Nonlinear-approximation technique for determining vertical ozone-concentration profiles with a differential-absorption lidar

A new technique is presented for the retrieval of ozone-concentration profiles (O(3)) from backscattered signals obtained by a multiwavelength differential-absorption lidar (DIAL). The technique makes it possible to reduce erroneous local fluctuations induced in the ozone-concentration profiles by signal noise and other phenomena such as aerosol inhomogeneity. Before the O(3) profiles are derived, the dominant measurement errors are estimated and uncertainty boundaries for the measured profiles are established. The off- to on-line signal ratio is transformed into an intermediate function, and analytical approximations of the function are then determined. The separation of low- and high-frequency constituents of the measured ozone profile is made by the application of different approximation fits to appropriate intermediate functions. The low-frequency constituents are approximated with a low-order polynomial fit, whereas the high-frequency constituents are approximated with a trigonometric fit. The latter fit makes it possible to correct the measured O(3) profiles in zones of large ozone-concentration gradients where the low-order polynomial fit is found to be insufficient. Application of this technique to experimental data obtained in the lower troposphere shows that erroneous fluctuations induced in the ozone-concentration profile by signal noise and aerosol inhomogeneity undergo a significant reduction in comparison with the results from the conventional technique based on straightforward numerical differentiation.     

Design for fully steerable dual-trap optical tweezers

A design for complete beam steering (in three dimensions) of one or two optical tweezers traps is presented. The two most important requirements for efficient and stable movement of an optical trap are identified. A detailed recipe for the construction of a movable optical tweezers trap that fulfills these requirements is given (exemplified with an inverted microscope). The system has been found to allow for precise and free movements of both traps in all three dimensions in a dual-trap optical tweezers configuration and to be robust and reliable, as well as forgiving of small misalignments in the optical system.     

Characterization and modeling of drift noise in Fourier transform spectroscopy: implications for signal processing and detection limits

A theoretical analysis of long-term drift noise in Fourier transform spectroscopy is presented. Theoretical predictions are confirmed by experiment. Fractional Brownian motion is employed as a stochastic process model for drift noise. A formulation of minimum detectable signal is given that properly accounts for drift noise. The spectral exponent of the low-frequency drift noise is calculated from experimental data. A frequency-dependent optimal spectrum averaging time is found to exist beyond which the minimum detectable signal increases indefinitely. It is also shown that the minimum detectable signal in an absorbance or transmission measurement degrades indefinitely with the time elapsed since background spectrum acquisition.     

基于ACPT技术的纳米振动测量激光干涉仪

为实现高精度、高稳定的微小振动测量,设计了一种改进型迈克尔逊干涉仪,对其所采用的光学结构和解调算法进行了分析.信号解调方案基于载波调制和交流相位跟踪（ACPT）技术,相对应的光路采用双压电陶瓷（PZT）和准平面猫眼动镜以构成参考臂和测量臂,设计以改善信噪比、抑制低频干扰、增强系统稳定性、提高测量分辨力为目标.建立了纳米振动发生装置,搭建了振动信号测量系统,研究了激光干涉仪对振动信号的输出特性.实验结果表明,测量动态范围达到120 dB,信噪比高于40 dB,系统分辨力达到0.25 nm.该干涉仪可有效抑制诸如温度、湿度、压强、低频振动等带来的相位噪声的干扰,满足高精度、稳定、可靠的要求.     

Tracking Control of a Nanopositioner Using Complementary Sensors

Piezoelectric tube actuators are widely used in atomic force and scanning tunneling microscopy (STM) for nanoscale positioning. There has been a consistent effort to increase the scan speed of these actuators using feedback control techniques. A feedback controller requires a measurement of the scanner's deflection, which is often provided by a capacitive sensor. Such measurements are corrupted by sensor noise, typically in the order of 20 pm/ radicHz rms. Over a bandwidth of 10 kHz, this translates into an rms noise of 2 nm, clearly inadequate for applications that require subnanometer positioning accuracy, e.g., STM. In this paper, we illustrate how the strain voltage induced in a free electrode of the scanner can be used as an additional displacement signal. The noise level corresponding to the strain signal is about three orders of magnitude less than that of a capacitive sensor, making it an ideal choice for nanopositioning applications. However, it cannot be used for dc and low-frequency measurements. A two-sensor-based controller is designed to use the capacitive sensor signal at low frequencies, and the strain displacement signal at high frequencies. By limiting the capacitive sensor feedback loop bandwidth to less than 100 Hz, the rms value of the noise is reduced to well below 1 nm. For almost the same noise level, the two-sensor-based control structure achieves a closed-loop bandwidth of more than three times that of the single-sensor-based controller.     

