The Role of Losses in the Definition of the Overmoded Condition for Reverberation Chambers and Their Statistics

International audienceIt is commonly acknowledged that in perfectly stirred reverberation chambers, the energy density of the electric field follows a χ26 law, as long as the overmoded condition applies. This concept, never defined properly, is often confused with the idea of a threshold on the modal density, regardless of the quality factor of the cavity. This interpretation is here proven to be inaccurate, as losses play a fundamental role in the nature of the field statistics and not, as often assumed, just in its scaling. In particular, it is shown how the overmoded condition should be stated mathematically, highlighting how the cavity quality factor and the number of eigenmodes excited cannot be regarded as quantities intervening independently on the field statistics, but should rather be considered jointly. These results are derived by means of a modal analysis, with a limited number of assumptions. A quantitative relationship is established between average modal overlapping and the rate of convergence of the electric energy density towards a χ26 law. Rather than setting an arbitrary threshold on modal overlapping as a necessary condition for an overmoded behavior, the statistical uncertainty due to the limited number of available field samples is shown to affect the very definition of the overmoded condition

Emulating an Anechoic Environment in a Wave-Diffusive Medium through an Extended Time-Reversal Approach

International audienceA generalized time-reversal (TR) technique for the generation of coherent wavefronts within complex media is presented in this paper. Although completely general, this method is primarily considered for testing purposes herein, where an equipment under test is submitted to a series of impinging wavefronts with varying features. Electromagnetic compatibility, antenna testing as well as telecommunications facilities where complex-wavefront schemes (e.g., multi-path configurations) are required, could benefit from the proposed approach. The main advantages and limitations of current standard TR approaches are reviewed in this respect, exposing their inadequacy for this particular context. The proposed alternative technique, named Time-Reversal Electromagnetic Chamber (TREC) is introduced and studied by means of a formal theoretical analysis, showing how a reverberation chamber (RC) supporting a diffused-field condition can be operated as a generator of deterministic pulsed wavefronts. The TREC is demonstrated to be capable of generating arbitrary wavefronts with a remarkable accuracy, allowing to revisit the RC as a deterministic facility: the main advantages of RCs and anechoic ones are merged, leading to a new facility capable of potentially generating in real-time pulsed wavefronts while using low input energies, without requiring neither mechanical displacements nor any special features of the sources

Measuring the internationalisation of EU corporate R&D: a novel complementary use of statistical sources

The report summarises the main results of a research activity aimed at testing a novel approach for the measurement of EU business R&D internationalisation. Such approach is based on the complementary use of two different sources of data: on the one hand, statistical data from private R&D expenditure taken from national surveys (BERD); on the other hand, data collected from companies' annual reports and accounts (as in the EU Industrial R&D Investment Scoreboard). The main objectives of the study were: i) to explore the methodological rationale for comparing the two sets of data; ii) to test the robustness of the novel methodology through an analysis applied to four EU countries (Belgium, Finland, Germany and Italy); iii) to provide indications of possible further research and follow up activities. The main results from the project are as follows: - BERD and Scoreboard values, though addressing slightly different concepts, are comparable and can be used in a complementary way. - Data regarding top EU R&D performers (that is, companies included in Scoreboard rankings who are the active part of the R&D internationalisation process) have to be considered from the starting point of such complementary use, instead of as final data at the country level resulting from official statistics. - Using top R&D performers� global values and adding aggregate values from national R&D statistics allows novel insights on the R&D internationalisation process to be given, at least for the four EU countries involved. - Further research could rely on the forthcoming Euro-Group Register under development at EUROSTAT, to obtain a clear view of intra-EU cross-country R&D flows.JRC.DDG.J.3 - Knowledge for Growt

Probability Distributions of Local Modal-Density Fluctuations in an Electromagnetic Cavity

International audienceResults from random-matrix theory are applied to the modeling of random fluctuations in the modal density observed in an electrically large cavity. By starting from results describing the probability distribution of the modal spacing between adjacent frequencies of resonance, or nearest-neighbor spacing, we introduce a simple procedure allowing to pass from the modal spacing to the local modal density as measured over a finite bandwidth. This local definition of the modal density is more consistent with the physics of reverberation chambers, since it has been recently shown that the deviation from asymptotic statistics of field samples is dependent on the number of modes overlapping within a modal bandwidth. It is shown that as opposed to current interpretation, the number of overlapping modes is a strongly fluctuating quantity, and that estimating it by taking the frequency derivative of Weyl's formula can lead to non-negligible errors and misunderstandings. Regarding these fluctuations as second-order effects is therefore not sound from a physical point of view, since the existence of modal depleted scenarios can easily explain the appearance of local anomalies in the field statistics, particularly, but not exclusively, in the lower frequency range of operation of reverberation chambers

