A low-voltage and high-frequency ac-dc transfer system usinginductive dividers

A setup for the calibration of high-input-resistance ac-measuring instruments for low voltages (mV) and high frequencies (1 MHz) is presented. The low voltages generated down to one hundredth of an ac standard voltage (0.5 V to 2 V) are traced by means of a one-decade inductive voltage divider (IVD) cascaded by a sub-IVD with a fixed ratio of 10-to-1. Construction details of the main IVD which are necessary to meet the requirements are given. The error characteristics are mainly determined by capacitive currents acting on stray and mutual inductances. The internally built-in resistive dc voltage divider and a special switch allow calibrations to be performed via effective ac-dc transfer at the standard voltage level. The first results for the calibration of a transfer standard, Fluke 792A, at 100 mV and 200 mV were in good agreement with results obtained with very sensitive multijunction thermal converters. Using such converters as standards in the new system will enable ac calibrations down to voltages below 1 mV     

An improved straddling method with triaxial guards for thecalibration of inductive voltage dividers at 1592 Hz

Calibrated inductive voltage dividers (IVDs) with the smallest possible uncertainties are required for the determination, maintenance, and dissemination of the ohm and the farad. An improved straddling method is described which uses triaxial guards to decrease systematic errors due to screen currents. The relative uncertainty with which IVD voltage ratios can be determined is calculated to be 1×10^-9 (1 σ) at a frequency of 1592 Hz     

1000V多盘感应分压器标准建立

中国计量科学研究院研制了一台1000V多盘感应分压器标准装置,用作工频1000V电压比例标准。该装置使用了一种基于互感器注入的误差补偿技术,可准确提供从1×10^-8到1的宽范围比例出。感应分压器的误差校验使用了包含三同轴屏蔽技术的改进型参考电势增量法,并设计了多比例参考互感器以便对感应分压器进行整体校验。所研制的感应分压器准确度优于1×10^-7,校准结果不确定度小于2×10^-8。     

Comparison of a Multichip 10-V Programmable Josephson Voltage Standard System With a Superconductor–Insulator–Superconductor-Based Conventional System

We developed a 10-V dc programmable Josephson voltage standard (PJVS) using a multichip technique. The PJVS was based on $hbox{NbN/TiN}_{x}/hbox{NbN}$ junctions and operated using a 10-K compact cryocooler. We carried out an indirect comparison with a superconductor–insulator–superconductor-based conventional Josephson voltage standard (JVS) by measuring the voltage of a 10-V zener diode reference standard. The combined standard uncertainty of the comparison was $u_{c} = 0.03 muhbox{V}(k = 1)$, and the relative combined standard uncertainty was $3 times 10^{-9}$.      

Improvement of the Voltage Dependence of High-Voltage AC–DC Transfer Differences at the NMIJ

A new ac–dc comparator system of 20–1000 V has been developed at the National Metrology Institute of Japan (NMIJ) for the calibration of ac–dc thermal voltage converters (TVCs), which are used as national ac–dc transfer standards. The ac–dc transfer differences of high-voltage transfer standards were evaluated by a traditional step-up procedure. The voltage dependence of the ac–dc transfer difference was observed in the earlier step-up procedures over 300 V, as reported by the latest international intercomparison. The experimental results for high-voltage TVCs with several different range resistors at the NMIJ suggest that the voltage dependence may primarily be caused by input connectors of range resistors and the change in the resistance value of the resistor and TVC modules due to heating from resistors. This paper describes the voltage dependence improvement of ac–dc transfer differences over 300 V at the NMIJ.      

Harmonic Power Standard at NIM and Its Compensation Algorithm

A new harmonic power standard has been developed at the National Institute of Metrology (NIM), Beijing, China, for the calibration of harmonic power analyzers under nonsinusoidal conditions at fundamental frequencies of 50 and 60 Hz. The standard is based on digital sampling techniques that do not require synchronization. A compensation algorithm is presented in this paper. A new definition of the uncertainty for harmonic measurement is proposed that is referred to the fundamental. A characterization signal and its application are introduced. Results have shown that over its operating range of up to 50 A, 500 V, and the sixtieth-order harmonic, the harmonic power standard has uncertainties $(k =2)$ of less than 30 $muhbox{V/V}$, 36 $mu hbox{A/A}$, and 42 $muhbox{W/VA}$ for voltage, current, and power measurements, respectively.      

