线相比,固定指向多波束天线结构指标在追求主反射面具有高精度的表面精度的同时,要保证其主波束指向精度,并实现馈源系统的精确定位。本文提出一种综合运用静态GPS测量系统、全站仪测量系统、激光跟踪仪测量系统、数字工业摄影测量系统等多种空间测量技术的方案。在该多波束天线安装过程中采用此测量方案,主反射面表面精度优于±0.15㎜,主波束指向精度优于±0.0122°,馈源系统定位精度优于±1.5㎜,满足结构安装指标要求。通过多波束天线系统测试电性能指标满足要求,充分验证了该方案的可行性、可靠性、高效率和高精度的特点。     

Chaotic multi-objective optimization based design of fractional order PIλDμ controller in AVR system

In this paper, a fractional order (FO) PI^λD^μ controller is designed to take care of various contradictory objective functions for an automatic voltage regulator (AVR) system. An improved evolutionary non-dominated sorting genetic algorithm II (NSGA II), which is augmented with a chaotic map for greater effectiveness, is used for the multi-objective optimization problem. The Pareto fronts showing the trade-off between different design criteria are obtained for the PI^λD^μ and PID controller. A comparative analysis is done with respect to the standard PID controller to demonstrate the merits and demerits of the fractional order PI^λD^μ controller.     

Detection and tracking of short duration variations of power system disturbances using modified potential function

This study introduces a modified potential function based modeling approach for real-time disturbance tracking. Real-time tracking requires the least complex techniques for processing and classification and still provide accurate results. Time required to detection of the power quality disturbance should be small enough so that the further mitigation action may be taken. The proposed FRTD algorithm is able to detect the disturbances in 3–5 samples. The proposed FRTD algorithm is principally based upon potential function, which has been modified so as to give the one-dimensional position vector corresponding to the voltage sag and swell, with complexity O(1) with no pre-process complexity. The method detects the transient in 3–5 samples at sampling frequency of 12.8 kHz. Potential function uses the current nonlinear drift velocity vector along with a prior knowledge of the power system disturbance to compute the unknown parameter in the form of diffusion matrix, D_t. The parameter D_t has been used to detect the events in the signal. Magnitude of D_t play vital role to detect the events. FRTD algorithm has been tested to track and detect the power system disturbances such as voltage sag, swell, and transient. The algorithm has been tested on hardware interfaced with MATLAB. 1000 samples of different types of power system disturbances such as voltage swell, voltage sag, oscillatory-transient and impulsive-transient have been tested. FRTD algorithm is having the efficiency of 99.83%.     

Switched‐capacitor‐voltage‐multiplier boost DC–AC inverter with adaptive stages

A multistage switched‐capacitor‐voltage‐multiplier inverter (SCVMI) is proposed with a variable‐conversion‐ratio phase generator and a sinusoidal pulse‐width‐modulation controller for boost DC–AC conversion and high‐efficiency regulation. Its power unit contains: SCVM booster and H‐bridge. The SCVM booster includes two m_c‐stage switched‐capacitor cells and two n_c‐stage switched‐capacitor cells in the interleaving operation to realize DC–DC boost gain of m_c × n_c at most. Here, the variable‐conversion‐ratio phase generator is suggested and adopted to change the running stage number and topological path for a suitable gain level of m × n (m = 1, 2, ?,m_c, n = 1, 2, ?,n_c) to improve efficiency, especially for the lower AC output. The H‐bridge is employed for DC–AC conversion, where four switches are controlled by sinusoidal pulse‐width‐modulation not only for full‐wave output but also for output regulation as well as robustness to source/loading variation. Some theoretical analysis and design include: SCVMI model, steady‐state/dynamic analysis, conversion ratio, power efficiency, stability, capacitance selection, output filter, and control design. Finally, the closed‐loop SCVMI is simulated, and the hardware circuit is implemented and tested. All the results are illustrated to show the efficacy of this scheme. Copyright © 2011 John Wiley & Sons, Ltd.     

Design and analysis of a novel SAW sensor for civil structural health monitoring

This paper presents a novel surface acoustic wave sensor for civil structural health monitoring. The sensing chip packaged in a shell is composed of two SAW delay line dice. One is utilized to sense the pressure induced by a gauge, and the other is used to sense the surrounding temperature for temperature compensation. Each SAW delay line die consists of an inter-digital transducer and two open grating reflectors. The SAW devices are analyzed by using coupling of modes model. Then, the devices in different parameters are fabricated on a 128°Y-X cut LiNbO₃ substrate by lift-off technique. Finally, a network analyzer is utilized to test the devices. Large signal-to-noise ratio, low loss and clear sharp peaks of the reflection coefficients S₁₁ in frequency and time domain can be observed. The experimental results matched well with the theoretical analysis by the finite element software COMSOL. The IDT finger pairs number, electrodes number of per reflector and acoustic aperture effect the reflection coefficient S₁₁ is discussed in detail.     

