J. Bohacek

AC measurements of the quantized Hall resistance

The quantized Hall resistance R/sub H/(i), for i=2, 4 and 6 has been measured over the frequency range from dc to 3 kHz using a standard four terminal-pair impedance bridge. For the ith plateau we found that R/sub H/(i) depended linearly on the magnetic flux density B and quadratically on the frequency f. That is, the relative difference /spl Delta/R/sub H/(i,f)/R/sub H/(i) between the ac and dc values is consistent with the empirical relationship k(i)Bf/sup 2/, where k(i) is constant. Below 3 kHz k(i) decreases by approximately a factor of six as i increases from 2 to 6. These results are consistent with inductances /spl ap/100 /spl mu/H, which depend on B, in series with each terminal of the quantum Hall device. From dc to 3 kHz we can derive the ac value of a standard resistor in terms of R/sub H/(2) with a type A relative uncertainty (1/spl sigma/) of 0.05/spl times/10/sup -6/ f/sup 2/ when f is expressed in kHz. >

AC QHE-based calibration of resistance standards

A coaxial transformer bridge for QHE-based calibration of resistance standards has been developed. An eight-decade two-stage inductive voltage divider forms ratio arms of the bridge and triple-series connection of the reference QHE sample to the bridge is employed. Frequency characteristics of LEP heterostructures have been measured by comparing them against resistors with calculable frequency dependences. Two quadrifilar, Gibbingss type resistors with nominal values of 12 906 /spl Omega/ and 6353 /spl Omega/ have been used in this measurement.

The European ACQHE project: modular system for the calibration of capacitance standards based on the quantum Hall effect

Starting in 1998 a project funded by the European Commission has been carried out in a co-operation of seven partners. The aim of this project was to establish a measurement system which allows the calibration of standard capacitors in terms of R/sub K-90/. The whole system comprises suitable quantum Hall samples, an automated bridge system and auxiliary devices to calibrate and characterize the whole set-up. The uncertainty for the calibration of a 10-pF capacitor is about 1 part in 10/sup 7/.

Octofilar resistors with calculable frequency dependence

The results are presented of calculation of the frequency dependence of an octofilar resistor, the resistive element of which is folded in such a way that it forms four long loops connected in series. Magnetic fluxes produced by the loops tend to cancel, similar to the fluxes produced by the two loops of the common quadrifilar reversed resistor. The folded length of the octofilar resistive element is one half of that of a quadrifilar resistor made from the same wire and having the same nominal resistance, and the octofilar resistor has a smaller frequency dependence. As an example, a 12906 Ω octofilar resistor is described, for which the frequency dependence of its parallel equivalent resistance is more than seven times smaller than that of a previously made 12906 Ω quadrifilar resistor in the frequency range 500 Hz to 5 kHz.

Calculable resistors of coaxial design

1000 Ω and 1290.64 Ω coaxial resistors with calculable frequency dependence have been realized at PTB to be used in quantum Hall effect-based impedance measurements. In contradistinction to common designs of coaxial resistors, the design described in this paper makes it possible to remove the resistive element from the shield and to handle it without cutting the outer cylindrical shield of the resistor. Emphasis has been given to manufacturing technology and suppressing unwanted sources of frequency dependence. The adjustment accuracy is better than 10 µΩ Ω−1.

An HF coaxial bridge for measuring impedance ratios up to 1 MHz

A four-terminal pair coaxial ac bridge developed for calibrating both resistance and capacitance ratios and working in the frequency range from 100?kHz up to 1?MHz is described. A reference inductive voltage divider (IVD) makes it possible to calibrate ratios 1:1 and 10:1 with uncertainty of a few parts in 105. The IVD is calibrated by means of a series?parallel capacitance device (SPCD). Use of the same ac bridge with minimal changes for calibrating the SPCD, IVD and unknown impedances simplifies the whole calibration process. The bridge balance conditions are fulfilled with simple capacitance and resistance decades and by injecting voltage supplied from the auxiliary direct digital synthesizer. Bridge performance was checked on the basis of resistance ratio measurements and also capacitance ratio measurements.

A QHE-based system for calibrating impedance standards

Coaxial transformer bridges applied to a newly formed system for calibrating impedance standards are described. In this system, an R-R bridge characterizes reference quantum Hall devices as well as directly links them to 12906.4 /spl Omega/ or 10000 /spl Omega/ standard resistors. These resistors, in turn, serve as reference standards in capacitance calibrations made by a multifrequency quadrature R-C bridge and a C-C bridge. Finally, by means of a multifrequency double-balance L bridge, 1- and 10-nF capacitors calibrated in this manner are linked to inductance standards whose values range from 1 to 100 mH.

Calibrating Standards of Mutual Inductance

As the final stage of development of a new system for impedance calibrations, a facility for calibrating standards of mutual inductance has been realized. Being based on the three-voltmeter method, this facility makes it possible to calibrate 1 mH to 100 mH standards at frequencies up to 5 kHz with uncertainties of the order of 0.01 %. An improved direct connection technique is used when routine calibrations of mutual inductors are made by means of a four terminal-pair LCR meter

Measurement of Power Factors of Standard Capacitors

In the paper, a method for measurement of power factors of standard capacitors is described. Resistors with calculable time constants are used as reference standards. The measurement may be made with an uncertainty of less than 1.10-5 in the range of 1000-10 000 pF and at 1592 Hz.

A new capacitance device for calibration of N:1 HF inductive voltage dividers

A new series-parallel capacitance device (SPCD) for calibration of high-frequency inductive voltage dividers with nominal ratios N:1, N = 1…10, is described. A fully coaxial design and well-defined calibration planes with four terminal-pair (4-TP) interface make it possible to evaluate SPCD ratios with uncertainties of only a few parts per million at a frequency of 1 MHz.