Ing. Jan Kučera, Ph.D.

At the Czech Metrology Institute (CMI) we have developed a precision modular sinewave generator for impedance ratio bridges. The generator was developed to improve previously available designs regarding amplitude and phase accuracy, linearity, absolute stability, stability of the ratio between two outputs and harmonic distortion. It generates up to 7 V-rms in a frequency range from 1 mHz up to 20 kHz, extendable to 100 kHz with small changes to the filters. The amplitude resolution is better than 0.01 mu V V-1 of full scale with an output voltage stability of 0.05 mu V V-1/30 min and a stability for the ratio between two outputs of 0.02 x 10(-6) over several hours. The generator can be powered from internal batteries and is controlled via optically isolated connections. The internal clock and voltage references can be replaced by external ones, optically coupled in the case of the clock. In this paper, we discuss experimental results obtained with the generator used as a signal source in digital impedance bridges with relative combined uncertainties from 10(-5) down to 10(-8). The generators have been used in a bridge to drive a quantum Hall resistor in the AC regime. The use of a generator with an AC quantum voltmeter will also be discussed. The generator is not only applicable in the field of AC impedance metrology but also for on-site comparisons of AC quantum voltage standards or, in general, where there is a need for precision voltage sources.

In this paper we describe ac and dc quantum Hall effect (QHE) measurements at temperatures above 1.5 K which is beyond the commonly used value for high precision measurements of the quantum Hall resistance (QHR). It is shown, that GaAs devices with high electron density of about 5x1011 cm2 can be used as reliable dc resistance quantum standards with quantization of RH(2) value with accuracy within few parts in 109 even at temperatures up to 4.2 K. The devices have exhibited well-developed quantum Hall plateaus in the ac regime too, again at temperatures up to 4.2 K. The measurement system consists of a self-made cryogenic probe with TO-8 type socket together with coaxial wiring and a digitally assisted bridge (DAB) suitable for ac quantum Hall effect measurements. The experiments were performed with devices placed in new double-shielded TO-8 holders. Our investigations and the evaluation of the measurement system are dedicated for the future realization of the quantum impedance standard based on ac QHR.

A national traceability chain from dc quantum Hall resistance (QHR) to capacitance has been established at Korea Research Institute of Standards and Science using various impedance standards and coaxial bridges. An ac-dc calculable resistance standard of nominal value of 12.9 k Omega was used as a transfer standard from dc to ac at the frequency of 1.541 kHz. Two ac coaxial bridges, developed by INRiM, were employed: a ratio bridge, linking the 103-k Omega ac resistor to the 12.9-k Omega ac-dc calculable resistor calibrated in terms of the dc QHR and a quadrature bridge, calibrating the 1000 pF capacitors versus the 103-k Omega ac resistors. The ratio bridge defines the 103-k Omega resistor as a two-terminal-pair (2TP) standard and the 12.9-k Omega resistor as a four-terminal-pair standard; a cable correction was applied to the 2TP defined standard. The stability of all standards has been monitored for more than 3 years to be within a few parts in 10(6). The capacitance value, obtained with the new traceability chain, shows an agreement (a few parts in 10(7)) in a comparison with the reference values, traced from the National Institute for Standards and Technology (USA).

A reconfigurable digital bridge for calibrating four terminal-pair (4-TP) defined impedance standards in the whole complex impedance plane by means of fully digital (FD) operation of the bridge and for R-R, C-C calibration and ac quantized Hall resistance experiments by involving reference inductive voltage divider in digitally assisted (DA) configuration has been built at CMI in recent years. For the purposes of the bridge, high-accuracy digital voltage sources operating in the audio frequency range with full isolation between modules and with battery operation mode have been developed. Together with coaxial switches with negligible crosstalk between channels, the voltage sources form the basis for full automation of the calibration process. The bridge has been tested in both FD and DA configurations. Depending on the configuration, different relative accuracies of the measurements can be achieved in the range from a few parts in 10⁸ to a few parts in 10⁵.

This paper deals with methods for calculating frequency characteristics of high frequency four-terminal-pair coaxial impedance standards. An analytical calculation for coaxial resistance and capacitance standards was performed at frequencies up to 200 MHz. For verification of the used method, numerical calculations based on domain FEM (Finite element method) in low and high frequency ranges are presently under investigation. The paper describes and summarizes first results, which will serve as a base for ongoing work on evaluating the effects of constructional limitations of four-terminal-pair impedance standards.

An universal coaxial multiplexer for switching one measurement device between several measurement points is described. The coaxial multiplexer consists of independent switch modules (channels), which are controlled via optical fibres. The off-state isolation of multiplexer was measured to be better than -185 dB at 1 kHz. Properties of the multiplexer enable to use it in automation of high precision 2-TP and 4-TP coaxial a.c. bridges or other precise measurement applications.

This paper describes improved calculable 4TP coaxial capacitance standards with nominal values 10 pF and 100 pF. Modifications of construction and internal wiring made possible to lower and better define mutual inductions between some standard's parts than in previous version. Frequency dependencies of the standards were calculated up to 1 MHz and first measurements shown a good consistency between theoretical calculations and measurements. Achievable relative accuracy of measurements with such a type of capacitors seems to be on the level 10-5 in frequency range at least up to 1 MHz.

