Fourier Analysis of Time-Stamped Signals Using a Small Number of Samples

This paper concerns the Fourier analysis of measurements taken at times that are not at all uniformly spaced yet are accurately known. Such can be the case when measurements are obtained by networked sensors with accurate or synchronized clocks using common Internet Protocol networks or other networks that do not guarantee timely data delivery. Such can also be the case when the object being measured is only sporadically available for measurement. A method for computing a complex spectrum from a minimal number of samples taken at nonuniform time intervals is described. The novelty of this paper lies in the method's ability to minimize the number of samples required to ensure a desired accuracy. A practical application of the method lies in doing spectral analysis of signals obtained from networked sensors where acquiring each sample is expensive in time, power, or money, and so, the number of samples taken is to be minimized. The method has been tested on two rather different sets of actual measurements: one taken via smart sensors connected to the Internet and the other taken by a number of observational astronomers using a variety of apparatus over decades.     

Timing Recovery for IEEE 1588 Applications in Telecommunications

IEEE 1588 is being proposed as a solution to transport synchronization information within telecommunication networks, both for infrastructure use and for use in applications where time and frequency are required. The requirements of synchronization in these cases, however, are quite different from the traditional use of IEEE 1588 for synchronization in control and instrumentation applications. For successful deployment in communication applications, it is necessary to consider the unique aspects of telecom sync and the interrelation of these factors with timing recovery using IEEE 1588. This paper discusses the error considerations that must be applied toward timing recovery in packet-timing implementations, particularly those targeted toward telecom sync. A review of basic rate-recovery concepts is used to identify the various sources of error and their effects. Simulation and experimental results are used to illustrate the impact of different factors on the recovered timing.      

A Practical Implementation of IEEE 1588-2008 Transparent Clock for Distributed Measurement and Control Systems

This paper addresses issues with time synchronization using the IEEE 1588-2008 for distributed measurement and control systems. A practical implementation of the transparent clock is presented with the overall system architecture and detailed operation of each building block. To verify the submicrosecond accuracy using the implemented devices, an experimental setup that was analogous to a practical distributed system has been built. Measured results from the experiment show that the time error is limited below 30 ns for nodes that were connected by three switches. It is remarkable that the results are observed in spite of large packet queuing delays that were introduced by a traffic generator. The discussion on sources of time error that was outlined here provides technical considerations to designing IEEE 1588 systems.      

Frequency-Domain Interpretation of Oscillator Phase Stability

A frequency-domain interpretation of the phase stability of an oscillator is discussed. From a knowledge of the time dependence of an oscillator phase during a time interval T* it is possible to give the characteristics of this oscillator, not only for this time interval, but also for subsequent time intervals. Since the use of a Fourier transform for the computation of a continuous power spectrum is unrealistic, a discrete-spectrum approach will be taken. Usually, in the calculation of power spectra, stationarity of the fluctuations is assumed, although experiment indicates that this is often not the case. A more realistic approach is adopted. Analytical phenomena and random walk are separated from white noise on the basis of statistical criteria using discrete Fourier transforms. The white noise is then interpreted in the frequency domain. Both random walk and specific signals are studied in the time domain and can be separated by digital filtering. Two different sets of experimental results are analyzed by this method, one derived from measurements on a quartz-crystal oscillator locked to a low-frequency transmitter and the second from measurements on an ammonia maser. In both cases, measurement precision and ease of prediction of the behavior of the oscillator are improved.     

One-Way Delay Measurement: State of the Art

Nowadays, the evaluation of performance measurement in computer networks is an important issue. To ensure the quality of service of the network communication, one of the most important network performance parameters is the one-way delay (OWD). For accurate OWD estimation, it is essential to consider some parameters that can influence the measure, such as the operating system and, in particular, the threads, which are concurrent with the measurement application. Moreover, OWD estimation is not an easy task, because it can be affected by synchronization uncertainties. This paper aims to review the different solutions proposed in the scientific literature for OWD measurement. These solutions adopt different methods to guarantee a reasonable clock synchronization based on the Network Time Protocol, the Global Positioning System, and the IEEE 1588 Standard. These different approaches are critically reviewed, showing their advantages and disadvantages.      

