离散频谱校正方法的综合比较

在综合分析五种离散频谱校正方法特点的基础上,通过仿真研究,得出在无噪声情况下比值校正法的校正精度高于其它几种方法在小噪声情况下,各种校正方法的校正精度大致相当;在大噪情况下,第二种相位差校正法(构造新序列,即将原时域序列前N/2点平移N/4点,将序列的前后N/4点置零,分别对原序列和新序列进行FFT分析,利用对应峰值谱线的相位差进行频谱校正的通用方法)的校正精度要高于其它几种方法.对于相隔较近的频率成分(不小于4个频率分辨率),两种相位差校正法的校正精度要高于其它几种方法.在小噪声的工程实际中,推荐简单方便的加Hanning窗的能量重心法(n=1或n=2)进行校正,幅值误差小于1%,频率误差小于0.02个频率分辨率,相位误差小于5度.     

A Method for Measurement of Losses in the Noise-Matching Microwave Network While Measuring Transistor Noise Parameters

A new method for measuring the loss of a tuner network used as the noise-source admittance transformer in a noise parameter test set is presented. Since the method is based on noise figure measurements, the tuner losses are determined on-line while performing measurements for determining transistor noise parameters. Experiments carried out on a coaxial slide-screw tuner by means of a computer-assisted measurement setup are reported.     

Perception of temporally filtered X-ray fluoroscopy images

For noisy X-ray fluoroscopy image sequences we quantitatively evaluated image quality after digital temporal filtering to reduce noise. Using an experimental paradigm called a reference/test adaptive forced-choice method we compared detectability of stationary low-contrast disks in filtered and unfiltered, computer-generated image sequences. In the first experiment, a low-pass first-order recursive filter used in X-ray fluoroscopy was found to be much less effective at enhancing detectability than predicted from the reduction of display noise variance, a common measurement of filter effectiveness. Detectability was reasonably predicted by a nonprewhitening human-observer model (NPW-HVS) that included an independently determined human temporal-contrast-sensitivity function. In another experiment, designed to test models over a range of temporal frequencies, we used paired high-pass and low-pass temporal filters that both reduced noise variance by 25%. The high-pass filter was artificially applied to the noise only and greatly improved detectability, while the low-pass filter had little effect. The human-observer model quantitatively described the measurements, but classical prewhitening and nonprewhitening signal detectors did not. As compared to the nonprewhitening, spatio-temporal matched filter, human-observer efficiency was low and variable at 2.1%, 2.9%, and 0.06% for 60 frames of unfiltered low-pass and high-pass noise, respectively. As compared to this detector, humans were not very effective at combining information across frames. On the other hand, signal to noise ratios (SNR's) from the human-observer model were comparable to human performance, and efficiencies were reasonably constant at 40%, 52%, and 32%, respectively. We conclude that it is imperative to include human-observer models and experiments in the analysis of noise-reduction filtering of noisy image sequences, such as X-ray fluoroscopy.     

Determination of the noise parameters of a linear 2-port

A new algorithm is given for the calculation of the four noise parameters of a linear 2-port from experimental measurements of its noise figure as a function of source admittance. A computer program for performing this calculation is briefly described (more detailed documentation is available).     

Degradation of Detectability of Signals in the Output of a Limited Amplifier Overloaded by a Sinewave

The output spectrum of a low-pass amplifier that has a limited linear region and is overloaded by the addition of a large sinewave to its input signals of interest, is determined from a simplified but realistic model. The validity of the model depends on both the signals of interest and the background noise being small compared with the linear range of the amplifier, and thus small compared with the sinewave that causes overloading. This enables the effect on these small components to be considered as that of a periodic gain modulation of the amplifier at twice the frequency of the interfering sinewave. The degradation in detectability of small signals is found to reach a plateau as the interfering wave increases. The degradation may be zero if the interference is at the high-frequency end of the input band and increases without limit as the interference approaches dc.     

Measurements of amplitude probability distributions and power of automobile ignition noise at HF

Measurements of the amplitude probability distribution (APD) of the envelope and measurements of other parameters of automobile ignition noise were made at frequencies between 24 and 30 MHz. The measurements were conducted at a quiet site where several single stationary vehicles were operated at engine speeds corresponding to idle and cruise. Measurements were also made at two distances near a freeway, for light traffic (approximately 20 vehicles per min) and for heavy traffic (approximately 45 vehicles per min). The principal instrumentation consisted of several phase-stable receivers with coherent quadrature detectors whose outputs were digitized at a rate of 200 samples per quadrature component per second and recorded on magnetic tape for computer processing. About 10 min of data (about 120 000 samples) were obtained during each measurement. The computer plotted the APD on a Rayleigh scale in dB relative to thermal noise and calculated the mean noise power available at the antenna terminals (related by a constant to the noise factor Fa) and Vd--the ratio in dB of rms to average voltage. The average power of the ignition noise increases with engine r/min and Vdusually decreases. Near a freeway, most of the noise is contributed by a small number of very noisy vehicles. The APD's for various situations are strikingly similar; all show that most of the noise envelope samples in a measurement are Rayleigh distributed. A small percentage of the noise samples do not follow the Rayleigh distribution, but are of higher amplitude. This percentage is higher for an engine at cruise speed than at idle speed, and higher for heavy traffic than for light traffic.     

