AC Bridge Methods for the Measurement of Three-Terminal Amittances

To avoid uncertainties due to finite lead impedances, a direct admittance is best defined as the transfer admittance of a two-terminal-pair network. The network may contain both voltage and current transformers. Unwanted loop currents may be suppressed by coaxial chokes and the equivalent circuit reduced to that of a three-terminal admittance. Two direct admittances may be compared with a voltage or current ratio, and two such ratios may be combined to give a general form of four-arm bridge. This general bridge network may be balanced in a way which leads to simple relations independent of the ground admittances between the four direct admittances. At least two separate balance conditions must be satisfied, but it is usually best to make three balances since the error in the main balance is dependent on the product of the errors in the other two. There is no restriction on the type of network which may be used to couple the source and the detector to the bridge, provided the ground balance conditions are satisfied. Networks may be chosen to provide for filtering, impedance matching, and the elimination of transformers. Examples are given of four-arm bridge networks which use adjustable decade ratio-transformers as the main balance controls and fixed ratio-transformers for multiplication or inversion.     

A Direct-Reading Current-Comparator Bridge for Scaling Four-Terminal Impedances at Audio Frequencies

A direct-reading current-comparator bridge circuit, for scaling four-terminal impedances which do not deviate from nominal by more than a few thousand parts per million, is described. The scaling is performed at constant voltage and the impedances are treated as true four-terminal devices in that, at balance, no current is drawn from the potential terminals. The principal feature of the circuit is the use of a compensation winding to excite the magnetic shield of the current comparator and thus suppress the effect of lead unbalances. Possible applications of the bridge include the scaling of four-terminal resistance standards between 1 milliohm and 100 ohms at frequencies ranging from 50 Hz to 1600 Hz. Construction details and calibration of a current comparator for use in a 10-to-1 ratio bridge are given.     

Strain-gauge differential circuits with current transformers

Conclusions Differential strain-gauge circuits with current transformers have several advantages as compared with ordinary bridges or bridges with voltage transformers, since the former circuits eliminate the effect on the measurement precision of the conductor resistances and the contact resistances of the current collectors and switching devices. Moreover, two strain gauges are sufficient for the operation of these circuits. The above differential circuits are more sensitive than bridge circuits,and they have a linear characteristic, with the output voltage being proportional to the transducer resistance. Less stringent requirements can be specified for the stability and value of the switching devices when these circuits are used for multipoint measurements. The circuits operate with alternating currents and, therefore, they possess all the advantages of ac strain-gauge circuits.     

A Direct-Current-Comparator Bridge for Measuring Shunts up to 20000 Amperes

A direct-current-comparator bridge for calibrating four-terminal resistors or shunts at currents up to 20 000 amperes is described. Measurements can be made at up to full rated current of the shunts so that the effects of the load coefficient are included. The resistor under test is compared with a reference resistor of higher value by measuring the ratio of the currents through the two resistors required to produce equal voltages across them. A comparator bridge with a range of 100 amperes and errors of less than 1 ppm has been described previously. Improvements to this bridge have been made, the main one being a reversing feature, which permits the currents through the resistors to be reversed in a few milliseconds. This makes an accurate measurement easier, particularly if there is a change of resistance due to heating. By connecting a second comparator in cascade, the range has been extended to 20 000 amperes at an overall ratio up to 2 × 106: 1, with only a slight loss of accuracy; the errors may be a few parts per million. Other applications of the measuring system are the accurate measurement of large currents or the calibration of transductors.     

Direct-Current Comparator Bridge for Resistance Thermometry

The design, construction, and performance of a dc bridge that covers the complete range of both platinum-and germanium-resistance thermometry in four ranges, 0-11 up to 0-11 000 ohms, is described. The bridge uses a potentiometric method based on the current comparator. In the design, emphasis has been placed on reducing the noise level, on obtaining a single-balance operation, and on providing a recorder output. Detector sensitivity and noise level (with a three-second time constant) permit full eight-decade bridge resolution to be achieved at a thermometer current of I mA on all ranges except the lowest, which requires 5 mA. Bridge errors are believed to be less than 10-7 of a reading or one step on the last dial, whichever is larger. A facility is provided for measuring the self-heating effect of the thermometer, even if the measured temperature is not constant. The bridge is also suitable for the measurement of four-terminal standard resistors over the same ranges.     