Performance measurement of IEEE 802.11b-based networks affected by narrowband interference through cross-layer measurements

Researches and development efforts in wireless networking and systems are progressing at an incredible rate. Among them, measurement and analysis of performance achieved at network layer and perceived by end users is an important task. In particular, recent advances concerning IEEE 802.11b-based networks seem to be focused on the measurement of key parameters at different protocol levels in a cross-layered fashion, because of their inherent vulnerability to in-channel interference. By adopting a cross-layer approach on a real network set-up operating in a suitable experimental testbed, packet loss against signal-to-interference ratio in IEEE 802.11b-based networks is hereinafter assessed. Results of several measurements aimed at establishing the sensitivity of IEEE 802.11b carrier sensing mechanisms to continuous interfering signals and evaluating the effects of triggered interference on packet transmission.     

A Fourier transform technique that measures phase delays between ultrasonic impulses with sufficient accuracy to determine residual stresses in metals

A Fourier transform technique is described which measures phase delay using short duration pulses or impulses. The method of data analysis is detailed and some simple test functions are used to estimate the accuracy and reliability of the resulting phase delays. It appears that the inherent accuracy associated with this technique is not much different from that of a zero cross-over technique using a chopped CW source and narrowband receiver.The computer software used to fully automate the Fourier transform technique is described. It is clear that the technique can be automated to give reliable measurements of phase delay, which for various reasons is not always possible using the narrowband CW technique.Measurements of ultrasonic phase delays for a 50 mm thick mild steel plate using the Fourier transform technique are given for compressional and shear waves with frequencies around 2 MHz and 5 MHz. Both EMA and piezoelectric transducers have been used. Corresponding measurements of phase delay using a chopped CW stimulus are shown to be in good agreement.     

Redundant Measurements in Microwave Research and Development

Capability and accuracy of microwave measuring methods and instruments can be substantially improved by the use of general-purpose digital computers. One of the fruitful applications is concerned with redundant measurements and their evaluation. In a redundant measurement more data are measured than would be necessary for a singular evaluation and the excess information is used to reduce measurement errors. Redundant measurements are advantageous for sophisticated microwave measurements where the final results can be determined only by tedious graphical or numerical transforms of measured data. Redundant measuring methods also can be used to verify network or device models and, to determine the parameters of these models. The evaluation of redundant measurements can be carried out in simple cases by averaging and generally by fitting the hypothetical parameters to the measured data (e.g., by least-square error method) or by using integral transform (e.g., Fourier transform) techniques. Digital computers enable one to make fast and accurate evaluations.     

An Implementation of IEEE 1588 Over IEEE 802.11b for Synchronization of Wireless Local Area Network Nodes

IEEE 1588 is a new standard to synchronize independent clocks running on separate nodes of a distributed measurement and control system. It is intended for high-accuracy implementations on compact systems such as a single subnet. This paper examines potential accuracy limitations introduced by the physical layer of the IEEE 802.11b wireless local area network. Experimental results are presented that show that these limitations do not preclude clock-synchronization accuracy of several hundred nanoseconds.     

High temperature, spectral-directional emittance of high purity nickel oxidized in air

Spectral-directional emittance measurements for 99.99% nickel, thermally oxidized in air, were performed at temperatures of 673, 773, and 873 K using an apparatus comprised of a Fourier transform infrared (FTIR) spectrometer, a blackbody radiating cavity (hohlraum), and a sample holder which allows directional measurements. The data cover the spectral range between 2 and 20 μm, and the directional range from a surface normal to 72° polar angle. The Ni sample had a nominal surface roughness of 4.1 μm and was heated for 1 h at the measurement temperatures prior to emission measurements. X-ray diffraction and EDS analyses were performed in order to characterize the sample surface. It was found that the normal emittance of oxidized nickel increases with temperature for the temperature range considered. Directional emittance shows slightly departure from pure metal behavior.     

