Emilia Nunzi

Detection of Anomalies in the Behavior of Atomic Clocks

In this paper, the problem of identifying variations in the nature of atomic clock noise is addressed. Two methods are proposed. One method is based on a generalized likelihood ratio test (GLRT), and the other is based on the dynamic Allan variance (DAVAR), which is a representation of the instantaneous clock stability that is able to point out possible nonstationary behaviors. Both methods efficiently track variations in the experimental clock data and, thus, appear as suitable tools for the detection of atomic clock anomalies

Windows for ADC dynamic testing via frequency-domain analysis

In this paper, the frequency-domain dynamic test of analog-to-digital converters (ADCs) is considered under the assumption of noncoherent sinewave sampling. A procedure is described which is based on the windowed discrete Fourier transform (WDFT), optimized for the achievement of high estimation accuracy. With this aim, the class of windows belonging to the set of discrete prolate spheroidal sequences is adopted for the reduction of the effects of spectral leakage. Practical suggestions are given for a straightforward applicability of derived results and for an efficient estimation of ADC spectral parameters. Finally, experimental results are presented in order to validate the proposed testing approach.

Frequency-domain based least-squares estimation of multifrequency signal parameters

This paper proposes a frequency-based least squares algorithm applied to the estimation of multifrequency signal parameters. It is based on the processing of the joint information carried by nearby frequency components of Fourier-transformed data. Theoretical and simulated analysis are carried out on the algorithm accuracy while its performance is compared to the corresponding Cramer-Rao lower bound.

Immunogenicity of intramuscular MF59-adjuvanted and intradermal administered influenza enhanced vaccines in subjects aged over 60: A literature review

Because of the age-related immune system decline, 2 potentiated influenza vaccines were specifically licensed for the elderly: Fluad®, an MF59-adjuvanted vaccine administered intramuscularly (IM-MF59), and Intanza 15mcg®, a non adjuvanted vaccine administered intradermally (ID). The objective of this paper was to conduct a systematic review of studies that evaluated antibody responses in the elderly following immunization with IM-MF59 or ID vaccines. The two potentiated vaccines induced immune responses satisfying, in most instances, the European Medicine Agency immunogenicity criteria, both against vaccine antigens and heterovariant drifted strains. Considering pooled data reported in the articles analyzed and papers directly comparing the 2 vaccines, the antibody responses elicited by IM-MF59 and ID were found to be generally comparable. The use of IM-MF59 and ID vaccines can be proposed as an appropriate strategy for elderly seasonal influenza vaccination although further studies are required for a more complete characterization of the 2 vaccines.

Statistical efficiency of the ADC sinewave histogram test

This paper presents an analysis of the statistical efficiency of the sinewave histogram test used for estimating the unknown transition levels of an analog-to-digital converter. Accordingly, at first, a closed-form determination of the Cramer-Rao bound is derived under the assumption of a noiseless stimulus signal. Both unbiased and biased versions of the bound are described in order to account for the eventual bias introduced by commonly employed estimators. Then, additive Gaussian noise is assumed, and comments are made about its effects on the maximum achievable accuracy.

A procedure for highly reproducible measurements of ADC spectral parameters

The evaluation of spectral parameters characterizing analog-to-digital converters (ADC) can be addressed by employing both parametric and non-parametric techniques. Whatever the adopted testing techniques, the IEEE standards 1057 and 1241, which list the most effective ADC testing procedure, recommend the use of coherent sampling. Such a condition can not be guaranteed a priori with respect to spurious tones eventually present in the ADC output spectrum and data windowing is usually employed to reduce these phenomena. However, standards do not provide clear criteria for choosing the window that assure the maximum parameter estimation accuracy. The European draft standard Dynad suggests the employment of one out of seven optimal windows, in accordance to the ADC resolution. However, each sequence is characterized by distinctive parameters. Thus test automation may be reduced. In this paper the use of a class of windows that are defined by setting the mainlobe width value and that can be easily calculated by employing standard computational tools is proposed. Such sequences maximize also the estimation accuracy for any given ADC resolution, thus improving both measurement reproducibility and test automation. Experimental results that validate the effectiveness of the proposed window class are presented.

Monitoring signal integrity of atomic clocks by means of the GLRT

The generalized likelihood ratio test (GLRT) is a statistical fault detection method used for revealing fault occurrences in electronic systems. In this paper, the GLRT technique is analysed and customized for a rubidium frequency standard in order to reveal mean or standard deviation changes in the clock frequency. Experimental results are presented that confirm the effectiveness of the technique also when it is applied to data acquired from a rubidium clock. Monte Carlo simulations are shown in order to characterize the proposed method and to give a simple interpretation of the obtained results. The effectiveness of the GLRT has been already compared with standard tools such as the Allan variance (Nunzi E et al 2007 IEEE Trans. Instrum. Meas. 56 523?8). In particular, the sensitivity of the method with respect to the jump size has been analysed. In this paper, the fault detection technique is characterized with respect to its readiness in terms of the number of samples employed for obtaining a failure occurrence when applied to clock frequency. Results obtained are employed for giving practical indications on the design of this failure test.

Stochastic and reactive methods for the determination of optimal calibration intervals

The length of calibration intervals or measurement instrumentations can be determined by means of several techniques. In this paper, three different methods are compared for the establishment of optimal calibration interval of atomic clocks. The first one is based on a stochastic model and provides the estimation of the calibration interval also in the transient situation, while the others, attain to the class of the so-called reactive methods, which determine the value of the optimal calibration interval on the basis of the last calibration outcomes. Algorithms have been applied to experimental data and obtained results have been compared in order to determine the most effective technique. Since the analyzed reactive methods presents a presents a large transient time, a new algorithm is proposed and applied to the available data.

Comparison of nonparametric frequency estimators

This paper deals with the performance of several up-to-date nonparametric frequency estimators. The algorithms of frequency estimation are introduced and their bias, standard deviation and consumption time are compared with regard to the most common signal parameters.

Fault detection in atomic clock frequency standards affected by mean and variance changes and by an additive periodic component: the GLRT approach

Atomic clocks are measurement instruments widely employed for satellite navigation purposes, that need to be constantly monitored in order to guarantee high accuracy of the end-user application. Many research activities successfully investigated techniques for increasing clocks accuracy. However, more demanding specifications require the analysis of new efficient techniques for monitoring the behavior on-board and for revealing eventual faults of atomic clocks. In this paper a statistical fault detection method is described and validated. The presented method could be employed for designing technical parameters of an algorithm for fault detection of on-board atomic clock and could be a good candidate as clock monitoring algorithm in the next space missions.