Optical frequency standards based on the /sup 199/Hg/sup +/ ion

We report on work directed toward the systematic evaluation of an optical frequency standard based on the /sup 2/S/sub 1/2/-/sup 2/D/sub 5/2/ transition of a single, laser-cooled, trapped /sup 199/Hg/sup +/ ion, whose resonance frequency is 1.06/spl times/10/sup 15/ Hz. For the purpose of the evaluation, a second /sup 199/Hg/sup +/ standard has been constructed. In the cooling-laser system built for the second standard, an injection-locking scheme has been applied to a CW Ti-sapphire laser. We also report optical frequency measurements of the clock transition performed over the past 21 months with the first standard. During this term, the variation of the clock transition frequency is found to be less than /spl plusmn/1/spl times/10/sup -14/.     

Evaluation of the second-order Doppler shift in the Mg frequencystandard

We report in this paper the first evaluation of the second-order Doppler shift in the Mg frequency standard. Three methods are considered: direct theoretical computation, Fourier analysis of the experimental Ramsey curve, and Fourier analysis of the discrimination curve. The results obtained are compared in a practical experimental case and turn out in satisfactory agreement, allowing a good confidence on the determination of the shift at the accuracy level of ±3×10^-13. The considerations reported in this paper may be of general interest for frequency standards based on atomic or molecular beams with transverse dimensions much larger than the transition wavelength     

(87)Rb versus (133)Cs in cold atom fountains: a comparison

We describe the operation of a laser cooled (87)Rb frequency standard and present a new measurement of the (87)Rb ground state hyperfine frequency with a relative accuracy of 2.4x10(-15), by comparison with a Cs fountain primary standard. The measured frequency is 6 834 682 610.904 333(17) Hz. An evaluation of the frequency shift induced by cold collisions gives Deltanu/nu(Rb)=(-7.2+/-20)x10(-24) n , where n is the average atomic density in cm(-3). With our present 1 sigma uncertainty of 10(-15), this measurement is still compatible with 0 and about 300 times smaller than for (133)Cs. We also report a test of a possible variation of the fine structure constant at the level of 2.7x10(-14) yr(-1), comparing Rb and Cs cold atom fountains.     

A Novel Technique for Blackbody Shift Evaluation in Cs With a Fountain Standard

The uncertainty in the evaluation of the frequency shift caused in the Cs hyperfine transition by the alternating current Stark effect due to blackbody radiation is presently the dominant term in the accuracy budget of Cs fountain frequency standards. The bias is calculated with a formula wherein it is proportional to a coefficient beta. The latter must be known to the nearest 1% if the accuracy goal is in the low 10 ^-16. This, in turn, has required 10^-16 uncertainty in frequency measurements used to evaluate beta. The novel technique discussed here relaxes this latter requirement by shifting the burden to the short-term stability of the experiment, hence allowing that kind of accuracy on beta even if the accuracy of the standard is lower. The method is based on fast temperature cycling of the drift tube while keeping everything else at a constant temperature. In this way, all frequency biases, other than the one under study, are rejected as common mode in a differential measurement, and the uncertainty on the relative shift can be better than the accuracy of the frequency standard     

高精密测温电桥的标定与评估

使用RTU和RBC两种精密仪器对中国计量科学研究院中温基准实验室的F900高精密测温电桥进行了标定和评估,分析了不同工作频率、不同输出电流在长杆标准铂电阻温度计测温范围内对测量精度的影响。结果表明:电桥在不同的工作频率下精度的差异比较明显。工作频率为25 Hz时,使用RTU和RBC进行标定的最大测量偏差分别约为0.01 mK和0.06 mK;工作频率为75 Hz时,使用RTU和RBC进行标定的最大测量偏差约为0.08 mK和0.2 mK,且相比出厂时有了较大的漂移。电桥在不同的输出电流下精度的差异非常小,在15 μK以内。采用RTU及RBC对F900测温电桥评价的结果之间相互印证,该结果对中温固定点基准复现过程的不确定度评定提供了数据支撑。     

A chirp-z transform-based synchronizer for power system measurements

In the last few years, increased interest in power and voltage quality has forced international working groups to standardize testing and measurement techniques. IEC 61000-4-30, which defines the characteristics of instrumentation for the measurement of power quality, refers to IEC 61000-4-7 for the evaluation of harmonics and interharmonics. This standard, revised in 2002, requires a synchronous sampling of voltage or current signal, in order to limit errors and to ensure reproducible results even in the presence of nonstationary signals. Therefore, an accurate estimation of the fundamental frequency is required, even in the presence of disturbances. In this paper, an algorithm to detect the fundamental frequency is proposed; it is based on the chirp-z transform (CZT) spectral analysis and is able to observe all standards in force because of its accuracy and working characteristics. Theoretical aspects are discussed and various experimental tests are reported, comparing the proposed method with a similar well-known one based on interpolated fast Fourier transform (FFT) analysis. Finally, a complete characterization of the proposed system was performed, and its accuracy was evaluated by means of a calibrator.     