Electronic compensation of inductive voltage dividers and standardvoltage transformers

A new compensating system is proposed to reduce the ratio error and the phase error of inductive voltage dividers and standard voltage transformers, operating at no load. An electronic amplifier reduces the input current, which is the main cause of error in low-frequency applications, while at the same time the input impedance is greatly increased     

New multifrequency method for the determination of the dissipation factor of capacitors and of the time constant of resistors

A new method for the simultaneous determination of the dissipation factor of capacitors and of the time constant of resistors by a multifrequency method is presented. This method is based on the measurement of impedance ratios at two or more frequencies. A bridge is set up with a programmable two-channel ac voltage source and the impedances to be compared. The impedance ratios are determined by synchronous sampling procedures and application of discrete Fourier transform. In a first experiment, a high-grade gas-filled 1 nF standard capacitor was compared with a stable 1 M/spl Omega/ ac resistor at frequencies between 31 Hz and 666 Hz. The estimated standard uncertainties (k=1) are 0.6/spl middot/10/sup -6/ for the dissipation factor of the capacitor and 0.4 ns for the time constant of the ac resistor.     

A Direct Current Comparator Bridge for High Resistance Measurements

A dc comparator bridge, which is suitable for measuring resistors from a fraction of an ohm to several megohms with partper-million accuracy or better is described. An analysis of errors which may occur when measuring high-value resistors and methods used for reducing these errors to negligible proportions are included. The bridge can be connected to make either four-terminal or three-terminal measurements. The four-terminal connection is best for measuring low-value resistors. When it was used for measuring high-value resistors, a systematic bridge error was detected, which was found to be primarily due to leakage paths through the insulation, permitting current to bypass one of the measured resistors. This error is avoided by using the three-terminal connection for measuring high-value resistors. The leakage resistance is then in parallel with a low resistance winding rather than one of the measured resistors. The bridge has automatic reversal and a chart recorder output, which continuously indicates the deviation from the set point. The comparator is adjustable in steps of 10-' of full scale, and measurements can be made at ratios up to 11.111/1.     

Bridge Circuit for Measuring Differences of Current or Voltage from a Preselected Value

A bridge circuit has been developed, using a Zener diode, which enables measurement of small changes in current or voltage from a preselected value. The circuit features increased sensitivity over a meter whose range includes the total current and is more easily used than a balanced bridge or potentiometer. The design equations are developed and the design procedure outlined for both the current and voltage difference measuring bridges, while a sample calculation is made for the current change measuring circuit. A bridge similar to that in the sample calculation was built and tested. The test circuit displayed less than 1 per cent nonlinearity of meter current vs change of input current for the design region of 460±5 ma. This shift was attributed to a slight heating of the Zener diode or a resistor at the higher test currents, but since the diode used in the test circuit had a smaller power rating than the type originally intended for use, it is not considered to be a serious limitation.     

Off-diagonal impedance in amorphous wires and its application to linear magnetic sensors

We investigated the magnetic-field behavior of the off-diagonal impedance in Co-based amorphous wires under sinusoidal (50 MHz) and pulsed (5 ns rise time) current excitations. For comparison, we measured the field characteristics of the diagonal impedance as well. In general, when an alternating current is applied to a magnetic wire, the voltage signal is generated not only across the wire but also in a pickup coil wound on it. These voltages are related to the diagonal and off-diagonal impedances, respectively. We demonstrate that these impedances have a different behavior as functions of axial magnetic field: the diagonal impedance is symmetrical, whereas the off-diagonal one is antisymmetrical with a near-linear portion within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate circular domains. In the case of the sinusoidal excitation without a dc bias current, the off-diagonal response is very small and irregular. In contrast, the pulsed excitation, combining both high- and low-frequency harmonics, produces the off-diagonal voltage response without additional biasing. This behavior is ideal for a practical sensor circuit design. We discuss the principles of operation of a linear magnetic sensor based on a complementary metal-oxide-semiconductor transistor circuit.     