Thermal Converters for Audio-Frequency Voltage Measurements of High Accuracy

The ac-dc differences of a reference group of thermoelements have been evaluated at audio frequencies to a few parts per million (ppm) at currents from 5 to 20 mA. A technique for comparing the ac-dc differences of two thermoelements with an uncertainty of about 2 ppm has been developed. Two 5 mA thermoelements are used with a plug-in set of resistors of computable reactances to form thermal voltage converters for voltage measurements. With this same technique adjacent ranges of these converters can be compared to step up from 0.5 to 500 V to better than 10 ppm.     

Inductive voltage divider of simple design for 120 V and 50 Hz to30 kHz

An inductive voltage divider (IVD) is presented, which-in spite of its simple design-meets the level of accuracy commonly needed in the field of ac-power measurement. The in-phase errors of about -(20±2) 10^-6 achieved at the power frequency of 60 Hz, -(130±5) 10^-6 at 4 kHz and +(70±35) 10^-6 at 32 kHz can be considered adequate, considering that the determination of the power factor implies greater uncertainties at higher frequencies. The achievement is due to the build-up of the IVD using two cores, a suitable capacitance distribution and a special technique of screening which reduces the stray inductance of the windings     

Automation and Evaluation of Two Different Techniques to Calibrate Precision Calibrators for Low Frequency Voltage Using Thermal Devices

Low frequency (LF) voltage and current are important parameters in electrical metrology. The standards for LF voltage and current are established by assigning AC–DC transfer difference to thermal devices, i.e. thermal converters or thermal transfer standard along with current shunts. Automated calibration systems have been developed based on Null method and measurement technique developed by Budovsky for calibration of precision calibrator in LF voltage and current against thermal devices. The technique based on the Algorithm developed by Dr. Ilya Budovsky (National Metrology Institute (NMI), Australia) has been compared with the conventional null technique. Indigenously developed software has been used to calibrate the precision calibrator in the entire LF voltage and current range using Holt thermal converters and current shunts. Calibration results at 1 V, 10 V in the frequency range from 10 Hz to 1 MHz as well as calibration results of 1 A in the frequency range from 40 Hz to 10 kHz are presented in this paper. These result shows that the measurement technique developed by Budovsky has reduced the complexity of AC–DC transfer measurements, measurement time and the uncertainty in measurement.     

Precision Differential Sampling Measurements of Low-Frequency Synthesized Sine Waves With an AC Programmable Josephson Voltage Standard

We have developed a precision technique to measure sine-wave sources with the use of a quantum-accurate ac programmable Josephson voltage standard. This paper describes a differential method that uses an integrating sampling voltmeter to precisely determine the amplitude and phase of high-purity and low-frequency (a few hundred hertz or less) sine-wave voltages. We have performed a variety of measurements to evaluate this differential technique. After averaging, the uncertainty obtained in the determination of the amplitude of a 1.2 V sine wave at 50 Hz is 0.3 $muhbox{V/V}$ (type A). Finally, we propose a dual-waveform approach for measuring two precision sine waves with the use of a single Josephson system. Currently, the National Institute of Standards and Technology (NIST) is developing a new calibration system for electrical power measurements based on this technique.      

Addressing the lower limit of breakdown delay time in vacuum

Based on the Joule mechanism of vacuum breakdown initiation, the dependence of a pulsed electric strength on the high-voltage pulse duration at a finite rate of electric field buildup in the vacuum gap is calculated. The results of calculations are confirmed by experimental data available for subnanosecond pulses. It is shown that the breakdown delay time has no lower limit at an electric strength of up to ～3 × 10¹⁰ V/m observed in experiment. The independence of the breakdown delay time of the electric field strength for pulse durations comparable with the pulse front width is explained by the finite rate of applied voltage buildup.     

A Dual-Mode Conditioning Circuit for Differential Analog-to-Digital Converters

Several devices such as load cells and pressure sensors, among others, provide differential outputs. Given that present high-resolution analog-to-digital converters (ADCs) have differential inputs, fully differential (F-D) circuits are required to adapt the sensor output to the ADC input. This paper proposes an F-D conditioning circuit that allows adjusting both differential- and common-mode signals to the levels required by the ADC. A design example is presented, and a prototype was built and tested. It transforms a differential input signal of $pm$25 mV with a common-mode voltage of 5 V to a differential output signal of $pm$5 and 2.5 V, respectively. It shows an input-referenced peak-to-peak noise of 120 nV, which results in a 112-dB dynamic range (18.7-bit noise-free resolution) for a signal bandwidth of 10 Hz.      

