Errors and uncertainties in the measurement of ultrasonic wave attenuation and phase velocity

This paper presents an analysis of the error generation mechanisms that affect the accuracy of measurements of ultrasonic wave attenuation coefficient and phase velocity as functions of frequency. In the first stage of the analysis we show that electronic system noise, expressed in the frequency domain, maps into errors in the attenuation and the phase velocity spectra in a highly nonlinear way; the condition for minimum error is when the total measured attenuation is around 1 Neper. The maximum measurable total attenuation has a practical limit of around 6 Nepers and the minimum measurable value is around 0.1 Neper. In the second part of the paper we consider electronic noise as the primary source of measurement error; errors in attenuation result from additive noise whereas errors in phase velocity result from both additive noise and system timing jitter. Quantization noise can be neglected if the amplitude of the additive noise is comparable with the quantization step, and coherent averaging is employed. Experimental results are presented which confirm the relationship between electronic noise and measurement errors. The analytical technique is applicable to the design of ultrasonic spectrometers, formal assessment of the accuracy of ultrasonic measurements, and the optimization of signal processing procedures to achieve a specified accuracy.     

"Real time" noise measurements with sensitivity exceeding the standard thermal noise limit

A possibility of "real-time" noise measurements with spectral resolution better than a standard thermal noise limit has been experimentally demonstrated at microwave frequencies. The enhancement in the sensitivity of spectral measurements was achieved due to more efficient use of the signal power via the power recycling technique. By utilizing such a technique, the noise floor of a 9 GHz "real time" measurement system was reduced by 3 dB below the standard thermal noise limit. This makes possible the characterization of intrinsic fluctuations in individual low-noise microwave components, such as ferrite circulators, without the need for cascading them or using the cross-correlation signal processing.     

Modeling phase noise in frequency dividers

A theory for modeling noise in frequency dividers and the measurements to support the theory are presented. The most complete measurements were made on ECL (emitter-coupled logic) dividers for which the primary noise contributions were from additive output noise and sampled additive input noise. Output phase power spectral density due to the latter varies as the square of the input frequency and is inversely proportion to the output frequency. The third most significant contributor was sampled output noise. A summary of available noise data is also given.     

A new method for estimating the aperture uncertainty of A/Dconverters

Aperture uncertainty measurements are strongly influenced by quantization, converter nonlinearity and test setup noise. The proposed method solves the difficulties arising from quantization, and nonlinearity. The method estimates the noise distribution function by fine adjustment of the input signal offset. The contribution of jitter-induced voltage noise is separated from additive noise in two different ways. Finally, the aperture uncertainty, assumed independent of the frequency, is estimated by varying the signal frequency. Experimental results obtained on 8 and 10 bit converters are discussed     

Frequency stability characterization from the filtered signal of aprecision oscillator in the presence of additive noise

The authors present a generalized theory to express the frequency stability characterization of a precision oscillator when its signal, perturbed by additive noise, is filtered. The general expressions for the power spectral density of the amplitude and phase fluctuations of the filtered signal are calculated as functions of the oscillator amplitude and phase fluctuations, the additive noise, and the filter characteristics. The results obtained for the phase fluctuations of the filtered signal are used to characterize the frequency stability of the oscillator. The contribution of white additive noise to the generalized Allan variance is expressed in terms of a parameter, the equivalent bandwidth. The contributions of other types of noise are also calculated. For the first-order low-pass filter, the contributions of all types of additive, amplitude, phase, and frequency noise are given. Experimental results show excellent agreement with the theoretical predictions     

Correlation between upper and lower sidebands

Experimental measurements supported by a simple model show that the upper and lower phase modulation (PM) noise sidebands are always equal and 100% correlated, independent of the noise power originating from multiplicative or additive processes. Similarly, we show that the upper and lower amplitude modulation (AM) noise sidebands are also equal and 100% correlated, independent of the noise power originating from multiplicative or additive processes, Moreover, the single sideband (SSB) PM noise is always equal to one-half the total PM noise. The same is true for the AM noise. Although the upper and lower PM or AM noise sidebands are equal and correlated for broadband additive noise, the phase between the AM and the PM sidebands varies randomly with time. These conclusions still hold even when the RF noise sidebands are not symmetric about the carrier.     