The Effect of a Bandpass Nonlinearity on Signal Detectability

When an angle-modulated signal plus noise constitute the input to a bandpass device exhibiting a nonlinear input-output power characteristic and AM to PM conversion, the noise component of the output has altered first- and second-order statistics. A method of evaluating the two-dimensional first-order statistics of this noise is presented. The effect on signal detectability of a nonlinearity inserted between two channel noise sources is studied; expressions for the mean square received phase error and probability of error (for coherent digital phase modulation) are derived. The hard limiting satellite channel, with Gaussian noise on the up and down links, is examined in detail, and it is demonstrated that the limiter can significantly affect signal detectability.     

New denoising scheme for magnetic resonance spectroscopy signals

A new scheme for denoising magnetic resonance spectroscopy (MRS) signals is presented. This scheme is based on projecting noisy MRS signals in different domains, consecutively, and performing noise filtering operations in these domains. The domains are chosen such that the noise portion, which is inseparable from the desired signal in one domain, is separable in the other. A set of stable, linear, time-frequency (SLTF) transforms with different resolutions was selected for these projections as an example. Scheme evaluation was performed using extensive MRS signals with various noise levels. Compared with one domain denoising, it was observed that the proposed scheme gives superior results that compensate for the excess computational requirements. The proposed scheme supersedes also the wavelet packet denoising schemes.     

A linear correction for principal component analysis of dynamic fluorescence diffuse optical tomography images

The analysis of dynamic fluorescence diffuse optical tomography (D-FDOT) is important both for drug delivery research and for medical diagnosis and treatment. The low spatial resolution and complex kinetics, however, limit the ability of FDOT in resolving drug distributions within small animals. Principal component analysis (PCA) provides the capability of detecting and visualizing functional structures with different kinetic patterns from D-FDOT images. A particular challenge in using PCA is to reduce the level of noise in D-FDOT images. This is particularly relevant in drug study, where the time-varying fluorophore concentration (drug concentration) will result in the reconstructed images containing more noise and, therefore, affect the performance of PCA. In this paper, a new linear corrected method is proposed for modeling these time-varying fluorescence measurements before performing PCA. To evaluate the performance of the new method in resolving drug biodistribution, the metabolic processes of indocyanine green within mouse is dynamically simulated and used as the input data of PCA. Simulation results suggest that the principal component (PC) images generated using the new method improve SNR and discrimination capability, compared to the PC images generated using the uncorrected D-FDOT images.     

Maximum likelihood estimation for InSAR image co-registration

This paper presents a closed-form robust phase correlation based algorithm for performing image registration to subpixel accuracy. The subpixel translational shift information is directly ob- tained from the phase of the normalized cross power spectrum by using Maximum Likelihood Esti- mation （MLE）. The proposed algorithm also has slighter time complexity. Experimental results show that the proposed algorithm yields superior registration precision on the Cram@r-Rao Bound （CRB） in the presence of aliasing and noise.     

基于凸锥分析的低概率检测方法研究

低概率检测(LPD)方法利用图像互相关矩阵的特征向量,在已知目标先验信息的情况下检测图像中小概率目标.但是,由于噪声的影响以及特征向量之间正交约束性,使其检测效果不理想.本文提出了利用凸锥分析(CCA)来改善LPD的方法 ;它避免由特征向量的正交性约束导致的虚警概率较高的不良结果,同时消除图像中的条带噪声的影响.最后,结合OMIS数据分析了这种方法检测小目标的效果.     

Functional MRI activity characterization using response time shift estimates from curve evolution

Characterizing the response of the brain to a stimulus based on functional magnetic resonance imaging data is a major challenge due to the fact that the response time delay of the brain may be different from one stimulus phase to the next and from pixel to pixel. To enhance detectability, this work introduces the use of a curve evolution approach that provides separate estimates of the response time shifts at each phase of the stimulus on a pixel-by-pixel basis. The approach relies on a parsimonious but simple model that is nonlinear in the time shifts of the response relative to the stimulus and linear in the gains. To effectively use the response time shift estimates in a subspace detection framework, we implement a robust hypothesis test based on a Laplacian noise model. The algorithm provides a pixel-by-pixel functional characterization of the brain's response. The results based on experimental data show that response time shift estimates, when properly implemented, enhance detectability without sacrificing robustness.     