Reduction of the cold collisions frequency shift in a multiple velocity fountain: a new proposal

In this paper, we propose a new scheme for operating an atomic fountain frequency standard. A sequence of time-spaced balls of atoms, launched over non-overlapping trajectories, makes possible a reduction of the atomic density and consequently the cold collision frequency shift by one order of magnitude without significant reduction of the useful signal and of the overall stability of the clock. The ultimate accuracy of a Cs fountain could be improved below the 10(-15) level currently achieved in operating clocks.     

A new algorithm for the normal distribution function

In this paper new algorithms for the rapid, efficient and accurate evaluation of the standard normal integral and its tail are developed. It is shown how the accuracy of the computation can easily be improved so as to achieve machine accuracy for the particular computer being used.     

A new algorithm for the normal distribution function

In this paper new algorithms for the rapid, efficient and accurate evaluation of the standard normal integral and its tail are developed. It is shown how the accuracy of the computation can easily be improved so as to achieve machine accuracy for the particular computer being used.     

Construction and evaluation of the first Brazilian atomic clock

For the past two years we have been implementing a program for the establishment of scientific time and frequency metrology in Brazil. The main objective of this program is to construct an atomic fountain and use it as a primary standard. As a first step toward this goal, we have constructed a (133)Cs beam optically pumped conventional clock. In this paper we describe the system and the results of its evaluation. The possible limitations of our short-term stability are discussed.     

基于非线性扫频的频率特性校准研究

在动态频率特性测量和校准应用中,传统的线性调频信号频谱均匀分布、可设计参数单一,难以同时兼顾测量精度与测量效率。本文提出了一种基于非线性扫频方式的动态频率特性校准方法,可广泛用于传感器与仪器仪表的频率响应校准与标定。非线性扫频信号可以实现激励信号能量在频域内的非均匀分布,从而在某些重要频段内实现较高精度测量的同时,在其他频段通过牺牲一定的测量精度来保证测量速度,从而进一步优化频率特性测量的整体性能。仿真结果表明,在相同的扫频时间内,非线性扫频方法相比于线性扫频方法,能够将特定频段内的测量精度进一步提高。     

A method for calibrating the line-focus-beam acoustic microscopy system

Absolute accuracy of the line-focus-beam (LFB) acoustic microscopy system is investigated for measurements of the leaky surface acoustic wave (LSAW) velocity and attenuation, and a method of system calibration is proposed. In order to discuss the accuracy, it is necessary to introduce a standard specimen whose bulk acoustic properties, (e.g., the independent elastic constants and density) are measured with high accuracy. Single crystal substrates of gadolinium gallium garnet (GGG) are taken as standard specimens. The LSAW propagation characteristics are measured and compared with the calculated results using the measured bulk acoustic properties. Calibration is demonstrated for the system using two LFB acoustic lens devices with a cylindrical concave surface of 1-mm radius in the frequency range 100 to 300 MHz.     

Numerical solution of elastic bodies in contact by FEM utilising equilibrium displacement fields

In this paper a new effective formulation of the computation of the statics of elastic bodies in contact is described and demonstrated. Line contact, plane strain, and negligible friction are assumed. The formulation is an extension of the standard finite element method (FEM). With the aim to utilise the Hertz theory directly, we use the exact solution of the elastic 2-dimensional halfspace loaded by Hertzian pressure distribution and enforce the contact condition by the method of Lagrange multipliers. In numerical examples we have focussed on the demonstration and evaluation of the accuracy of the new formulation for selected applications compared with the state of the art node-to-segment contact algorithm implemented in the software system ADINA. Proposed formulation is more accurate for problems where Hertz contact dominates the strain state, especially for small number of elements, whereas we obtained a fairly good agreement with ADINA for a more general bending problem.     

