Interior imaging using spatial frequency sampling

A new acoustic synthetic aperture geometry is demonstrated in which the image field is sampled in the spatial frequency domain. This means the sampled field is recorded in a particularly convenient form for presentation to the back propagation algorithms used to reconstruct the field at the object plane. The method has applications in interior visualization. It is difficult to image the interior of solid objects using lens based imaging systems because a different lens geometry must be used for each distinct object material. The system presented here overcomes such problems since both the angular variation of the transmission coefficient at the object-water interface and the aberrations introduced by the velocity mismatch at the object surface may be readily compensated for in the back propagation routine. Experimental results are presented illustrating the detection of four half wavelength diameter defects, spaced by three wavelengths, at a depth of eight wavelengths below the surface of an aluminum block.     

Effects of ultrasound frequency, temporal sampling frequency, and spatial sampling step on the quantitative ultrasound parameters of articular cartilage

Quantitative ultrasound imaging may provide a technique for diagnosing initial signs of osteoarthritis (OA), such as surface fibrillation of articular cartilage. Because subchondral sclerosis and osteophyte formation occur in OA as well, ultrasonic analysis of subchondral bone could yield useful diagnostic information. In this study, we investigated whether low-frequency (5 MHz) ultrasound, typically used in bone diagnostics, would be feasible for evaluating the integrity of the surface of the cartilage. The reflection parameters in the time and frequency domains, the ultrasound roughness index, and the wavelet-based parameters were evaluated using ultrasound transducers operating at 5, 10, and 50 MHz frequencies. The effects of variable size of spatial sampling steps and of temporal sampling frequencies were also investigated. Custom-made phantoms and cartilage samples with various surface characteristics were analyzed. The reflection parameters detected the surface degradation with all ultrasound frequencies. The roughness of the surface could only be evaluated reliably with the 50 MHz-focused transducer. In conclusion, simultaneous analysis of the reflection parameters of the cartilage and the subchondral bone is feasible at low (5 MHz) ultrasound frequencies. However, reliable evaluation of the microtopography of the cartilage requires use of a higher ultrasound frequency.     

Frequency domain analysis of microstrip filters and antennae using an adaptive frequency sampling moment method

In this paper, an adaptive frequency sampling technique is applied to the moment method for the analysis of microstrip filters and patch antennae. The analysis of microstrip low-pass filter and patch antenna in the frequency domain has been usually done with uniform frequency step. An adaptive frequency sampling technique can significantly reduce the time taken for the analysis through the frequency range without reducing the accuracy of the results.     

利用双音频信号实现多点远程采样

文章介绍了一种利用双音频信号传输数据以实现计算机控制多点远程采样的系统。具有结构简单、误码率低、可靠性高等特点     

A method for using a time interval counter to measure frequency stability

An interval timer is used in a single-mixer frequency-stability measurement system in place of an event timer. The dead-time problem is avoided with the aid of a reference pulse train and an algorithm for ambiguity resolution. The noise floor test and the unfolding algorithm are described. Advantages of the technique are high precision, convenient interfacing, and low cost. A disadvantage is the vulnerability of the technique to missing data.     

Directional velocity estimation using a spatio-temporal encoding technique based on frequency division for synthetic transmit aperture ultrasound

This paper investigates the possibility of flow estimation using spatio-temporal encoding of the transmissions in synthetic transmit aperture imaging (STA). The spatial encoding is based on a frequency division approach. In STA, a major disadvantage is that only a single transmitter (denoting single transducer element or a virtual source) is used in every transmission. The transmitted acoustic energy will be low compared to a conventional focused transmission in which a large part of the aperture is used. By using several transmitters simultaneously, the total transmitted energy can be increased. However, to focus the data properly, the signals originating from the different transmitters must be separated. To do so, the pass band of the transducer is divided into a number of subbands with disjoint spectral support. At every transmission, each transmitter is assigned one of the subbands. In receive, the signals are separated using a simple filtering operation. To attain high axial resolution, broadband spectra must be synthesized for each of the transmitters. By multiplexing the different waveforms on different transmitters over a number of transmissions, this can be accomplished. To further increase the transmitted energy, the waveforms are designed as linear frequency modulated signals. Therefore, the full excitation amplitude can be used during most of the transmission. The method has been evaluated for blood velocity estimation for several different velocities and incident angles. The program Field II was used. A 128-element transducer with a center frequency of 7 MHz was simulated. The 64 transmitting elements were used as the transmitting aperture and 128 elements were used as the receiving aperture. Four virtual sources were created in every transmission. By beamforming lines in the flow direction, directional data were extracted and correlated. Hereby, the velocity of the blood was estimated. The pulse repetition frequency was 16 kHz. Three different setups were investigated with flow angles of 45, 60, and 75 degrees with respect to the acoustic axis. Four different velocities were simulated for each angle at 0.10, 0.25, 0.50, and 1.00 m/s. The mean relative bias with respect to the peak flow for the three angles was less than 2%, 2%, and 4%, respectively.     