Precision measurement method for cryogenic amplifier noise temperatures below 5 K

We report precision measurements of the effective input noise temperature of a cryogenic (liquid-helium temperature) monolithic-microwave integrated-circuit amplifier at the amplifier reference planes within the cryostat. A method is given for characterizing and removing the effect of the transmission lines between the amplifier reference planes and the input and output connectors of the cryostat. In conjunction with careful noise measurements, this method enables us to measure amplifier noise temperatures below 5 K with an uncertainty of 0.3 K. The particular amplifier that was measured exhibits a noise temperature below 5.5 K from 1 to 11 GHz, attaining a minimum value of 2.3 K/spl plusmn/0.3 K at 7 GHz. This corresponds to a noise figure of 0.034 dB/spl plusmn/0.004 dB. The measured amplifier gain is between 33.4 dB/spl plusmn/0.3 dB and 35.8 dB/spl plusmn/0.3 dB over the 1-12-GHz range.     

Threshold vector field detectors

The problem of threshold or weak-signal detection in highly nonGaussian EMI is extended to vector fields, and narrow-band signals and interference. The emphasis is on a canonical formulation, illustrated by a number of specific examples. Spatial sampling with adaptive beam forming, as well as temporal sampling and all relevant vector field components, must be included for maximum processing gain. New results for a canonical theory of these vector detection cases are presented. Jointly and asymptotically locally optimum algorithms and performance measures are obtained. These results provide statistical-physical models of the EMI environment, and they include first-order probability distributions of vector EMI noise fields and received processes, with specific examples of EMI fields generated by randomly distributed electric and magnetic dipole sources, as well as more general sources. The effects of beamforming, selfdirecting beams, multiple field components, fading, and Doppler `smear' on signal detectability are included     

Fast, sensitive magnetic-field sensors based on the Faraday effectin YIG

Magnetic-field sensors based on the Faraday effect in ferrimagnetic iron garnets are investigated in terms of their sensitivity, speed, and directionality. Signal-to-noise measurements at 80 Hz on small (typically 5-mm-diameter×3-mm-long) samples of yttrium iron garnet (YIG) yield noise equivalent magnetic fields of 10 nT/√Hz. Frequency-response measurements exhibit virtually flat response to approximately 700 MHz     

Averaging logarithms for detection and estimation (Corresp.)

In a number of applications in signal processing it is desirable to estimate the logarithm of the average of noisy physical measurements by averagingMindependent logarithms of the measurements. Only a modest number of bits are necessary in the logarithmic averaging process, which is used in logarithmic radar receivers to improve the effective dynamic range where post-detection averaging occurs, and in ensemble averaging spectral coefficients prior to the detection decision. A knowledge of the estimation error and of the detectability loss becomes important in these applications. This correspondence extends and generalizes Green's work in detection using averaged logarithms and presents an averaging loss methodology for continuousM. It gives numerical detectability loss results for a representative detection criterion. It contains further results on estimation accuracy attainable from summed logarithms, including numerical results for estimation error as a function ofMand SNR.     

数字粉红噪声生成研究

介绍了利用DSP方法和Voss算法生成数字粉红噪声的原理,并对生成的粉红噪声性能参数进行了比较和评价,推荐了各种方法的应用场合。     

A novel prediction method of acoustic magnetic noise based oninduction motor's NHCC function

When an induction motor is driven by a pulsewidth modulation (PWM) inverter, acoustic magnetic noise caused by harmonics of the induction motor input is a serious environmental problem. In this paper, a novel prediction method of acoustic magnetic noise is proposed. Acoustic magnetic noise emitted from an induction motor is considered as the function of harmonic components of induction motor current. The acoustic magnetic noise versus harmonic current characteristic (NHCC) function is defined. This NHCC function describes the response of acoustic magnetic noise to a single-frequency harmonic current. Based on the NHCC function, acoustic magnetic noise can be predicted very easily and accurately, simply by the calculation of harmonic components of the induction motor current. By making use of this method, the difficulties of the conventional prediction method, such as the complex calculations of magnetic force, mechanical resonant frequencies, and sound radiation efficiency can be avoided. This novel method can be used as a tool for environmental noise management and evaluation of low-acoustic magnetic noise characteristics of different inverter PWM control methods     

Long term noise measurements and median time to failure test for the characterization of electromigration in metal lines

The high sensitivity, which can be obtained by means of noise measurements, is especially useful for the characterization of the early stages of the electromigration phenomenon. In addition, with the noise being strictly dependent on the sample microstructure, it is obvious to expect that it can be used for monitoring the different stages of the degradation process. In this paper, the results obtained by performing noise measurements during lifetime tests are presented, with the aim of investigating the evolution of the electromigration noise during the entire life of the sample. Conventional Median Time to Failure (MTF) tests have been performed on samples belonging to the same set. One of the aims of this work is to investigate the possibility of establishing a new failure criterion based on noise measurement capable of providing, in a shorter time, the same type of information normally obtained from MTF tests.