Istituto Elettrotecnico Nazionale Research Articles

Comparison between frequency standards in Europe and the USA at the 10−15 uncertainty level

Istituto Elettrotecnico Nazionale Galileo Ferraris (IEN), National Institute of Standards and Technology (NIST), National Physical Laboratory (NPL), Laboratoire National de Métrologie et d'Essais—Observatoire de Paris/Systèmes de Référence Temps Espace (OP) and Physikalisch-Technische Bundesanstalt (PTB) operate cold-atom based primary frequency standards which are capable of realizing the SI second with a relative uncertainty of 1 × 10−15 or even below. These institutes performed an intense comparison campaign of selected frequency references maintained in their laboratories during about 25 days in October/November 2004. Active hydrogen maser reference standards served as frequency references for the institutes' fountain frequency standards. Three techniques of frequency (and time) comparisons were employed. Two-way satellite time and frequency transfer (TWSTFT) was performed in an intensified measurement schedule of 12 equally spaced measurements per day. The data of dual-frequency geodetic Global Positioning System (GPS) receivers were processed to yield an ionosphere-free linear combination of the code observations from both GPS frequencies, typically referred to as GPS TAI P3 analysis. Last but not least, the same GPS raw data were separately processed, allowing GPS carrier-phase (GPS CP) based frequency comparisons to be made. These showed the lowest relative frequency instability at short averaging times of all the methods. The instability was at the level of 1 part in 1015 at one-day averaging time using TWSTFT and GPS CP. The GPS TAI P3 analysis is capable of giving a similar quality of data after averaging over two days or longer. All techniques provided the same mean frequency difference between the standards involved within the 1σ measurement uncertainty of a few parts in 1016. The frequency differences between the three fountains of IEN (IEN-CsF1), NPL (NPL-CsF1) and OP (OP-FO2) were evaluated. Differences lower than the 1σ measurement uncertainty were observed between NPL and OP, whereas the IEN fountain deviated by about 2σ from the other two fountains.

Radiometric reference for weak radiations: comparison of methods

The parametric down conversion of photons generated in a non-linear crystal gives rise to two correlated photons. When associated with a system of coincidence counting, this phenomenon allows one to measure the quantum efficiencies of detectors working at a normal rate of photon counting, without the need for reference sources or detectors. At the Institut National de Métrologie (INM) (France), this method was implemented with the aim of developing a new standard detector for the absolute measurement of very weak radiation. The validation of this method is presented from an international comparison between the laboratories of the INM (France) and the Istituto Elettrotecnico Nazionale Galileo Ferraris (IENGF) (Italy) and from a comparison of methods at INM using the French national standard detector, the cryogenic radiometer.

New Calibration Method for Coaxial Microcalorimeters

The most critical point in the realization of an electrical power standard based on a microcalorimeter is the accurate determination of the microcalorimeter losses and their effect on the measurand, i.e., the effective efficiency of the sensor under calibration. At the Istituto Elettrotecnico Nazionale (IEN) Galileo Ferraris, a procedure has been introduced that makes the microcalorimeter constant determination more accurate and reliable. Experiments with the 7-mm coaxial microcalorimeter in 50 MHz-18 GHz range have shown a significant improvement in the uncertainty for the Italian national power standard at the 0.2% level.

The IEN CCC Bridge to Scale the Quantized Hall Resistance to 1-<tex>$Omega$</tex>Standards

A resistance bridge based on a cryogenic current comparator has been built at the Istituto Elettrotecnico Nazionale "G. Ferraris" (IEN). The use of this bridge simplifies the scaling down from the quantized Hall resistance to decadic standard resistors, at the same time reducing the measurement uncertainty. The bridge is described and the result of a comparison at the level of 100 /spl Omega/, with the traditional scaling method used at IEN is reported. The two methods agree well within their uncertainties, for which detailed uncertainty budgets are given.