光丝同轴激光增材制造研究进展

增材制造作为实现三维结构快速成形的技术,广泛用于航空航天、汽车交通等领域.当前,激光熔丝增材制造多依托于传统的激光焊接设备,采用旁轴送丝方式,在增材过程中,需调整激光头方向保证送丝和焊接头行进的相对方位,增大了复杂构件增材制造系统控制难度,损失了加工自由度.随着激光加工设备和技术的发展,近年来出现了一种可以解决上述问题...     

调幅信号谱相关密度分析中白噪声影响的研究

重点研究了加性高斯白噪声对调幅信号谱相关密度分析结果的影响。平稳信号的谱相关密度仅存在于频率轴上，但是由于谱相关密度分析为非线性二次变换，信号和噪声的交叉项也将干扰分析结果。本文通过对加性高斯白噪声干扰的调幅信号的理论分析，得到了加性噪声干扰在谱相关密度双频率平面上的分布特点，总结了该类型循环平稳信号谱相关密度图的分布规律，最后利用仿真信号验证了理论分析的正确性。     

噪声对阶跃信号上升时间测量不确定度的影响

为了更准确地反映阶跃信号上升时间测量过程中可能存在的不确定度来源、提高测量结果的可信度,对噪声引入的阶跃信号上升时间测量不确定度分量开展了数值仿真和试验分析。结果表明,信噪比对上升时间测量不确定度存在显著影响,在信噪比较低的情况下,噪声引起的上升时间测量不确定度成为主要来源。通过比较Savitzky?Golay平滑算法和移动平均平滑算法在阶跃信号上升时间计算中的应用效果,发现采用适当的平滑算法可以降低噪声引入的不确定度分量,其中,移动平均平滑算法的效果更好。本研究有助于提高动态信号测量结果的可靠性和准确性,对航空、航天、汽车等领域的动态信号测试不确定度分析具有重要的参考价值。     

Single- and multiple-pulse noncoherent detection statistics associated with partially developed speckle

Single- and multiple-pulse detection statistics are presented for aperture-averaged direct detection optical receivers operating against partially developed speckle fields. A partially developed speckle field arises when the probability density function of the received intensity does not follow negative exponential statistics. The case of interest here is the target surface that exhibits diffuse as well as specular components in the scattered radiation. An approximate expression is derived for the integrated intensity at the aperture, which leads to single- and multiple-pulse discrete probability density functions for the case of a Poisson signal in Poisson noise with an additive coherent component. In the absence of noise, the single-pulse discrete density function is shown to reduce to a generalized negative binomial distribution. The radar concept of integration loss is discussed in the context of direct detection optical systems where it is shown that, given an appropriate set of system parameters, multiple-pulse processing can be more efficient than single-pulse processing over a finite range of the integration parameter n.     

Measuring ultrashort optical pulses in the presence of noise: an empirical study of the performance of spectral phase interferometry for direct electric field reconstruction

We have measured the performance of a real spectral phase interferometry for direct electric field reconstruction (SPIDER) apparatus operating under suboptimal conditions. We analyzed the errors in SPIDER's measurements of the temporal phases and intensities of 50-fs ultrashort laser pulses as a function of the additive noise in the detected signal. It was found that SPIDER performs exceptionally well, particularly in the case of additive noise. Specifically, a signal with 10% noise yields a pulse that has a mere 2% error in its intensity profile and a phase that differs from the nominal value by 0.2 rad. Furthermore, we quantified SPIDER's performance with limited detector resolution and as a function of signal averaging.     

Non-Gaussian Noise at the Output of a Resonant Gravitational Antenna in the Fast Filtering Regime. Information Aspect

The information characteristics of additive noise at the output of a resonant gravitational antenna are considered. Fisher information for the Explorer resonant gravitational antenna operating in the fast filtering regime is calculated under conditions of parametric a priori uncertainty from experimental data. The expediency is discussed of using an amplitude–frequency algorithm for suppressing correlated non-Gaussian noise when processing the output signal of a resonant gravitational antenna operating in this regime.     

