The Electrical Metrology Department performs three main research activities:
- DC electrical metrology;
- AC, RF and microwave electrical metrology;
- Electrochemical metrology.
Specifically,it improves, maintains and disseminates the national standards of current, DC voltage, resistance, capacitance, inductance, AC-DC transfer, power, energy, high frequency quantities and electrolytic conductivity. The Department is also involved in the development of new measurement systems, takes part in and coordinates international comparisons, as an active member of the CCEM, CCQM, EUROMET and CODATA.
Accurate measurement of DC currents, in the picoampere range, is
important for the calibration of instruments like electronic device curve
tracers, dosimeters for ionizing radiation, charge amplifiers.
The Italian national standard of DC current is being extended down to the fA
range. Two current sources have been developed. The first one, working in the
100 pA - 100 fA range, is based on charges and discharges of a
gas-dielectric capacitor. The second, working in the 10 µA - 10 pA
range, makes use of a calibrated resistor. Current is traceable to national
standards of voltage, and resistance or capacitance and frequency. Both will be
employed in the EUROMET EM-S24 international intercomparison.
The new system for generation of DC currents in the 100 pA-100 fA range.
The set-up of the system dedicated to the measurement of resistance in the range 1 µΩ to 10 Ω with current up to 2000 A has been completed and characterized. It can measure very low value resistors by direct comparison to 1 Ω and 10 Ω standards with an accuracy of the order of a few parts in 10-6.
The measurement system for the calibration of low value resistors.
A system for calibration of clamp-meters in AC current (50 Hz) in the field (0-1500) A, was developed. The system consists of a supply transformer with secondary circuit that can supply up to 1500 A. The supplied current is measured by a standard current transformer, a standard resistor of 50 mΩ and a voltmeter. The current clamp is applied to a copper wire with suitable section and mounted on a oscillating support. The traceability levels span from 1.5·10-3 at 100 A to 4.2·10-3 at 1500 A.
Improvements have been carried out on the electrolytic conductivity measurement system for participating in the International Key Comparison CCQM-K36 (Pilot Laboratory DFM-Denmark), on electrolytic conductivity measurement of diluted solutions with nominal conductivity of 0.5 and 0.005 S/m. Studies of planar electrode geometry have been carried out with the Applied Electromagnetics Department in order to realise a new electrolytic primary cell. The advantages of this new structure are an easier production and measurement set-up, along with the possibility of accurate measurements on highly diluted solutions. In particular, the electrode geometry can be designed in order to adapt the measured resistance value to the best instrument specifications.
The primary power standard is now available from direct current to 40 GHz for three different 50-ohm transmission lines (7 mm, 3.5 mm and 2.92 mm). The standard is obtained with the microcalorimeter technique and the transfer device is now a thermoelectric power sensor for all frequency band and transmission line types. Modified commercial thermocouples turned out to be more efficient than the bolometric detectors and have allowed a significant accuracy improvement. S-parameter measurements, range from 100 kHz to 50 GHz. This calibration technique and its traceability are maintained only for 7, 3.5, 2.92, 2.4 mm coaxial lines.
Upper figure: view of modified power sensor.
Lower figure:accuracy improvement of the power standard for the 3.5 mm coaxial line.
In the framework of the EUROMET.EM-K11 international comparison, accurate calibrations of AC-DC transfer standards at millivolt level have been performed by means of low voltage single junction thermal converters and resistive dividers. For the same purpose, a new calibration method using a wide-band transformer for AC-AC comparisons of transfer standards has been investigated.
AC-AC comparisons of a transfer standard at millivolt range by a wide-band transformer.
A new resistive divider has been built, which
allows the voltage section of the primary system for power to operate
in the most suitable ranges and for a wider frequency range. The traceability
chain from the national standards to the three-phase measurement system
and some comparison programs have been revised.
A system for characterisation of high precision ADCs and DACs based on a sampling
voltmeter and a high-resolution generator has been set up. The main
output of the generator is adjusted by a second output with the same
resolution but with lower weight. Automatic adjustment either increases
the accuracy or decreases the distortion respectively for the static
and the dynamic characterisation.
A new four terminal-pair transformer ratio bridge, constructed for resistance and capacitance scaling in the audio frequency range, has been built: it will become part of the traceability chain for the reproduction of the farad from the quantized Hall resistance. The ratio uncertainty is lower than 1·10-7.
The new four terminal-pair bridge.
A new set-up for traceable measurement of voltage, current and power noise in the audio frequency range has been constructed, with sensitivity better than 10-23 W/Hz. It will be employed in measurement of noise spectra of active and passive devices and in the measurement of Johnson noise of impedance standards.