Resistance Ratio Bridge Research Articles

On the calibration of DC resistance ratio bridges

Current comparator bridges are employed for the realization of the resistance scale from the quantum Hall effect in several National Metrology Institutes and calibration centers. Quantum resistance standards under development, based on novel materials and tabletop dry cryostats, make the more achievable DC current comparator bridges (DCCs) a viable alternative to the more accurate but more expensive cryogenic current comparator bridges (CCCs). A DCC ratios’ calibration against a reference CCC is a straightforward way to improve the DCC’s performances and the resistance scale overall accuracy.The paper reports the calibration results of two DCCs on the ratios employed in a 1Ω to 10kΩ resistance scale traceable to a 12.906kΩ quantized Hall resistance, showing a good reproducibility and stability of the DCC readings over the measurement period and supporting the possibility of a DCC errors’ correction and of a realization of the primary resistance scale at the 10-8 level.

Design Considerations for a CCC Bridge With Complete Digital Control

The design considerations for a completely digitally controlled cryogenic current comparator resistance ratio bridge, including the feedback loops for voltage and current balance, are described. The numerical algorithms have been modeled and optimized, and the overall system performance is illustrated with example measurement data. It is shown that the final performance of the bridge is limited only by the accuracy of the current comparator, the noise sources associated with the current and voltage null detectors, and the resistors under test. An explanation of how the effect of leakage resistance in the system is minimized is also given.

Evaluation of a Modified ADC-Based Thermometry Bridge

This article presents the modification and testing of an ADC-based thermometry bridge. The instrument under investigation is an Anton Paar MKT 50 Millikelvin Thermometer (developed at the IFE, TU-Graz) based on a precision analog-to-digital converter (ADC). During preliminary testing, it was found that the MKT 50 performs better than its declared uncertainty (1 mK equal to 1 ppm when using a 100 Ω PRT) and is comparable to thermometry resistance ratio bridges typically used in secondary thermometry laboratories (with typical uncertainties from 0.1 mK to 1 mK). The modifications to the original bridge were undertaken by the development team of the MKT 50 at the Graz University of Technology, Austria. Measurements and evaluation of the modified instruments were performed at the MIRS/UL-FE/LMK. For the MKT 50 to be used in thermometry laboratories as a reference unit, measuring parameters of the instrument had to be changed. During the first modification, the upper limit of the instrument range was decreased from 400 Ω to 133 Ω, this is a preferred range for standard platinum resistance thermometers (SPRTs). This also meant an increase in the measuring current from 0.5 mA to the more frequently used 1 mA. A modification of the programmable ADC control unit increased the resolution from 24 bit to 27 bit. By adding a switch, the use of an external standard resistor was enabled. After this stage of the modification, the first tests on the instrument were performed. The second stage was aimed at the removal of noise sources. The instrument was prepared in such a way that it only used two input channels, one connected to the SPRT and the other to the standard resistor. Also, the components of the ADC were upgraded to further reduce noise. The elimination of one input channel sped up measurements, making the PC software capable of taking several readings in a shorter time period. All tests were performed in laboratory conditions, where precision AC and DC resistance ratio bridges are typically used. The non-linearity was assessed by the use of an automated resistance bridge calibrator (RBC, Model RBC100), while the noise value was determined both from the standard deviation of RBC measurements as well as from comparison measurements of two standard resistors. All tests were repeated several times to assure confidence in the results. With its lowered range of 133 Ω and an increased resolution of 27 bit, the instrument non-linearity, its value below 2 μΩ, was comparable to primary resistance ratio bridges such as the ASL F900 or MI 6015T. However, the noise of the instrument remained relatively high at 4 μΩ. Since the modified MKT-50 is a much faster instrument than AC and DC bridges, averaging was used for true comparison. Measurements done with the modified MKT 50 were also averaged every 15 s (the time a classic resistance ratio bridge takes for one measurement). When measurements with the MKT-50 were averaged, the noise of measurements would be comparable to primary resistance ratio bridges.

Realization of the farad from the dc quantum Hall effect with digitally assisted impedance bridges

A traceability chain for the derivation of the farad from dc quantum Hall effect has been recently implemented at INRIM. The main components of the chain are two coaxial transformer bridges: a resistance ratio bridge and a quadrature bridge, both operating at 1541 Hz.Both bridges are energized and controlled by a polyphase sinewave direct-digital-synthesizer; the automated control of relative amplitudes and phases of the generator voltage outputs permits principal and auxiliary balances of each bridge to be achieved. Such a technique, which may be called digital assistance, is implemented for the first time in an ohm–farad traceability chain.The relative standard uncertainty in the realization of the farad, at the level of 1000 pF, is estimated to be 64 × 10−9. A first verification of the realization is given by a comparison with the maintained INRIM capacitance standard, where an agreement within a relative combined standard uncertainty of 420 × 10−9 is obtained.