OFDM削波噪声迭代消除算法

高峰均功率比(PAPR)的OFDM信号通过功率放大器的时候会产生非线性干扰,同时降低了放大器的工作效率。传统削波算法可以降低信号的PAPR,但是会带来较大的频谱扩展。作为一种新的削波算法,误差削波可以在降低OFDM信号PAPR值的同时不带来任何频谱扩展。但是这种削波会给信号带来更大的带内干扰噪声。提出了一种OFDM削波噪声迭代估计和消除算法,它能有效的消除由于误差削波带来的噪声。新方法通过建立削波噪声模型,在接收端根据该噪声模型用迭代的方法重新产生削波噪声。仿真结果表明,使用削波噪声消除算法后,使得系统的误码率性能接近未削波信号的水平。     

基于实验室重现噪声的高速列车车厢主动降噪实验研究

为降低高速列车运行时的车厢内低频噪声,研究了车厢内大范围区域的噪声主动控制问题.针对高速列车运行实测噪声频谱与目标降噪区域尺寸(1.8 m×2.5 m×1.3 m),设计48通道的前馈主动控制系统.按照比较匀称的排列方式,次级声源布放在车厢内除底部的其余5个面上,误差点分布在目标区域.通过测量各声学路径传递函数,离线计...     

Stability of microelectromechanical devices for electricalmetrology

Microelectromechanical systems (MEMS) have been recently proposed for realizing several references in electrical metrology. Such devices are formed from micromachined electrodes of which at least one is supported by a compliant structure such that an electrostatic force between two electrodes displaces the moving electrode. The properties of these electromechanical devices can be very stable if they are fabricated from single-crystalline silicon and sealed hermetically in a low-pressure atmosphere. In comparison to several semiconducting reference devices, micromechanical components are large in size and consume a negligible power. Thus, a low 1/f noise level is expected. The proposed MEMS electrical references include a DC and an AC voltage reference, an AC/DC converter, a low-frequency voltage divider, a microwave and millimeter-wave detector, a DC current reference, etc. Measurements on a prototype for a MEMS DC reference are discussed. The stability is presently limited by charge fluctuations on the native oxides of electrode surfaces. Preliminary results show relative fluctuations below 1 μV/V     

Electrical characteristic analysis using low-frequency noise in low-temperature polysilicon thin film transistors

This study carried out an electrical characteristic analysis using low-frequency noise (LFN) in top gate p-type low-temperature polysilicon thin film transistors (LTPS TFTs) with different active layer thicknesses between 40 nm and 80 nm. The transfer characteristic curves show that the 40-nm device has better electrical characteristics compared with the 80-nm device. The carrier number fluctuation, with and without correlated mobility fluctuation model in both devices, has modeled well the measured noise. On the other hand, the trap density and coulomb scattering in the 40-nm device are smaller compared with the 80-nm device. To confirm the effectiveness of the LFN noise analysis, the trap densities at a grain boundary are extracted using in both devices the similar methods of Proano et al. and Levinson et al. That is, coulomb scattering, caused by the trapped charges at or near the interface, has a greater effect on the device with inferior electrical properties. Based on the LFN and the quantitative analysis of the trap density at a grain boundary, the interface traps between the active layer and the gate insulator can explain the devices' electrical degradation.     