Linking the results of Key Comparisons CCEM-K8 and EUROMET.EM-K8

Key comparisons CCEM-K8 and EUROMET.EM-K8 on DC voltage ratio were carried out in parallel, both having the Istituto Elettrotecnico Nazionale G Ferraris (IEN) as the pilot laboratory. Two Datron 4902S voltage dividers were used as travelling standards, each divider being dedicated to one comparison. On these dividers the measurements of the ratios 1000 V/10 V and 100 V/10 V were mandatory and were used to evaluate the degrees of equivalence of the participants with respect to the reference value of their own comparison.The reference value of a key comparison carried out by a Consultative Committee of the CIPM, like CCEM-K8, is called the Key Comparison Reference Value (KCRV), and the degrees of equivalence of a regional comparison, like EUROMET.EM-K8, must be translated in terms of the KCRV. The two comparisons under study concern the measurement of voltage ratios, hence no national standard is involved. Then any participant in both comparisons can be considered equally able, in principle, to perform an unbiased measurement of the travelling standard. Since the number of participants in each comparison is significant and different methods of measurement were used, it is unlikely that the corresponding comparison reference value is affected by systematic errors. The degrees of equivalence with respect to the KCRV of the participants in EUROMET.EM-K8 are then the same as those with respect to the reference value of EUROMET.EM-K8.The extended linking document reports a verification of such an assumption, based on the results of the common laboratories of the two comparisons.Main text. To reach the main text of this paper, click on Linkage Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The linkage report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the Mutual Recognition Arrangement (MRA).

Report on Key Comparison EUROMET.EM-K8 (DC voltage ratio)

The purpose of the regional comparison EUROMET.EM-K8 was to compare the scaling capabilities up to 1000 V DC of the European national metrology institutes, thus extending to Europe the evaluation of equivalence of voltage ratio measurements obtained by the comparison CCEM-K8. Twenty laboratories participated, with the Istituto Elettrotecnico Nazionale Galileo Ferraris (Torino, Italy) acting as pilot laboratory. The circulation of the travelling standard, a resistive voltage divider, started in July 1998 and ended in January 2002. The technical protocol requested the measurement of voltage ratios 1000 V/10 V and 100 V/10 V, the measurement of other ratios being optional. Nineteen laboratories reported measurements of the mandatory ratios. For these ratios, the measurement methods used are described and the process to evaluate the comparison reference value and the degrees of equivalence is reported in detail. The relative uncertainty of the comparison, given by the combination of the uncertainty of the reference value and the instability of the transfer standard, was about 0.26×10-6 for ratio 1000 V/10 V and about 0.16×10-6 for ratio 100 V/10 V, at 95% level of confidence. The results for the optional ratios are reported in an appendix. With some exceptions, the agreement between the results is satisfactory.Main text.To reach the main text of this paper, click on Final Report.Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the Mutual Recognition Arrangement (MRA).

Precision impedance measurements by the three-voltage method with a novel high-stability multiphase DDS generator

The system for the intercomparison of unlike impedances of the Istituto Elettrotecnico Nazionale Galileo Ferraris, based on the three-voltmeter method and four-terminal pair definition of the standards, has now been provided with a new direct-digital synthesis generator, having multiple outputs, isolation among the channels, and high amplitude and frequency stability. Experimental results obtained using the new generator are shown and compared with those achieved with the previous system configuration.

A new microcalorimeter for measurements in 3.5-mm coaxial line

A new microcalorimeter has been realized for implementing the national power standard up to 26.5 GHz in a 3.5-mm coaxial line at the Istituto Elettrotecnico Nazionale (IEN) Galileo Ferraris, Turin, Italy. The system is based on a dry thermostatic cell controlled by Peltier elements. The thermal load consists of a twin sensor system that is alternately supplied with high frequency test power and low frequency or direct current (DC) reference power through adiabatic coaxial lines. This microcalorimeter was originally designed for calibrating bolometric mounts. Recently, it has been modified for thermocouple power sensors, which can operate from DC to 26.5 GHz in a 3.5-mm coaxial line.

Extension of the IEN traceability for ac voltages below 200 mv

This paper describes the system, the standards, and the procedures developed at the Istituto Elettrotecnico Nazionale Galileo Ferraris for the extension of traceability to low voltage ranges down to 1 mV. Specific new features are the technique for the generation of low voltage, the procedures for building the traceability down to 1 mV, and the method for the evaluation of the load correction based on perturbations with known admittances.

Final report on the subsequent bilateral comparison of cryogenic radiometers CCPR-S3 between the BIPM and the IEN

In 1994 the Consultative Committee for Photometry and Radiometry (CCPR) decided to start an international comparison of cryogenic radiometers. The BIPM acted as pilot laboratory. Following the publication of the results of this comparison in 2000, the Istituto Elettrotecnico Nazionale (IEN, Italy) expressed the wish to repeat the comparison. The report describes the results of this subsequent bilateral comparison between the BIPM and the IEN.The comparison was carried out by using silicon trap detectors as transfer instruments, to be calibrated at a number of laser wavelengths at the IEN and the BIPM.The results obtained by the IEN in the subsequent comparison are in much better agreement with the BIPM results than in the previous comparison. The BIPM providing a link to the previous large-scale comparison, this leads also to a considerably better agreement between the IEN and the other participants of the first comparison.Main text.To reach the main text of this paper, click on Final Report.Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by CCPR, according to theprovisions of the Mutual Recognition Arrangement (MRA).