An Automatic System for AC/DC Calibration

An automatic ac/dc difference calibration system using direct measurement of thermoelement EMFs is described. The system operates over a frequency range from 20 Hz to 100 kHz, covering the voltage range from 0.5 V to 1 kV. For all voltages, the total uncertainty (including the uncertainty of the specific reference thermal converters used) is 50 ppm at frequencies from 20 Hz to 20 kHz, inclusive, and 100 ppm at higher frequencies up to 100 kHz. In addition to ac/dc difference testing, the system can be used to measure some important characteristics of thermoelements, as well as to calibrate ac voltage calibrators and precision voltmeters. Results of intercomparisons between the new system and the manual NBS calibration system, using single-range, coaxial-type, thermal voltage converters as transfer standards, are reported. The results indicate that the ac/dc differences measured are accurate to well within the combined total uncertainty limits of the two systems.     

High precision measurement of DC voltage ratios from 20 V/10 V to1000 V/10 V

This paper presents a new method of calibrating de voltage ratios from 20 V: 10 V to 1000 V: 10 V. A new battery-powered differential voltage detector has been developed to reduce the uncertainty of the measurement system. An absolute self-calibration process was used and traceability to a voltage ratio standard is not necessary. The uncertainties of dc voltage ratios (20 V to 1000 V): 10 V were less than 2 × 10^-7     

A Two-Way Josephson Voltage Standard Comparison Between NIST and NRC

A two-way Josephson voltage standard (JVS) direct comparison between the National Institute of Standards and Technology (NIST) and the National Research Council (NRC) has been conducted. The process consists of two comparisons: first, using the NRC JVS with the NRC's measuring system (hardware and software) to measure the 10 V provided by the NIST JVS and then using the NIST JVS measuring system to measure the 10 V provided by the NRC JVS. The results of the two comparisons are in agreement to within 0.7 nV, and their mean indicates that the difference between the two JVSs at 10 V is $-$0.28 nV, with a pooled combined uncertainty of 2.07 nV $(k = 2)$ or a relative uncertainty of 2.1 parts in $10^{10}$.      

Single-Junction Thermal Voltage Converters With Reduced Uncertainties at Frequencies up to 1 MHz

This paper presents a new realization of standards for ac–dc difference at voltages ranging from 1 to 4 V and frequencies of up to 1 MHz. This work has mainly been focused on improving the determination of the components of ac–dc difference that most significantly contributed to the uncertainty of the previous realization. These components are the ac–dc difference due to skin effect, transmission line effect, and the interaction between parasitic inductances and capacitances, which is described here as the L/C effect. As a result, combined $1 - sigma$ uncertainties as low as 4.3 $muhbox{V/V}$ at 1 MHz have been achieved.      

Novel Digital Voltage Ramp Generator for Use in Precision Current Sources in the Picoampere Range

We report on the development of a novel highly linear voltage ramp generator to be used for the traceable calibration of picoamperemeters. The generator is based on two digital-to-analog converters, one of them being used to compensate for the differential nonlinearity of the other one. The generator is completely controllable by a computer, its voltage slope $dV/dt$ is adjustable between 1 and 1000 mV/s. During a ramp running between $-$10 and $+$10 V, the slope shows a relative variation of only $1.3 cdot 10^{-5}$ (relative standard deviation). Due to a small output filter time constant of only 10 ms, we are, for the first time, able to conduct dynamic measurements of picoamperemeters with no sacrifice of linearity.      

Improved High-Input-Impedance mV-Amplifiers With Gain Factors From 10 to 900

New millivolt-amplifiers (mV-amplifiers) driving planar multijunction thermal converters (PMJTCs) at their output have been developed to measure the output voltage of micropotentiometers $(muhbox{pots})$. Using a twin-stage design with gains of 10, 20, and 30 per stage, total gain factors ranging from 10 to 900 are achieved. This results in a usable range of input voltages from 100 $muhbox{V}$ up to 100 mV at frequencies ranging from dc to 1 MHz. A novel offset compensation results in small reversal differences, and this allows for reliable operation at higher gain factors. There is a sophisticated protection circuit to avoid damage of the PMJTC in the case of overload. The ac–dc transfer difference is flat within 100 $muhbox{V/V}$ up to 100 kHz.      

A reevaluation of the NIST low-frequency standards for AC-DCdifference in the voltage range 0.6-100 V

A reevaluation of the NIST standards of ac-dc difference was undertaken in an effort to reduce the calibration uncertainty offered by NIST for thermal voltage converters (TVC's) at frequencies below 100 Hz. This paper describes the measurements taken in support of this effort, as well as the devices used for the reevaluation process and the analysis of the uncertainty of the measurements. This reevaluation of the NIST low-frequency standards will permit a significant reduction in uncertainty for ac-dc difference calibrations at 10 Hz in the voltage range from 0.6-100 V