材料发射率测量技术及其应用

论述了发射率在航天、航空、国防及国民经济各领域中的重要作用和意义,综述了近年来材料发射率测量技术的研究发展现状。简单介绍了基于红外傅里叶分析光谱仪建立的光谱发射率测量系统,进而描述了发射率的应用情况。展望了发射率测量技术的目前存在的问题及发展趋势。     

基于约瑟夫森量子电压的谐波电压测量方法研究

约瑟夫森量子电压目前大多应用于正弦信号的测量,极少有针对谐波信号的研究。对可编程约瑟夫森电压基准应用于谐波检测的可行性进行了研究,实现谐波电压向量子电压过渡。在使用阶梯波交流量子电压进行谐波电压测量时,由于过渡过程和吉布斯现象的存在,部分数据出现较大波动;若将该部分处于过渡过程的数据纳入傅里叶变换,会导致恢复得到的信号幅值和相位失真。针对这一问题,提出了一种新的谐波电压计算方法:加权傅里叶变换。该方法通过将处于过渡过程的数据权重置零的方式,实现了谐波电压的准确测量。实验结果表明,谐波电压幅值和相位测量结果较理想,幅值测量标准差均小于1 μV,相位测量标准差均小于15 μrad。     

Integrating-sphere system and method for absolute measurement of transmittance, reflectance, and absorptance of specular samples

An integrating-sphere system has been designed and constructed for multiple optical properties measurement in the IR spectral range. In particular, for specular samples, the absolute transmittance and reflectance can be measured directly with high accuracy and the absorptance can be obtained from these by simple calculation. These properties are measured with a Fourier transform spectrophotometer for several samples of both opaque and transmitting materials. The expanded uncertainties of the measurements are shown to be less than 0.003 (absolute) over most of the detector-limited working spectral range of 2 to 18 microm. The sphere is manipulated by means of two rotation stages that enable the ports on the sphere to be rearranged in any orientation relative to the input beam. Although the sphere system is used for infrared spectral measurements, the measurement method, design principles, and features are generally applicable to other wavelengths as well.     

A computer-controlled submillimeter Fourier spectrometer

A system for submillimeter-wave Fourier transform spectroscopy (FTS) has been automated for the measurement of the power absorption coefficient of liquids in the wavenumber range 10-400 cm^-1. The interferometer is interfaced with a RM380Z Research Machines microcomputer. Software and hardware have been developed to automate the process of collecting and storing the data. Some points of relevance to the analysis of interferometric data are reviewed including the use of phase modulation, the resolution of spectra computed using the discrete Fourier transform, and the necessity of proper optical filtering in spectrometers which use Fourier transform techniques. Spectra measured using the FTS system described have been compared with results obtained using a tunable submillimeter laser and a good agreement was found between the two sets of data     

Quick scanning exafs: First experiments

By speeding up a conventional X-ray monochromator by a factor of 100 using the quick scanning EXAFS (QEXAFS) mode the data collection time required for a typical EXAFS scan was reduced to a couple of seconds, making XANES scans in a split second possible. Typical absorption spectra of Fe and Cu foils taken in 3.5–10 s covering an energy region of up to 800 ev are shown. It is demonstrated that the normalized Cu-EXAFS and its Fourier transform agree very well with the results of a conventional step by step measurement. The new approach allows the use of fluorescence detection and hereby makes possible the time dependent investigation of the changes in structure or valence state of a wide variety of samples which are inaccessible for transmission spectroscopy.     

Technical Evaluation and Optimization of Phasor Measurement Unit Using CSIR-NPL PMU Calibrator System to Ensure Reliability