Constant Current Drive for Resistive Sensors Based on Generalized Impedance Converter

A generalized impedance converter (GIC) is presented in this paper as a programmable current source. A study of its properties and limitations is shown. Its current efficiency is compared with that of Howland topologies, and it is improved in this paper. The contribution of the operational amplifier static offsets to the current source is obtained by presenting typical values of these parameters, and an equivalent circuit with its differential and common modes is presented. The calculated and experimental GIC input impedances are obtained, showing its bandwidth limitation and proposing the possibility of extending the input impedance frequency response without varying its low-frequency value. In this paper, the GIC is used to provide constant current to a series sensor loop and a magnetoresistive current sensor (Wheatstone bridge), which are located within the GIC itself. In both cases, the GIC works under the dc regime, unlike usual ac use.      

A Wide-Range High-Voltage Capacitance Bridge with One PPM Accuracy

An impedance bridge for high-voltage capacitance and related measurements to an accuracy of 1 ppm is described and the background research and development which made it possible is documented. The bridge is of the transformer-ratioarm type, the principal components of which are a comparator and several two-stage transformers. The bridge can be used to measure capacitance ratios over a range from 1/1 to 107/1 with a resolution between 0.1 and 0.25 ppm. The highest accuracies are obtained at the principal power frequencies of 50-60 Hz, but the bridge is usable up to 400 Hz. The factors which limited the accuracy of previously developed bridges of this type were reexamined and their influence reduced. Two independent methods were developed for the calibration of the bridge.     

Inductive Voltage Dividers with Calculable Relative Corrections

Inductive voltage dividers are now used in measurement laboratories for the production of audio-frequency voltage ratios with errors (deviations from turns ratio) of a few parts in ten million of input. A major component of error arises from the interaction between distributed shunt impedances and leakage impedances in the windings. Correction for this systematic error in dividers of special design can result in an order-of-magnitude improvement in accuracy. A solution using network equations has been obtained for the corrections to the relative errors of inductive voltage dividers of specific design. Earlier theoretical considerations, confirmed by measurements of limited accuracy, indicated an S-shaped curve of ratio error vs nominal ratio, and quadrature component vs nominal ratio. The results from recent calculations are in agreement with the earlier measurements and provide much better definition of the true shape of the S-curve. An algebraic equation has been derived for the limiting form of this S-shaped characteristic curve. A resistance analog of an inductive voltage divider was constructed to represent the lumped circuit parameters equivalent to the distributed shunt admittances and the winding impedances. Measurements of the gross errors in the analog have yielded experimental results in excellent agreement with those calculated from the network equations.     

Adjustable Complex-Ratio Transformer

A four-terminal network in which the complex output-to-input ac current or voltage ratio may be adjusted is described. The design of an experimental Adjustable Complex-Ratio Transformer for use at a frequency of 60 Hz, in which the output-to-input current ratio (Io/Ii = ?+j?) can be adjusted with two sets of 5 decade dials to 0.1 percent accuracy, is presented. The application of the instrument for the direct reading of the difference of two alternating currents is discussed.     