An automated cryogenic current comparator resistance ratio bridge for routine resistance measurements

A design of an automated cryogenic current comparator (CCC) resistance ratio bridge for routine measurements in a national metrology institute or standards laboratory is described. It employs a type II CCC for use in a low loss liquid helium storage vessel and can be continuously operated from the mains power via specialized isolated supplies. All parameters including the servo control loops are digitally controlled. Noise sources in the system are analysed using the Allan deviation and it is demonstrated that non-white noise sources can be eliminated by choosing an appropriate current reversal rate.

Improvement of Resistance Ratio Bridge Using Cryogenic Current Comparator by Operating with Low-Noise Voltage Amplifier Using Superconducting Quantum Interference Device for Low Resistance Measurements

We improved a resistance ratio bridge using a cryogenic current comparator (CCC bridge) for low-resistance measurement by operated with a laboratory-developed low-noise voltage amplifier using a superconducting quantum interference device (SQUID voltage amplifier). The combined uncertainty of the resistance ratio measurement of 10 mΩ/1 Ω was evaluated to be about 0.03 ppm.

Low-Noise Voltage Amplifier Using RF-Superconducting Quantum Interference Device

We developed a low-noise voltage amplifier using a commercially available rf-superconducting quantum interference device (rf-SQUID) operated with a feedback circuit, which increased the input impedance to approximately 200 MΩ. Its voltage resolution in the frequency range less than 1 Hz was measured to be approximately 0.4 nV. It was designed for a voltage detector of a resistance ratio bridge using a cryogenic current comparator (CCC-bridge).

Cryogenic Current Comparator Bridge Supplying Current up to 1 A and Current Dependence Measurement of 1 Ω Resistor

We developed a resistance ratio bridge using a cryogenic current comparator (CCC bridge), which supplied current of up to 1 A. We used it to measure the current dependence of 1 Ω resistors from 50 mA to 1 A by comparing them to a 100 Ω resistor with a small power coefficient. The typical value of the combined standard uncertainty of the measurements was estimated to be approximately 0.03 ppm.

Comparison of resistance standards between the National Institute of Metrology (China) and the Electrotechnical Laboratory (Japan)

The resistance standard maintained at the Electrotechnical Laboratory (ETL, Japan) is based on the resistance of the second plateau of the quantized Hall resistance, RH(2). A resistance ratio bridge with a cryogenic current comparator (CCC) is used for the measurements. The typical relative standard uncertainty in measuring a 1 Ω resistor is estimated to be about 4 × 10-9. The resistance standard maintained at the National Institute of Metrology (NIM, China) is based on the average value of six wire-wound 1 Ω resistors. It was monitored using the ETL quantum Hall effect (QHE) resistance standard in 1992, 1996 and 2000 by bringing 1 Ω resistors from the NIM to the ETL or from the ETL to the NIM. The drift rate of the standard was measured to be -0.024 µΩ/year. The NIM has developed a CCC bridge. The typical relative standard uncertainty in measuring a 1 Ω resistor using this CCC bridge and the QHE system of the NIM is estimated to be about 7 × 10-9. The NIM QHE system was compared with the ETL QHE resistance standard by bringing three 1 Ω resistors from the ETL to the NIM. Differences in the measurements were (+3.9 ± 8.9) nΩ, (+1.0 ± 9.9) nΩ and (-0.9 ± 9.1) nΩ, respectively.

Improved Quantum Hall Standard of Resistance at Electrotechnical Laboratory

We improved the quantum Hall standard of resistance at the Electrotechnical Laboratory to measure 1 Ω, 100 Ω and 10 kΩ resistors. It is composed of an 11 T superconducting magnet system with a 3He insert, a resistance ratio bridge using a cryogenic current comparator (CCC bridge) and a 100 Ω transfer resistor. The 100 Ω transfer resistor is directly compared to the second or fourth plateau of the quantized Hall resistance using the CCC bridge, whose combined uncertainty is estimated to be about 4 ×10-9. The 1 Ω, 100 Ω and 10 kΩ resistors are compared to the 100 Ω transfer resistor using the same CCC bridge with different settings, whose combined uncertainty is estimated to be about 3 ×10-9 or less. Total uncertainty of measuring the resistor is given as the root sum squares of these combined uncertainties and estimated to be about 5 ×10-9, because these two comparisons are independent of each other. This is the first report of the procedure for resistance measurement and its uncertainty.

A cryogenic current comparator bridge for resistance measurements at currents of up to 100 A

The use of cryogenic current comparators at NPL has been limited so far to currents of 100 mA. A new current comparator with a maximum current capacity of 100 A has been developed and forms the basis of a resistance ratio bridge for low value resistors with a relative uncertainty of approximately 10/sup -7/ or better.