Theoretical study of the Dick effect in a continuously operatedRamsey resonator

It is well established that passive frequency standards operated in pulsed mode may suffer a degradation of their frequency stability due to the frequency (FM) noise of the Local Oscillator (LO). In continuously operated frequency standards, it has been shown that a similar degradation of the frequency stability may arise, depending on the used modulation-demodulation scheme. In this paper, we report a theoretical analysis on the possible degradations of the frequency stability of a continuous fountain due to the LO FM noise. A simple model is developed to evaluate whether or not aliasing persists. This model is based on a continuous frequency control loop of a frequency standard using a Ramsey resonator. From this model, we derive a general formula, valid for all usual modulation-demodulation schemes, for the LO frequency fluctuations due to aliasing in closed loop operation. We demonstrate that in an ideal situation and for all usual modulation waveforms, no aliasing occurs if the half-period of modulation equals the transit time of atoms in the Ramsey resonator. We also deduce that in the same conditions, square-wave phase modulation provides the strongest cancellation of the LO instabilities in closed loop operation. Finally, we show that the “Dick formula” for the specific case of the pulsed fountain can be recovered from the model by a sampling operation     

Microwave leakage-induced frequency shifts in the primary frequency standards NIST-F1 and IEN-CSF1

In atomic fountain primary frequency standards, the atoms ideally are subjected to microwave fields resonant with the ground-state, hyperfine splitting only during the two pulses of Ramsey's separated oscillatory field measurement scheme. As a practical matter, however, stray microwave fields can be present that shift the frequency of the central Ramsey fringe and, therefore, adversely affect the accuracy of the standard. We investigate these uncontrolled stray fields here and show that the frequency errors can be measured, and indeed even the location within the standard determined by the behavior of the measured frequency with respect to microwave power in the Ramsey cavity. Experimental results that agree with the theory are presented as well.     

磁选态铯钟内辐射频移及其对准确度的影响

对磁选态铯原子频标中存在的各种辐射频移进行了研究,包括Bloch-Siegert效应,微波频谱不纯及调制影响,Stark效应与黑体辐射以及Majorana跃迁的影响。同时分析了产生各种频移的物理原因,估算了各种频移及其不确定度的大小,分析了它们对频标准确度的影响,提出了减小各种频移和误差以提高频标准确度的方法。     

GA-BP Air Quality Evaluation Method Based on Fuzzy Theory

With the rapid development of China’s economy, the scale of the city has been continuously expanding, industrial enterprises have been increasing, the discharge of multiple pollutants has reached the top of the world, and the environmental problems become more and more serious. The air pollution problem is particularly prominent. Air quality has become a daily concern for people. In order to control air pollution, it is necessary to grasp the air quality situation in an all-round way. It is necessary to evaluate air quality. Accurate results of air quality evaluation can help people know more about air quality. In this paper, refers to previous research results and different evaluation methods, combined with artificial neural network, fuzzy theory, genetic algorithm, GA-BP hybrid algorithm based on fuzzy theory is proposed to evaluate air quality. At the same time, for the problem that the two-grade standard of air quality annual evaluation is not suitable for practical application, the four-grade standard for annual air quality evaluation has been proposed, and its practicality has been verified through experiments. By setting contrast experiments and comparing the air quality evaluation model based on standard BP algorithm, it is proved that the fuzzy GA-BP evaluation model is better than the standard BP model, both in efficiency and accuracy.     

直升机旋翼锥体频闪灯校准技术研究

仿真模拟试验器校准频闪灯传统校准方法需要操作者目视判断，试验结果的重复性较差，且存在评定所得不确定度较大的问题。为此，研究了航空专用测试设备直升机旋翼锥体频闪灯的一种实验室校准方法。该方法采用脉冲频率法以光电转换形式建立校准装置，对频闪灯的闪烁频率等技术参数进行校准。实验结果表明：在准确度大幅提高的同时，不确定度更小。与传统方法相较而言，本方法操作简单快捷，通用的标准设备即可满足校准要求，量值溯源更可靠、更容易，同时也符合计量相关法规的要求，具有一定的推广及应用价值。     

NIST Sampling System for Accurate AC Waveform Parameter Measurements

This paper summarizes efforts at the National Institute of Standards and Technology (NIST) to develop a waveform sampling and digitizing system with accuracy comparable to that of an ac-dc thermal transfer standard for ac voltage measurements over the frequency range of 10 Hz to 1 MHz. In the frequency range from 1 kHz to 1 MHz, the sampler's gain flatness is better than that available from the best commercial digital multimeter. In ac-ac comparisons referenced to 1 kHz, the system agrees with a NIST-calibrated thermal transfer standard to within 17 muV/V from 20 Hz to 100 kHz for measurements made at both 1 and 0.1 V. The sampler's excellent dynamic linearity and flat input impedance are also discussed