A Skeptic's View of Unstirred Components

International audienceIn the practice and research on reverberation chambers, the concept of unstirred field components have often been invoked as soon as empirical distributions fail to comply with goodness-of-fit tests applied to data collected at a frequency of operation far beyond the lowest usable frequency. The current explanation for this phenomenon is that under certain conditions, the field samples generated by means of a stirring procedure are not characterized by a zero average-value, but actually present a deterministic offset term that is linked to a line-of-sight contribution. In this paper, we prove that this practice is not sound, as it does not acknowledge the fact that even a very low but non-zero residual correlation between the samples is enough to put in jeopardy the validity of the limits imposed by most hypothesis-test statistics, and hence their ability in properly detecting any constant contribution. An alternative approach is here proposed, based on the analysis of the variability of the line-of-sight contribution estimate, capable of accounting for the residual correlation in a reverberation chamber. Experimental results are presented to support the validity of our approach, exposing the critical use of goodness-of-fit tests as currently applied

Studio e progetto di sistema DSP per la cancellazione di rumore ambiente in caschi Bluetooth

Il lavoro svolto ha come obiettivo il denoising, ovvero la soppressione o quantomeno la riduzione del rumore: applicheremo le soluzioni dedotte ai segnali audio, illustrando le tecniche che meglio si adattano alle nostre specifiche di progetto. Saranno presi in considerazione gli aspetti teorici del degrado dell'audio e valutato attentamente lo stato dell’arte delle tecniche di denoising, passando in rassegna i metodi generali per l’elaborazione dei segnali vocali. Lo scopo di questo lavoro di tesi consiste nello studio e nella scelta del miglior sistema possibile di cancellazione del rumore e nel valutarne i suoi criteri di applicabilità ed adattamento al fine di progettare un sistema DSP per la cancellazione di rumore ambiente che giunge al canale audio integrato in un sistema di interfono wireless Bluetooth. In particolare, verranno da noi descritte due metodologie differenti che permettono l’ottenimento di buoni risultati in termini di rapporto segnale – rumore e illustrati gli algoritmi fondamentali utilizzati per permettere l'elaborazione tramite le tecniche proposte. Il lavoro si conclude con la realizzazione di un filtro digitale FIR su piattaforma DSP, al fine di creare le basi per una futura implementazione di un filtro adattivo che possa, attraverso operazioni in tempo reale, garantire una migliore comunicazione attraverso il canale dell'interfono operante secondo la più recente tecnologia wireless Bluetooth

Average Number of Significant Modes Excited in a Mode-Stirred Reverberation Chamber

International audienceAlthough the number of significant modes is intuitive, this concept has never been clearly defined, and this, mainly because of the unbound number of modes involved in modal overlap. In the present paper, we show that, for a perfect stirring process, the effect of modal overlap can be modeled as an equivalent filtering formulation. By introducing the statistical-bandwidth concept we show that the electromagnetic field statistics due to an infinite number of modes can be summarized by a finite number of significant modes. The case of the electric-energy density in an mode-stirred reverberation chamber (MSRC) has been considered and a new expression of its variability has been established. The good agreement found between the new expression and experimental and simulation results support the several concepts introduced in this paper

GOODNESS-OF-FIT TESTS IN RADIATED SUSCEPTIBILITY TESTS

International audienceGoodness-of-fit (GoF) tests are commonly used to assess the degree of perfection of a reverberation chamber to fulfill the overmoded condition. One may expect the outcome of GoF tests to provide some hints on the value of practical interest. The present paper shows that no correlation is found between the outcome of GoF tests and the range of values taken by the maximum power which is of practical interest in radiated susceptibility tests

Increasing peak-field generation efficiency of reverberation chamber

International audienceThe use of time-reversal techniques has been shown to allow focusing energy in a spot about half a wavelength wide. The fact of being able to concentrate energy into a reduced volume of space implies higher power densities and, ultimately, higher field levels. The use of this feature for improving the ability of a reverberation chamber in converting energy into high-intensity fields is investigated here. Experimental results are compared to those predicted by a simple asymptotic model, revealing the role played by losses and frequency bandwidth and how the performance of time-reversal techniques depends on these parameters