It is almost a natural law: when attempting to improve something, the effort to do this increases disproportionately to the benefit. This also applies when it comes to reducing measurement uncertainties: to become twice as precise, usually a four times longer measurement time is required. With the newly developed system for resistance measurements, however, a four times smaller uncertainty is achieved in half the measurement time.
Electric resistance is one of the most important measurands, as many physical quantities are determined via a resistance measurement. The extreme precision with which resistances are calibrated is achieved by traceability to fundamental constants via the quantum Hall effect in combination with a complex metrological set-up employing so-called cryogenic current comparators which use superconducting effects.
Thereby, measurement uncertainties of less than 10–9 are achieved – but at the same time, measurement times of almost one hour have to be tolerated. In the case of the cryogenic current comparator system developed at PTB, there is no such disadvantage.
This is, basically, achieved by two innovations: firstly, the frequent change of the direction of the measurement current – which is indispensable for the measurement procedure used – can be performed with considerably increased velocity. This was achieved by the development of a fast current source which can be triggered digitally, and by the use of the most recent magnetic field sensors which have been developed at PTB and are based on super-conducting quantum interferometers. Secondly, a new detector with a higher bandwidth has been developed for the measurement of the bridge voltage, which amounts to a few nanovolts only. In addition, its disturbing backaction on the interferometer has been considerably reduced.
The improvements result in a four times lower uncertainty, which is reached in half the measurement time. At present, the system is combined with a new lowtemperature magnet for the quantum Hall effect so that – after more than twenty years of successfully using the previous quantum resistance standard – PTB now has the latest standard of this type worldwide at its disposal.
Due to the cooperation with PTB, Magnicon GmbH in Hamburg – which developed and manufactured the required electronic components on behalf of PTB – is now able to offer both the highly sensitive fast voltage detector as an independent measuring instrument and the complete cryogenic current comparator systems.
The efficiency of the new system, which was demonstrated by PTB at an international technical conference, has already resulted in the first orders for the medium-sized enterprise.
Martin Götz
Department 2.6 Electrical Quantum Metrology
Phone: +49 (0) 531 592-2104
E-mail: martin.goetz(at)ptb.de
Götz, M.; Drung, D.; Pesel, E.; Ahlers, F.-J.: Settling Behavior of the Bridge Voltage in Resistance Ratio Measurements with Cryogenic Current Comparators. IEEE Trans. Instrum. Meas.
Drung, D.; Storm, J.-H.: Ultra-low Noise Chopper Amplifi er with Low Input Charge Injection. IEEE Trans. Instrum. Meas.