非整周期采样信号相位差估计的相频匹配方法

To improve the precision and anti-interference performance of phase difference estimation for non-integer periods  sampling signals,a phase and frequency matching based phase difference estimation method  was proposed.The phase difference estimation  was obtained by means of the segmented Hilbert transform to suppress the effect of non-integer periods  sampling sinusoidal signals on the Hilbert transform.The simulation results demonstrate that compared with the methods of cross-correlation,Hilbert transform and data extension-based correlation, the proposed method has better phase difference estimation performance for non-integer periods sampling signals in terms of its estimation precision and anti-interference performance.Its phase difference estimation errors are closer to Cramer-Rao lower bound (CRLB).The measurement experiments  on the flow rates of Coriolis mass flowmeter validate the effectiveness of the proposed method.     

Two-color interferometer for surface characterization using two frequency doublers and a four-phase-step method

A high-accuracy, two-color interferometer for characterizing a processed surface is described. It is composed of two frequency doublers. It is not affected by air turbulence and mechanical vibration because it is a complete common-path interferometer, and the effects from different reflectivities of various materials are canceled by a four-phase-step technique that shifts the phase of the interference fringe in steps of π/2. The processed surface is measured with subnanometer resolution.     

Dual random phase encoding: a temporal approach for fiber optic applications

We analyze the dual random phase encoding technique in the temporal domain to evaluate its potential application for secure data transmission in fiber optic links. To take into account the optical fiber multiplexing capabilities, the noise content of the signal is restricted when multiple channels are transmitted over a single fiber optic link. We also discuss some mechanisms for producing encoded time-limited as well as bandwidth-limited signals and a comparison with another recently proposed technique is made. Numerical simulations have been carried out to analyze the system performance. The results indicate that this multiplexing encryption method could be a good alternative compared with other well-established methods.     

An adaptive sampling method for variable-fidelity surrogate models using improved hierarchical kriging

Variable-fidelity (VF) modelling methods have been widely used in complex engineering system design to mitigate the computational burden. Building a VF model generally includes two parts: design of experiments and metamodel construction. In this article, an adaptive sampling method based on improved hierarchical kriging (ASM-IHK) is proposed to refine the improved VF model. First, an improved hierarchical kriging model is developed as the metamodel, in which the low-fidelity model is varied through a polynomial response surface function to capture the characteristics of a high-fidelity model. Secondly, to reduce local approximation errors, an active learning strategy based on a sequential sampling method is introduced to make full use of the already required information on the current sampling points and to guide the sampling process of the high-fidelity model. Finally, two numerical examples and the modelling of the aerodynamic coefficient for an aircraft are provided to demonstrate the approximation capability of the proposed approach, as well as three other metamodelling methods and two sequential sampling methods. The results show that ASM-IHK provides a more accurate metamodel at the same simulation cost, which is very important in metamodel-based engineering design problems.     

Designing two mixed quick switching sampling plans for linear profiles

This study compares two proposed mixed quick switching sampling (QSS) plans for linear profiles as the quality characteristic. For the QSS plans, we recommend a binomial attribute plan for normal inspection and then a variable sampling plan for tightened inspection based on capability index C_puA of linear profiles with one-sided specifications. The difference between the two proposed QSS plans is in the tightened inspection. Tightened inspection of the first proposed plan is a single sampling using C_puA index, but tightened inspection of the second plan is a multiple dependent state repetitive (MDSR) plan based on C_puA index. The optimal parameters are obtained by nonlinear optimization. Simulation study for selecting parameters is conducted with various combinations of specified acceptable quality level (AQL), limited quality level (LQL), producer's risk, and consumer's risk. Simulation results confirm that the second proposed QSS plan which applies variable MDSR at tightened inspection performs better than another proposed plan. Hence, the approach of the second proposed plan is demonstrated in an illustrative example.     