Resistance Ratio Bridge Using Cryogenic Current Comparator with DC-Superconducting Quantum Interference Device Magnetometer

A resistance ratio bridge using a cryogenic current comparator (CCC) with a dc superconducting quantum interference device (dc-SQUID) was developed. Its structure, driving circuit and principle of operation were based on those of a resistance ratio bridge using a CCC with a commercially available rf-SQUID. Because of the small intrinsic noise of the dc-SQUID, its resolution was determined not by the dc-SQUID noise but by the thermal noise of the resistor being tested. The ratio of a resistor having a nominal resistance of 100 Ω and a resistor with that of 1 Ω was measured with an uncertainty of 1.1×10-8.

Comparison of quantum Hall effect resistance standards of the BNM/LCIE and the BIPM

A comparison of quantum Hall effect (QHE) resistance standards has been carried out by taking a complete QHE system from the BIPM to the Laboratoire Primaire d'Electricite-Magnetisme (BNM/LCIE). From December, 14-17, 1993, measurements of the same 100 /spl Omega/ standard resistor were carried out in terms of the quantized Hall resistance (QHR) standards of the two laboratories. This exercise followed a preliminary comparison in July 1993 in which the ratio of the 100 /spl Omega/ standard to a 1 /spl Omega/ standard, and that of the 100 R standard to a 10 k/spl Omega/ standard were measured at the BNM/LCIE using the resistance-ratio bridges of both laboratories. All comparison results agree to within 4 parts in 10/sup 9/. In particular, the values attributed by the two laboratories to the 100 R standard, based on the mean of measurements made on the i=2 plateaus of GaAs-based heterostructures, agreed to (1.2/spl plusmn/3.0) parts in 10/sup 9/. >

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 Omega /100 Omega , 6453.2 Omega /100 Omega , and 10 k Omega /100 Omega . 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 Omega /1 Omega CCC bridge. >

An automated cryogenic current comparator resistance ratio bridge

The method of measurement of two-terminal cryogenic and four-terminal room-temperature resistors using a DC cryogenic current comparator bridge is well established. A bridge which is based on this technique and is fully automated with a computer controlling both the measurement current and the readings of the bridge imbalance current is described. The bridge is designed to measure integer ratios of 1:1 to 1:25 of four-terminal room-temperature resistors in the range 1 Omega to 10 k Omega . An overall accuracy of 1 part in 10/sup 9/ for differences in the ratio from nominal of up to 100 p.p.m. is desired. Some initial determinations of the resolution have been made for a measurement of 10 Omega :100 Omega . These determinations were carried out without the balance servo, and the output from the null detector was measured with a digital voltmeter connected to the computer. An analysis of a set of 11 reversals gave a standard deviation for a single reversal of 1 part in 10/sup 8/ and therefore a standard error of the mean of 3 parts in 10/sup 9/. >

An AC-bridge for low-frequency measurements of the quantized Hall resistance

A simple circuit which makes possible the use of a resistance-ratio bridge based on a cryogenic current comparator (CCC) with both AC and DC is described. The different sources of uncertainty associated with the use of AC in a CCC bridge are discussed. It is shown that they should have an effect which does not exceed a few parts in 10/sup 9/ of the resistance-ratio being measured, if the frequency is limited to a few hertz. This analysis is confirmed by experimental results of resistance-ratio measurements between the quantized Hall resistance (QHR) and a 100 Omega resistance standard carried out at DC, 1, 2, and 4 Hz. These measurements are, to the author's knowledge, the first accurate DC measurements of the QHR. They demonstrate that the quantization of the Hall resistance, observed with AC and for the frequency range studied here, remains complete to within a few parts in 10/sup 9/ or better. >

Self-balancing resistance ratio bridge using a cryogenic current comparator

A cryogenic current comparator resistance bridge, developed to measure the resistance ratios of 1, 10, 100, and 6453.2 Omega against 1 Omega , is described. It is shown that the precision of the 1- Omega comparison is better than 0.01 p.p.m. This is attained by using an improved feedback system in the secondary current source.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A resistance ratio bridge based on a cryogenic current comparator for measuring the quantized Hall resistance

A description is given of a resistance bridge based on a cryogenic current comparator covering a range of 1-12 906 Omega . Type-B ('systematic') uncertainty contributions are carefully evaluated and are less than 0.004 p.p.m. (1 sigma ). Type-A or random contributions are 0.006 p.p.m. ( sigma , 20 min.) in a transfer of the quantized Hall resistance R/sub H/ (i=2) to 100 Omega . Measurements relating h/2e/sup 2/ to Omega /sub VSI/ maintained by the Van Swinden Laboratory are presented, showing agreement to 0.09 p.p.m. with the results of other national standards institutes. >