Design of triple‐band antenna using S‐shaped patch fed by cross strip line for WLAN and WiMAX applications

This paper presents a simple triple‐band S‐shaped patch antenna fed by a cross strip line for both WLAN and WiMAX applications. It is operated at the triple bands of 2.4 and 5.2 GHz for WLAN and 3.5 GHz for WiMAX. The antenna, designed on an FR4 substrate with a thickness of 1.6 mm and relative permittivity of 4.4, is fed by a 50‐Ω microstrip line, and is of size 25 × 35 mm. The simulated and measured results of |S₁₁|, gains, and radiation patterns are presented. The measured results show that the triple‐band antenna achieves a broad operating bandwidth of 2.36–2.54, 3.27–3.69, and 5.16–5.48 GHz for a 10‐dB return loss (i.e. |S₁₁|<−10 dB). The gains of the antenna measured at 2.4, 3.5, and 5.2 GHz frequencies are 1.87, 1.95, and 3.82, respectively. The radiation pattern of the antenna is omnidirectional. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

SVPWM逆变器供电时FSCW-PMSM损耗特性研究

为探究空间矢量脉宽调制(space vector pulse width modulation,SVPWM)逆变器引入的时间谐波电流对分数槽集中绕组永磁同步电机(fractional-slot concentrated-winding permanent magnet synchronous machine,FSCW-PMSM)损耗特性的影响,文中提出一种SVPWM逆变器与FSCW-PMSM联合系统中电机损耗谐波特性分析方法。首先,理论分析了考虑时间谐波电流时电机损耗的时空谐波特性。其次,以一台三相双层绕组10极12槽PMSM为例,搭建电机与SVPWM逆变器的场路联合仿真模型,求解样机在恒转矩和恒功率调速时损耗的谐波特性,揭示各谐波损耗的产生机理。结果表明：转子谐波损耗由基波电流与次谐波、iZ±p次齿谐波和(f_c±4f)/f、(f_c±2f)/f次时间谐波电流与p、Z±p次空间谐波引起;定子铁芯谐波损耗由基波磁场、谐波励磁磁场及(f_c±4f)/f、(f_c±2f)/f次时间谐波电流引起。该结论适用于其他极槽组合的FSCW-PMSM。最后,通过实验验证了该分析方法的有效性。     

Lp-stability of estimation errors of kalman filter for tracking time-varying parameters

The Kalman filtering algorithm, owing to its optimality in some sense, is widely used in systems and control, signal processing and many other fields. This paper presents a detailed analysis for the L_p-stability of tracking errors when the Kalman filter is used for tracking unknown time-varying parameters. The results of this paper differ from the previous ones in that the regression vector (in a linear regression model) or the output matrix (in state space terminology) is random rather than deterministic. The context is kept general so that, in particular, the time-varying parameter is allowed to be unbounded, and no assumption of stationarity or independence for signals is made.     

Experimental and Numerical Investigations on Hydrodynamics of Nanofluid Flowing in Coiled Tubes

Abstract

A numerical model was developed to analyze the hydrodynamics of TiO₂-water nanofluids flowing through straight and coiled tubes in laminar flow regime. Pressure drop of nanofluid predicted from this model was validated with experiments for identical dimensions of tubes and process conditions. The velocity contours of flow in straight as well as coiled tube were examined. The present study revealed that there was 74% decrease in pressure drop when the straight tube diameter was increased from 0.003?m to 0.0072?m. There was 46–51% decrease in friction factor when curvature ratio (D_c/d_t) was increased from 7.54 to 12.24 for 200<De?<?850.     