Microwave interferometry: application to precision measurements and noise reduction techniques

A concept of interferometric measurements has been applied to the development of ultra-sensitive microwave noise measurement systems. These systems are capable of reaching a noise performance limited only by the thermal fluctuations in their lossy components. The noise floor of a real time microwave measurement system has been measured to be equal to -193 dBc/Hz at Fourier frequencies above 1 kHz. This performance is 40 dB better than that of conventional systems and has allowed the first experimental evidence of the intrinsic phase fluctuations in microwave isolators and circulators. Microwave frequency discriminators with interferometric signal processing have proved to be extremely effective for measuring and cancelling the phase noise in oscillators. This technique has allowed the design of X-band microwave oscillators with a phase noise spectral density of order -150 dBc/Hz at 1 kHz Fourier frequency, without the use of cryogenics. Another possible application of the interferometric noise measurements systems include “flicker noise-free” microwave amplifiers and advanced two oscillator noise measurement systems     

Nonlinear processing and fractional-order filtering in a joint fractional fourier-transform correlator: performance evaluation in multiobject recognition

We investigated the correlation performance of a joint fractional Fourier-transform correlator (JFRTC) using computer simulation results. We present a mathematical analysis suggesting use of processing techniques based on a nonlinear transformation and fractional-order fractional-power fringe-adjusted filter to attain improved performance in terms of discrimination sensitivity and input space-bandwidth utilization. Optimal noise performance for the JFRTC is predicted in the presence of additive white Gaussian noise. An all-optical implementation scheme based on incoherent erasure in a photorefractive crystal is proposed.     

Correction of Systematic Errors of Ratiometric Measurement Transducers

New methods are considered for correcting additive and multiplicative systematic errors of ratiometric measurement transducers utilizing sensors whose output signal u(x) is proportional to their power supply voltage or current. It is shown that synthesized algorithms for processing the intermediate results of measurements provide a reduction in the additive errors by several orders of magnitude and almost completely eliminate multiplicative errors.     

Total absorption radiometers for precision measurement of low power levels

We investigate a self-calibrated, total absorption radiometer with 100% quantum efficiency in the visible region. The fundamental components of its residual systematic errors are presented: the limit of additive residual systematic noise does not exceed 0.06% at a wavelength of 632.8 nm. The radiometer is applied to high-accuracy spectrophotometric measurement of reflection and transmission coefficients. The standard deviation of these measurements is less than 0.01%.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 32–35, August, 1994.     

A study of noise phenomena in microwave components using an advanced noise measurement system

A novel 9 GHz measurement system with thermal noise limited sensitivity has been developed for studying the fluctuations in passive microwave components. The noise floor of the measurement system is flat at offset frequencies above 1 kHz and equal to -193 dBc/Hz. The developed system is capable of measuring the noise in the quietest microwave components in real time. We discuss the results of phase and amplitude noise measurements in precision voltage controlled phase shifters and attenuators. The first reliable experimental evidences regarding the intrinsic flicker phase noise in microwave isolators are also presented.     

Image reconstruction in three-dimensional magnetostatic permeability tomography

Magnetostatic permeability tomography is an imaging technique that attempts to reconstruct the permeability distribution of an object using magnetostatic measurement data. The data for image reconstruction are external magnetic field measurements on the surface of the object due to an applied magnetostatic field. Theoretically, the normal and tangential components of the magnetic field in the surface uniquely define the internal isotropic permeability distributions. However, the inverse permeability problem is an ill-posed nonlinear problem. Regularization is needed for a stable solution. In this paper, we present a numerical method to solve the reconstruction problem in three dimensions using a regularized Gauss-Newton scheme. We have solved the forward problem using an edge finite-element method and we have employed an efficient technique to calculate the Jacobian matrix. The permeability of the object is assumed to be linear and isotropic. We present the reconstruction results for the permeability using synthetically generated data with additive noise.     

Processing photofluorographic images by means of decomposition into empirical modes

It is shown that the efficiency in processing photofluorographic images depends directly on the noise suppression level. It is proposed that a method based on decomposition into empirical modes should be used for noise suppression in photofluorographic images. A noise suppression algorithm based on threshold processing of the frequency components of a photofluorographic image is developed. A quantitative estimate of the level of noise suppression is presented.     

Computer device for monitoring the insulation of mains

The block diagram of a device is described for evaluating the insulation quality of mains from the results of measurements of the differential attenuation coefficient of the first and second harmonics of the cathodic protection current. Mathematical expressions are derived for the additive and multiplicative components of the measurement error.Translated from Izmeritel'naya Tekhnika, No. 5, pp. 42–44, May, 1994.