The linear sampling method for a mixed scattering problem

This paper is concerned with the scattering problem of time-harmonic acoustic plane waves by a mixed scatterer, which is a combination of an open crack and an impenetrable obstacle. Firstly, the well-posedness of the solution to the direct scattering problem is established using the variational method. Then, a uniqueness result for the inverse scattering problem is proved, that is, both of the crack and the impenetrable obstacle can be uniquely determined simultaneously by the knowledge of the far-field pattern. Furthermore, a mathematical basis is given to reconstruct the shape of the crack and the impenetrable obstacle using the linear sampling method, and some numerical examples are given to establish the viability of our approach.     

Investigation of an adaptive sampling method for data interpolation using radial basis functions

An adaptive sampling method is presented for optimizing the location of data points in parameter space for multidimensional data interpolation. The method requires a small number of points to begin, and achieves a compromise between space‐filling updates and local refinement in areas where the data are nonlinear, as measured by the Laplacian. A smooth separation function quantifies the sample spacing, and this is blended with the Laplacian to form a criterion on which to assess potential new sample positions. Validation results are presented using two‐dimensional analytic test cases, which demonstrate that the method can recover known optimal designs and gives improvement over data‐independent approaches. In addition, a detailed analysis of the various model parameters is presented. Initial findings are very promising, and it is hoped that further work using the method to generate an aerodynamic database using CFD simulations will lead to a reduction in the number of points required for a given modelling accuracy. Copyright © 2010 John Wiley & Sons, Ltd.     

A low-cost frequency multiplier for synchronous sampling ofperiodic signals

The theory of digital signal processing indicates clearly that the measurements performed on periodic signals by means of digital techniques are affected by an error due to the nonsynchronization between the sampling frequency and the signal frequency. The authors present a low-cost device, based on a frequency multiplier, that allows the generation of sampling pulses synchronized with the signal frequency. The behavior of this device is analyzed in order to assure its ability to work with input signals in the frequency range typical of the mains. As an example of the application of this device, the results of the Fourier analysis of a multifrequency signal are given, showing the high accuracy that can be attained     

Terahertz encoding approach for secured chipless radio frequency identification

In this article, we present a new family of chipless tags, which permit encoding of digital data in the terahertz domain. These devices consist of stacked dielectric media whose thicknesses are of the same order as terahertz wavelengths. Since the information is encoded in the volume of these multilayer terahertz tags, they can easily be associated with classical identification techniques (e.g., barcode, radio frequency identification), where information is encoded at the surface of the tag, to provide higher data security. The principle of this encoding approach is studied and experimentally demonstrated in this paper. A 2 bit tag prototype has been realized and measured for validation purposes.     

Imaging strongly scattering media using a multiple frequency distorted Born iterative method

The distorted Born iterative (DBI) method is a powerful approach for solving the inverse scattering problem for ultrasound tomographic imaging. This method alternates between solving the inverse scattering problem for the scattering function and the forward scattering problem for the total field and the inhomogeneous Green's function. The algorithm is initialized using the basic Born inverse solution. One fundamental problem is the algorithm diverges for strongly scattering media. This is caused by the limitation of the Born assumption in estimating the initial step of the algorithm. We present a multiple frequency DBI approach to alleviate this problem, thus extending the applicability of the DBI method to the level of dealing with biological tissue. In this multiple frequency approach, a low frequency DBI-based solution, is used to initialize the algorithm at higher frequencies. The low frequency allows convergence of the algorithm to a contrast level that is close to the true level, however, with a poor spatial resolution. The high frequency improves the spatial resolution while preserving convergence because the difference between the true contrast and the initial contrast is relatively small. We present numerical simulations that demonstrate the ability of this method to reconstruct strongly scattering regions.     

利用复相关法估计信号的平均频率

文章介绍了利用复相关估计信号平均频率的方法，并就其采样周期的选取与频率估计误差间的关系作了分析，最后给出最佳采样频率以及平均频率测量误差的实测结果。     

Characterizing frequency stability: a continuous power-law modelwith discrete sampling

This paper examines several aspects of the Allan variance and the modified Allan variance. New expressions for these variances are derived for noise processes that produce power spectral densities with both integer and noninteger powers (α) in their functional dependence on f. A single expression, continuous over α, is presented for each of these variances. Also investigated are the effects of discrete sampling and finite data length. Discrete equations are developed and compared with more familiar continuous expressions. In addition, the uncertainty of the estimates for the Allan variance and the modified Allan variance for fully overlapping data usage is presented. The uncertainties can be calculated for arbitrary α. The results presented are compared with computer simulations and found to be in excellent agreement