Robust chaotic communications exploiting waveform diversity. part 2: Complexity reduction and equalisation

The symbolic dynamics method of communication using chaotic systems has previously been shown to offer performance better than conventional binary phase shift keying (BPSK), while also providing similar spectrum efficiency. This is achieved by exploiting diversity in the waveform, through a mechanism similar to partial response signalling. In order to achieve this performance, a correlation method for detection has been proposed, but complexity was high and only performance in additive white Gaussian noise (AWGN) channels has been considered. It is demonstrated that the complexity can be reduced from requiring 1024 correlations per symbol down to only 32, while degrading performance only by 0.6 dB. The ability to further reduce the occupied bandwidth is investigated. Spectrum occupancy equivalent to a root-raised cosine-filtered BPSK signal is demonstrated, and performance is maintained when the reference waveforms are similarly filtered to maintain the signal match. Finally, a novel equalisation technique that incorporates a decision feedback structure into the correlation detector is proposed. Performance in multipath channels is investigated and shown to be effective.     

风机盘管机组低频噪声故障分析及改善

为了降低风机盘管机组的低频噪声水平,采用鱼骨刺图、专家判断、Scatter plots,Two-Sample TTest,One-way ANOVA和仿真分析等方法对FCU03机组的低频噪声进行分析。结果表明,低频噪声不会随机外静压的变化而变化;机组的低频噪声与风速相关,低速档工作时,低频噪声问题更明显;提高风机座板的刚度,能够有效地降低风机盘管机组的低频噪声水平,提高产品质量。     

Ultra low-noise current sources

The solid state DC current sources available on the market are not suitable for applications in low noise measurement systems because of the high level of low-frequency noise introduced in the measurement chain. The most important cause of low-frequency noise in such instruments is the solid state device used as a voltage reference (usually a Zener diode). This problem has been solved, in the instrument described in this paper, by using a new circuit topology in which the solid-state voltage reference has been substituted by a low-noise battery. The instrument, capable of supplying a current as high as 100 mA, is characterized by a low-frequency noise level some orders of magnitude lower than that of similar commercial instrumentation     

阻尼处理降低重型卡车驾驶室低频噪声

提出一种基于模态分析的重卡驾驶室低频噪声控制方法,将其应用于一款在怠速工况下低频轰鸣噪声较严重的重型卡车上。首先对车内噪声的频谱特性进行测试分析,获取低频噪声的频率成分信息。然后对驾驶室白车身进行模态分析,确定噪声峰值频率与车身结构振动关系,并对该部位采用阻尼处理以降低结构辐射噪声。对处理后的试验车进行噪声评价测试,结果表明主要频率处的噪声峰值降低了5.4 dB(A)～7.5 dB(A),试验车驾驶室低频噪声得到有效控制。     

自适应卡尔曼滤波在桥梁健康监测系统中的应用

针对桥梁健康监测中的桥梁结构的应变模态指标，一般采用振弦式应变传感器来测量，其噪声源对桥梁检测来说属于低频振动，低频噪声会影响到采集信息的准确性。为此，将自适应卡尔曼滤波用于应变模态指标的信号处理中，根据桥梁检测信号的特征建立自适应动态滤波模型，并用该模型对甘肃某大桥的实际应变监测数据进行仿真。仿真结果表明，该模型能有效抑制低频噪声，正确反映桥梁应变模态指标的变化，提高信号的测量精度，为桥梁健康评估提供可靠数据。     

A comparative evaluation of several algorithms for phase aberrationcorrection

A common framework is presented to classify several phase correction techniques. A subset of these techniques are evaluated through simulations which utilize 2-D phase aberration profiles measured in the breast. The techniques are compared based on their ability to reduce phase errors, stability, and sensitivity to noise and missing elements in the transducer array. Significant differences are observed in these measures of performance when the size and location of the aperture area used to generate a phase reference signal are varied. Techniques that utilize a small correction reference region are more susceptible to noise and missing elements than techniques which use larger reference regions. The algorithms encounter problems in 2-D phase correction when making the transition from one row to the next, due to the low interelement correlation at the transition points. It is shown that the magnitude of the interelement correlation is the key parameter governing phase correction performance     