Final report on key comparison CCEM-K8 (dc voltage ratio)

The purpose of CIPM comparison CCEM-K8 was to compare the scaling capabilities up to 1000 V dc of national metrology institutes. Fifteen institutes participated, with the Istituto Elettrotecnico Nazionale Galileo Ferraris (Turin, Italy) acting as pilot laboratory. Circulation of the travelling standard, a resistive voltage divider, started in October 1998 and ended in June 2001. The technical protocol requested the measurement of voltage ratios 1000 V/10 V and 100 V/10 V, the measurement of other ratios being optional. For the mandatory ratios, the measurement methods used by the participants are described and the evaluation of the key comparison reference value and the degrees of equivalence is reported in detail. For these ratios, the relative uncertainty of the comparison, given by the combination of the uncertainty of the reference value and the instability of the transfer standard, was about 0.23×10-6 at 95% level of confidence. Results for the supplementary ratios are reported in an appendix. With certain exceptions, agreement between the results is satisfactory; in some cases the measurement uncertainty was underestimated.

Tracking generator of calibrated harmonics

A tracking generator of calibrated harmonics has been tested at the Istituto Elettrotecnico Nazionale Galileo Ferraris. This system acquires the sinusoidal input supplied by a precision ac calibrator, and the internal DSP produces, by means of a digital-to-analog converter (DAC), an output signal combination of calibrated harmonics components, which is added to the sinusoidal signal of the calibrator. In this way, an ordinary 12-bit DAC can produce a precision signal with an assigned THD less than 10%. Some algorithms for generating the signal and for tracking with high accuracy the internal reference to the external input signal have been developed, which allow the frequency and phase difference to be recovered within 10/sup -4/ rad in less than 0.1 s.

Intercomparison of a correlated-photon-based method to measure detector quantum efficiency.

We report on the absolute calibration of photodetector quantum efficiency by using correlated photon sources, performed independently at two laboratories, the National Institute of Standards and Technology and the Istituto Elettrotecnico Nazionale (IEN). The goal is to use an interlaboratory comparison to demonstrate the inherent absoluteness of the photon correlation technique by showing its independence from the particular experimental setup. We find that detector nonuniformity limited this comparison rather than uncertainty inherent in the method itself. The ultimate goal of these investigations is development of a robust measurement protocol that allows the uncertainties of individual measurements to be determined experimentally and verified operationally. Furthermore, to demonstrate the generality of the procedure, the IEN measurement setup was also used to calibrate a fiber-coupled avalanche photodiode module. Uncertainties are evaluated for the detector both with and without fiber coupling and differences are discussed. The current IEN setup using a thinner and higher transmittance nonlinear crystal for the generation of correlated photons shows a significant improvement in overall accuracy with respect to previously reported results from IEN [Metrologia 32, 501-503 (1996)].

Writing on Water: A Terra Nova Book

Review: Writing on Water: A Terra Nova Book Edited by David Rothenberg and Marta Ulvaeus Reviewed by Enzo Ferrara Istituto Elettrotecnico Nazionale, Italy David Rothenberg and Marta Ulvaeus (Eds.). Writing on Water: A Terra Nova Book. Cambridge, MA: MIT Press, 2001. 288pp. ISBN 0-262-18211-4 (Hardcover). US $24.95 The Terra Nova project aims to assess the artistic and cultural aspects of environmental issues, provided that they are scientific and political. The second book in the series, Writing on Water, is an effective selection of short works. It covers almost everything about water, from the practice of resource management and protection to the mysteries that water hides. Geology, ecology, chemistry, sociology, economics, myth, and history are included. The book is divided into five sections, with each one devoted to a specific theme. Several questions are posed and analyses of human relationships with water are offered. A scientific and philosophical approach is taken, mainly through introspective debates. Reflections are complemented with black and white photographs. Source and Substance, the pilot chapter, contains a scientific essay on the properties of aqueous solutions, remarking on how our understanding still fails as the investigation gets deeper. Political reflections on watersheds follow, along with poetry celebrating water as the ancestor of life. Teaching of the Flow warns us to respect the environmental equilibrium of water as taught by reflections upon the ancestral odysseys of men and salmon meeting in fisheries, or by the question of the immorality of weather modification proposed about the flood that struck Rapid City in 1972. Self- reflections are suggested when we are asked how much awareness exists in us about some kinds of subtle pollution, unconsciously circulating underground. Diving Deep is on the hidden, profound soul of water. Or, metaphorically, it is about the abyss where humans choose to keep their fears and hopes. An amusing novel here treats the inner conflict of scientific and poetical moods. Different rationales are found, but continue to be sought amid the covert aqueous straits of a mountain lake. Swept Away deals with the cycling rhythm of life, quiet as a shore can be or tumultuous as a swift river. One contribution smartly invokes correspondences