The technological enhancements and new startups in India have led to a tremendous increase in the power demand. The grids are being made smart, more efficient and reliable to meet not only the growing energy demands but also to prevent blackouts. The synchrophasor technology is evolving, explored and used by the power sectors for real-time measurements with phasor measurement unit (PMU) as the main elemental device. Industries developing PMUs must ensure that it should comply the IEEE C37.118.1-2011 and C37.118.1a-2014 synchrophasor standards, as the corrective action taken by power grid in real-time environment will depend on the PMU data. The optimization of PMU ensures that the measurement results are as per the IEEE synchrophasor standards leading to improvement in its performance. Here, we had evaluated the performance of an industrial PMU using CSIR-NPL PMU calibration system. The evaluation was done under static and dynamic conditions. This analysis represents the capability of PMU testing and calibration at CSIR-NPL and also ensures that the PMU that will be used in real time on the fields reports precise data. PMU-Cal facility can handle all the steady-state and dynamic-state tests for both M class and P class configured PMU at all desired frame rates. This system is traceable to its respective primary standards and is functional to cater the needs of power sector in terms of PMU calibration and testing.     

Frequency and fundamental signal measurement algorithms for distributed control and protection applications

Increasing penetration of distributed generation within electricity networks leads to the requirement for cheap, integrated, protection and control systems. To minimise cost, algorithms for the measurement of AC voltage and current waveforms can be implemented on a single microcontroller, which also carries out other protection and control tasks, including communication and data logging. This limits the frame rate of the major algorithms, although analogue to digital converters (ADCs) can be oversampled using peripheral control processors on suitable microcontrollers. Measurement algorithms also have to be tolerant of poor power quality, which may arise within grid-connected or islanded (e.g. emergency, battlefield or marine) power system scenarios. This study presents a 'Clarke-FLL hybrid' architecture, which combines a three-phase Clarke transformation measurement with a frequency-locked loop (FLL). This hybrid contains suitable algorithms for the measurement of frequency, amplitude and phase within dynamic three-phase AC power systems. The Clarke-FLL hybrid is shown to be robust and accurate, with harmonic content up to and above 28% total harmonic distortion (THD), and with the major algorithms executing at only 500 samples per second. This is achieved by careful optimisation and cascaded use of exact-time averaging techniques, which prove to be useful at all stages of the measurements: from DC bias removal through low-sample-rate Fourier analysis to sub-harmonic ripple removal. Platform-independent algorithms for three-phase nodal power flow analysis are benchmarked on three processors, including the Infineon TC1796 microcontroller, on which only 10% of the 2000 mus frame time is required, leaving the remainder free for other algorithms.     

Imaging through an aberrating medium with classical ghost diffraction

We describe a method for image transmission through an aberrating medium by means of a modified configuration for conventional ghost diffraction with classical incoherent beams. On the basis of optical coherence theory, we show that the effects of phase disturbances, be they deterministic or random, can be canceled out in our method and the squared modulus of the Fourier transform of the object is obtained in terms of intensity-correlation measurements. From the measurement data, the object can be reconstructed using standard phase retrieval algorithms.     

Instrumental phase-based method for Fourier transform spectrometer measurements processing

Phase correction is a critical procedure for most space-borne Fourier transform spectrometers (FTSs) whose accuracy (owing to often poor signal-to-noise ratio, SNR) can be jeopardized from many uncontrollable environmental conditions. This work considers the phase correction in an FTS working under significant temperature change during the measurement and affected by mechanical disturbances. The implemented method is based on the identification of an instrumental phase that is dependent on the interferometer temperature and on the extraction of a linear phase component through a least-squares approach. The use of an instrumental phase parameterized with the interferometer temperature eases the determination of the linear phase that can be extracted using only a narrow spectral region selected to be immune from disturbances. The procedure, in this way, is made robust against phase errors arising from instrumental effects, a key feature to reduce the disturbances through spectra averaging. The method was specifically developed for the Mars IR Mapper spectrometer, that was designed for operation onboard a rover on the Mars surface; the validation was performed using ground and in-flight measurements of the Fourier transform IR spectrometer planetary Fourier spectrometer, onboard the MarsExpress mission. The symmetrization has been exploited also for the spectra calibration, highlighting the issues deriving from the cases of relevant beamsplitter emission. The applicability of this procedure to other instruments is conditional to the presence in the spectra of at least one spectral region with a large SNR along with a negligible (or known) beamsplitter emission. For the PFS instrument, the processing of data with relevant beamsplitter emission has been performed exploiting the absorption carbon dioxide bands present in Martian spectra.