快速傅里叶变换FFT及其应用

利用快速傅里叶变换FFT将图像信号从空间域转换到频域进行分析,使快速卷积、目标识别等许多算法易于实现;然后根据图像信号的灰度结构特征和频谱分布,用Butterworth带通滤波器和二维维纳滤波器进行滤波处理,去除图像信号中的低频干扰和噪声信号;再利用傅里叶反变换将信号还原。结果显示,处理后的模拟远程高空卫星照片轮廓清晰可见。     

Micro-prober for wafer-level low-noise measurements in MOS devices

Low-frequency noise measurements represent an interesting investigation technique for the characterization of the quality and reliability of microelectronic materials and devices. Performing meaningful noise measurements at low and very low (f<1 Hz) frequencies, however, may be quite challenging, particularly because of the many sources of interference that superimpose on the noise signal. For this reason, packaged samples are preferred because they allow accurate shielding from the external environment, and because keeping the sample in close proximity to the low-noise biasing system and amplifier reduces microphonic and electromagnetic disturbances. Notwithstanding this, the possibility of performing low-frequency noise measurements at wafer level would be quite interesting, both because of the ease of obtaining wafer-level samples from industries with respect to packaged samples, and because this would avoid possible packaging-process induced device degradation. The purpose of this work is to demonstrate that it is, in fact, possible to design and build a dedicated probe system for performing high-sensitivity, low-frequency noise measurements on metal-oxide-semiconductor devices at wafer level.     

高架轻轨箱梁噪声辐射现场实测分析

对武汉轻轨一号线某高架段25 m跨单箱单室预应力钢筋混凝土简支箱梁的噪声辐射进行了现场测试。将采集的噪声信号利用快速傅里叶变换技术通过相干函数进行一定程度的净化,从而获得真实的结构辐射噪声。绘制了箱梁结构表面辐射声压级等值线图。比较分析了综合噪声和结构辐射噪声的A声级。分析结果表明:近轨时箱梁结构各部位的辐射噪声明显大于远轨时箱梁结构各部位的辐射噪声。在近轨和远轨两种工况下,底板和翼缘的辐射噪声都大于腹板。箱梁各部位的辐射声压级在跨长方向和桥宽度方向上都表现出非单一的变化规律,需结合列车运行工况具体分析。轻轨箱梁结构辐射噪声级在61~66 d B(A)之间,实际工程中为了获得真实的结构辐射噪声级需将测得的综合噪声级减去约2 d B(A)(背景噪声影响)。实测分析结果可让工程技术人员了解箱梁噪声辐射情况,为后期箱梁的降噪设计提供参考依据。     

Quantitative measurement of the magnetic moment of individual magnetic nanoparticles by magnetic force microscopy

The quantitative measurement of the magnetization of individual magnetic nanoparticles (MNPs) using magnetic force microscopy (MFM) is described. Quantitative measurement is realized by calibration of the MFM signal using an MNP reference sample with traceably determined magnetization. A resolution of the magnetic moment of the order of 10(-18) A m(2) under ambient conditions is demonstrated, which is presently limited by the tip's magnetic moment and the noise level of the instrument. The calibration scheme can be applied to practically any magnetic force microscope and tip, thus allowing a wide range of future applications, for example in nanomagnetism and biotechnology.     

2-D companding for noise reduction in strain imaging

Companding is a signal preprocessing technique for improving the precision of correlation-based time delay measurements. In strain imaging, companding is applied to warp 2-D or 3-D ultrasonic echo fields to improve coherence between data acquired before and after compression. It minimizes decorrelation errors, which are the dominant source of strain image noise. The word refers to a spatially variable signal scaling that compresses and expands waveforms acquired in an ultrasonic scan plane or volume. Temporal stretching by the applied strain is a single-scale (global), 1-D companding process that has been used successfully to reduce strain noise. This paper describes a two-scale (global and local), 2-D companding technique that is based on a sum-absolute-difference (SAD) algorithm for blood velocity estimation. Several experiments are presented that demonstrate improvements in target visibility for strain imaging. The results show that, if tissue motion can be confined to the scan plane of a linear array transducer, displacement variance can be reduced two orders of magnitude using 2-D local companding relative to temporal stretching.