Identifying the Magnetic Part of the Equivalent Circuit of n-Winding Transformers

Representation of multiwinding transformers by equivalent circuits has been recently improved, and it is for the identification of components of these circuits. In this paper, the focus is on magnetic coupling with its related losses. A general method, based on external impedance measurements, is followed to determine inductances, coupling ratios, and resistances included in these equivalent circuits. Justification for impedance measurements, choice of measured impedances, and precautions regarding short-circuit compensation are discussed. For illustration, two components are tested, and their equivalent circuits are established     

Leakage current detection in cryogenic current comparator bridges

Several tests have been developed to locate leakage currents in cryogenic current comparator (CCC) resistance ratio bridges used at NIST to measure ratios of 1000 Ω/100 Ω, 6453.2 Ω/100 Ω, and 10 kΩ/100 Ω. The major advantage of the tests is that they can be performed in situ using the sensitivity of the CCC bridge. These test procedures have been used to reduce the leakage error uncertainty of CCC ratio measurements, linking working standards to the quantized Hall resistance (QHR) and to the NIST calculable capacitor experiment. CCC bridges require that the current which passes through a standard resistor must equal the current through the appropriate CCC winding to very high precision. This can be difficult to verify at or below 1 pA because a large number of possible leakage paths exist. Errors due to six important leakage current paths are given, and the calculated changes in the resistance ratio are compared with measurements made with a controlled leakage resistance in a 100 Ω/1 Ω CCC bridge     

A Simple Method for the Calibration of Traditional and Electronic Measurement Current and Voltage Transformers

The calibration of measurement transformers represents a classical task in the practice of electrical measurements. Most commercial instruments that are expressly designed for this purpose found their working principle on a scheme that is based on the idea of Kusters and Moore. Although they can assure very high accuracy, the need to employ a high-performance electromagnetic circuit makes them very expensive and usually not suitable for measurements at frequencies that are higher than 50 or 60 Hz. For this reason, these kinds of instruments cannot be employed for the calibration of the new generation of current and voltage transducers, such as electronic measurement transformers, whose employment is growing in all the applications where wide bandwidth is required. In this paper, a new method for the calibration of electromagnetic voltage and current measurement transformers (VTs and CTs) and electronic voltage and current measurement transformers (EVTs and ECTs) is discussed, and a deep metrological characterization is carried out. The novelty of the proposed method is represented by a completely different approach to the measurement of the ratio and phase errors of the measurement transformers. The method is based on the proper digital signal processing of the signals that are collected at the secondaries of the transformer under test and of a reference transformer when the same signal is applied to their primary. Since no auxiliary electromagnetic circuits are required, this solution can be easily implemented in a simple and cost-effective way. In spite of its simplicity, the tests that are developed on a prototype clearly point out that the proposed system is suitable for the calibration of measurement transformers with precision class up to 0.1 in the frequency range from 50 Hz to 1 kHz.     

Experimental studies on the optimization of the accelerating field distribution in an IH lineac

A new method is being studied to control the accelerating field distribution in an IH lineac with a high energy gain. In spite of its high shunt impedance, a shortcoming of an IH structure is that a concentration of accelerating field occurs at the low-energy section of the cavity. One of the practical methods to control the accelerating-gap voltages is adjusting the electric-capacity distribution of the structure. This method, however, is not always suitable for an IH lineac when the particle-energy gain is larger than the incident particle energy.The basic idea of our experiments is to utilize an inductive method to “tune” the accelerating field distribution. In order to test the applicability of this method, accelerating field distributions and shunt impedances are measured in terms of these “tuner” configurations by using $\text{1}\text{4}$ scale models. The results are compared with the calculations based on a four-terminal network simulation.     

An improved model of mutual admittances of tapped windings

The calculation of the equivalent admittances of windings tapped in sections, which are used as elements of high-accuracy measuring instruments (e.g., in inductive voltage dividers, magnetic current comparators, and transformers with minimum ratio errors), is considered. A comprehensive mathematical model of equivalent admittances is presented. It is derived from the measured mutual admittances of particular sections. The equivalent admittances of this model are dependent on the actual values of the mutual admittances as well as on the actual tap voltages. With the equivalent admittances it is possible to analyze, in a simple way, the ratio errors and the input admittance due to parasitic admittances and to calculate the accurate values of correcting admittances