A four-terminal pair coaxial a.c. bridge was developed for calibrating both resistance and capacitance ratios and working in the frequency range from 100 kHz up to 1 MHz. Use of the same a.c. bridge with minimal changes for calibrating reference dividers and unknown impedances simplifies the whole calibration process. 1:1 and 10:1 ratio calibrations are performed with uncertainty of a few parts in 1e5. Bridge performance was checked on the basis of resistance ratio measurements and also capacitance ratio measurements.

Alternating current measurements of double-shielded quantum Hall devices have a fascinating property of which only a quantum effect is capable: it can detect its own frequency dependence and convert it to a current dependence which can be used to eliminate both of them. According to an experimentally verified model, the residual frequency dependence is smaller than the measuring uncertainty of 1.3 × 10-9 kHz-1.In this way, a highly precise quantum standard of impedance can be established, without having to correct for any calculated frequency dependence and without the need for any artefact with a calculated frequency dependence. We hope that our results encourage other national metrology institutes to also apply it to impedance metrology and further explore its beautiful properties.

A new series-parallel capacitance device for calibration of high-frequency inductive voltage dividers is described, which makes calibrations of ratios N:1, N = 1, …, 10 at a frequency of 1 MHz with uncertainties of less than one thousandth of a percent possible.

1000 Ohm and 1290.64 Ohm coaxial resistors with calculable frequency dependence have been realized. 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.

Four terminal-pair coaxial capacitance standards with nominal values of 1 pF and 10 pF have been realized, thin cylindrical electrodes of which are formed by copper foils glued on insulating supports. Parasitic inductances of both standards have been calculated and contribution of these inductances to frequency dependences of the standards has been evaluated. The values of the 1 pF and the 10 pF standard change by 0.04 % and -0.05% respectively when frequency changes from 1 kHz to 1 MHz.

Coaxial resistors with a calculable frequency dependence have been realized at PTB as a part of the chain from the dc quantized Hall resistance to the capacitance standard. The construction of resistors with nominal value of 1000 Ohm and 1290.64 Ohm is described. A modified construction of the original Haddad resistor allows a full dismantling while the outer shielding is intact, and the resistance is adjustable within 10 parts in million.

Quadrifilar resistors with nominal values of 100 Ω, 1000 Ω and 10000 Ω, as well as a 100 Ω coaxial resistor, have been realized and their frequency dependences have been calculated from their constructional parameters. With the aim of experimentally verifying correctness of mathematical models used in these calculations, frequency dependences of ratios of parallel equivalent resistances of the realized resistors have been measured by means of a four terminal-pair transformer bridge.

Common optical methods for dimension measurement apply an image sensor with a lens. The presence of the lens in the camera increases the total dimensions of the measuring setup. Furthermore, the aberrations of the lens represent a source of uncertainty. In order to simplify the measuring setup and to reduce its overall dimensions, the method proposed in this paper applies a measuring setup that contains no lens - it employs an image sensor without a lens and two point light sources that illuminate the measured object.

Pro měření kmitočtových závislostí Rogowskiho cívek byly navrženy dvě různé metody. První z nich je založena na využítí kalibrátoru Fluke 5100B, číslicového multimetru Datron Wavetek 1291 a lock-in zesilovače SR 830. Při kmitočtech vyšších než je horní mez kmitočtového pásma kalibrátoru se používá přesný měřič LCR Agilent 4284A.

A new construction of permuting capacitors device (PCD) for calibrating 10:1 high-frequency IVDs is described. SMD reed contacts are used for PCD capacitors switching. By means of a method based on the PCD and on voltage injection, calibration uncertainties better than 20 ppm (k=2) were achieved.

A new construction of permuting capacitors device (PCD) for calibrating 10:1 high-frequency IVDs is described. SMD reed contacts are used for PCD capacitors switching. By means of a method based on the PCD and on voltage injection, calibration uncertainties better than 20 ppm (k=2) were achieved.

In the paper, A Novel Optical Method of Dimension Measurement of Objects with Circular Cross-section is presented. The application of this method using linear CCD sensor and the optical triangulation method for dimension measurement of objects with circular cross-section is described. Also, several issues related to its implementation are discussed and its performance is illustrated using experimental results.

Two different methods have been proposed for measuring frequency dependences of Rogowski coils. First of them is based on employment of a Fluke 5100B calibrator, a Datron Wavetek 1291 digital multimeter and a SR 830 DSP lock-in amplifier. At frequencies exceeding the upper limit of the frequency range of the calibrator, measurements are made by means of an Agilent 4284A precision LCR meter, which is operated in the mode of measurement of self-inductance.

Three versions of inductively coupled ratio arms have been prepared in the form of two-stage inductive voltage dividers. In a frequency range up to 1 MHz ratio errors of these dividers have been measured by comparison with a reference divider composed of eleven cyclically permuted capacitors.