Oriented growth of Bi3.25La0.75Ti3O12 thin films on RuO2/SiO2/Si substrates by using the polymeric precursor method: Structural, microstructural and electrical properties

c-axis oriented Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were grown on a RuO₂ top electrode deposited on a (100) SiO₂/Si substrate by the polymeric precursor method. X-ray diffraction and atomic force microscope investigations indicate that the films exhibit a dense, well crystallized microstructure having random orientations with a rather smooth surface morphology. The electrical properties of preferred oriented Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films deposited on RuO₂ bottom electrode leaded to a large remnant polarization (P _r ) of 17.2 μC/cm² and (V _c ) of 1.8 V, fatigue free characteristics up to 10¹⁰ switching cycles and a current density of 2.2 μA/cm² at 5 V. We found that the polarization loss is insignificant with nine write/read voltages at a waiting time of 10,000 s. Independently of the applied electric field the retained switchable polarization approached a nearly steady-state value after a retention time of 10 s.     

Dielectric properties of nanocrystalline barium zirconate titanate synthesized by glycine-nitrate autocombustion

Abstract

Dielectric properties of nanocrystalline barium zirconate titanate (Ba(Zr_xTi_1-x)O₃; BZT for x?=?0.1 and 0.3) synthesized by glycine-nitrate autocombustion method were investigated in this study. The phase formation examined by TGA-DTA, XRD, FT-IR and Raman spectroscopy confirmed that high purity single-phase BZT with perovskite structure was obtained by using glycine-to-nitrate molar ratio of 2.2:4 and calcining in air at 1100?°C for 4?h. TEM analysis showed that BZT had agglomerate particles consisted of primary spherical nanocrystals with the size of 8-11?nm. The diffuse phase transition behavior of BZT ceramics increased with increasing Zr concentration and for x?=?0.3, the Curie temperature; T_c, shifted to below room temperature. The BZT ceramics for x?=?0.1 had relatively high dielectric constant (ε), 13007, low T_c, 76?°C and comparable dielectric loss (tan δ) at T_c, 0.012 which caused by the high degree of Zr diffuseness into the perovskite structure. These results suggest that glycine-nitrate autocombustion is the effective method for preparing high quality BZT ceramics.     

High frequency response of Bragg reflector type film bulk acoustic wave resonator

Abstract

We fabricated a Bragg reflector type FBAR using AlN piezoelectric with quarter wavelength thickness, where the Bragg reflector was composed of W-SiO₂ pairs. By numerical simulation considering actual acoustic losses of each layer, we analyzed the frequency response of the resonator and could explain it using an equivalent circuit with parasitic elements. The Effective electromechanical coupling constant (K ² _eff ) and the Quality factor (Q_s ), figures of merit of the resonator, were about 1.1% and 307, respectively.</ab>     

Simulation of slow current transients and current collapse in GaN FETs

Two-dimensional transient simulations of GaN MESFETs are performed in which a three-level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor and a deep acceptor are considered. When the drain voltage V _D is raised abruptly (while keeping the gate voltage V _G constant), the drain current I _D overshoots the steady-state value, and when V _D is lowered abruptly, I _D remains a low value for some periods, showing drain-lag behavior. These are explained by the deep donor’s electron capturing and electron emission processes. We also calculate a case when both V _D and V _G are changed abruptly from an off point, and quasi-pulsed I-V curves are derived from the transient characteristics. It is shown that the drain currents in the pulsed I-V curves are rather lower than those in the steady state, indicating that so-called current collapse could occur due to deep levels in the semi-insulating buffer layer. It is also shown that the current collapse is more pronounced when V _D is lowered from a higher voltage during turn-on, because the trapping effects become more significant.     

Effect of substrate surface modification using Ag nano-dots on the improvement of J c and microstructures in YBa2Cu3O7 thin films grown on LaAlO3 (100) by pulsed laser deposition

YBa₂Cu₃O₇ (Y123) thin films were grown by pulsed laser deposition (PLD) on LaAlO₃ (100) substrates whose surfaces were modified by a discontinuously layer of Ag nano-dots. The Y123 films were characterised by atomic force microscope, X-ray diffraction, scanning electron microscope, and DC magnetization measurements. Effect of substrate surface modification using various densities of the Ag nano-dots on the improvements of critical current density J _c and microstructures in the Y123 films has been studied systematically. The results showed that at fixed physical deposition conditions J _c increased with the number of Ag shots, n. Zero field J _c at 77 K increased from 10⁶ to 3.3 × 10⁶ A/cm², and from 1.5 × 10⁷up to 4 × 10⁷A/cm² for 5 K as the number of Ag shots increased from zero to 150. However, a fluctuation of J _c was observed for n < 60 at 77 and 40 K in both low and high fields. Detailed microstructure analysis revealed that ab misalignment was gradually improved as Ag nano-dots density gradually increased and believed to be responsible for the J _c enhancement.