Automated system for inductance realization traceable to AC resistance with a three-voltmeter method

An automatic system for the comparison of unlike impedances in the range 1 /spl Omega/-10 k/spl Omega/, based on the three-voltmeter method, has been constructed and is undergoing extensive tests as a system for realization of the inductance scale at the Istituto Elettrotecnico Nazionale Galileo Ferraris (IEN), in the range 100 /spl mu/H-10 H at frequencies of 120 Hz and 1 kHz. The main features of the system are a two-phase direct-digital synthesis generator for energizing the standards, complete four-terminal pair definition of the standards with automatic servosystems, the use of a single high-accuracy voltmeter and a switching unit, and optical links for the distribution of frequency references. Measurements at the metrological level have been performed at 1 kHz with inductors in the range 10 mH-1 H.

Traceability of the 10-kΩ standard at IEN

The traditional scaling method used at the Istituto Elettrotecnico Nazionale (IEN) for the calibration of the 10-k/spl Omega/ standard is compared with more straightforward techniques: one based on the linearity of a DVM and the other on a commercial current comparator bridge. For the three methods, the measurement results and the uncertainty budgets are reported. The agreement is better than 1/spl times/10/sup -7/.

Quantum efficiency and dead time of single-photon counting photodiodes: a comparison between two measurement techniques

An experimental set-up based on correlated photons has been developed at the Istituto Elettrotecnico Nazionale G. Ferraris (IEN) for the absolute measurement of the quantum efficiency and dead time of single-photon counting photodiodes. The overall accuracy is improved with respect to previously reported results, by using a thin and highly transmitting non-linear crystal. For comparison, a conventional set-up has been developed at 632.8 nm with a He-Ne laser, a trap detector and calibrated neutral-density filters. Measurements at different count levels have been performed with both techniques and results are compared for the quantum efficiency in the limit of zero count, η0, and for the dead-time measurement, tD. Contributions to the final uncertainty are analysed for both techniques. The results obtained are η0 = 0.3372 (standard uncertainty u(η0) = 0.0016) and tD = 33.8 ns (u(tD) = 2.0 ns) in the case of the two-photon technique and η'0 = 0.3376 (u(η'0) = 0.0036) and t'D = 31.7 ns (u(t'D) = 5.4 ns) in the case of the conventional technique.

Stability evaluation of high-precision multifunction instruments for traceability transfer

In the traceability transfer process from the national standards of the Istituto Elettrotecnico Nazionale "Galileo Ferraris" to standards of calibration laboratories accredited by Servizio di Taratura in Italia, the use of high-precision multifunction instruments, in particular digital multimeters, as reference standards turned out to be a good transfer method. In fact, their use provides swift and less expensive transfer at an uncertainty level comparable to that one obtained using high-precision individual standards, such as 10 V DC voltage standards, standard resistors, or AC/DC transfer standards. In this paper, how this transfer process is performed and what results are obtained, are described.

Quantum-efficiency measurement of photodetectors by means of correlated photons

We report the first results obtained at the Istituto Elettrotecnico Nazionale Galileo Ferraris of quantum-efficiency measurements of actively quenched avalanche photodiodes in the photon-counting regime. The measurements were performed with the technique of correlated photons generated by a parametric downconversion process in a nonlinear crystal of lithium iodate, an absolute technique that does not require any standard. We pay special attention to the problem of alignment of the optical bench and to the implementation of a parametric amplifier scheme with a diode-laser source. The technique is described, and results of quantum-efficiency measurements of two photodiodes are presented.

Parametric amplification for radiance measurements

This paper presents the first results of parametric amplification obtained at the Istituto Elettrotecnico Nazionale G. Ferraris (IEN) at a wavelength of 633.5 nm in a crystal of lithium iodate, pumped by an argon laser at a wavelength of 351 nm and injected by a diode laser at 788.2 nm in the noncollinear phase-matching case. Amplified and spontaneous parametric signals around 633 nm were measured in a photon-counting setup, at different powers of both the argon and the diode laser. This experiment is preliminary to the absolute radiance measurement of a thermal source.