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2010 Conference on Precision Electromagnetic Measurements (CPEM 2010), pp.
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Progress in the development of low-frequency quantum-based AC
Power Standard at NRC Canada
Djokic, Branislav
https://publications-cnrc.canada.ca/fra/droits
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2010 Conference on Precision Electromagnetic Measurements
June 13-18, 2010, Daejeon Convention Center, Daejeon, Korea
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PROGRESS IN THE DEVELOPMENT OF LOW-FREQUENCY QUANTUM-BASEDAC POWER STANDARD AT NRC CANADA
Branislav Djokic
Institute for National Measurement Standards National Research Council of Canada
Ottawa, Ontario K1A 0R6, Canada
Abstract
Progress in the development of a new low-frequency quantum-based AC Power Standard at the National Research Council of Canada (NRC) is described in the paper. The power standard is based on programmable Josephson voltage standard (PJVS) for providing AC voltage with quantum accuracy, and includes electronic signal generators and amplifiers, inductive voltage dividers (IVDs) and current transformers (CTs) for scaling output signals to low levels. Differential digital sampling is used for comparison of low-level output signal replicas with the PJVS output.
Introduction
Josephson junction arrays have been used for high-accuracy AC voltage generation and measurement in the last two decades [1]-[3]. Programmable Josephson Voltage Standards (PJVS) have been recently applied to development of low-frequency AC power standards with quantum accuracy [4]-[6].
An AC power standard performing a continuous calibration of the digital sampling system in every cycle of the measured signals was developed at PTB. A recently developed 10 V Josephson Waveform Synthesizer (JWS) has been incorporated into the PTB primary standard for AC electrical power [4]. The 10 V SINIS binary arrays are used for in-situ calibration of the digital sampling voltmeter (DVM), with corrections applied over short time intervals.
NIST implemented a quantum-based system for electrical power with a 1.2 V SNS ternary-array PJVS generator [5]. The PJVS is combined with differential digital sampling [6] for reducing the uncertainties created by the PJVS voltage step transitions. The system uses a multichannel DSP-based generator of spectrally pure waveforms, and a voltage amplifier with permuting-resistance dividers.
The development of a quantum-based AC power standard at the National Research Council of Canada has also started [7], in order to improve the uncertainty and achieving direct traceability to a
quantum standard. This paper describes the progress towards the NRC quantum-based AC power standard.
Design Details
A block diagram of the NRC PJVS-based AC power standard is shown in Fig.1. A high-stability oscillator OSC provides reference frequency for the PJVS step voltages, and a signal source SG generating low-distortion waveforms. The waveforms are low-level signals with known amplitudes and phase angles relative to each other. They are driving voltage and transconductance amplifiers, and providing corrections for the composite voltage amplifier Av
consisting of two inverting amplifiers with a gain of -10, connected in series. The Av 120 V output is
compared with the PJVS 1.2 V output by means of two inductive voltage dividers (IVDs) of the ratio 10:-1, each connected across one voltage amplifier.
Fig.1 Block Diagram of the new PJVS-based AC Power Standard
The two IVDs are of two-stage design for power frequencies. They support two different voltages, one IVD supports 13.2 V and the other supports 132 V.
The output current of 5 A or of 1 A is scaled down in ratio 500:1 or 100:1, and then converted into voltage by two current transformers CT1 and CT2, and a reference resistor RR of 120 ȍ based on principles
described in [8].
A precision relay switch Sw and high accuracy digital samplers DVM in the differential sampling mode [6] are used for comparisons between the PJVS output
234 Monday Tuesday W ednesday Thursday Friday
voltage and the low-level voltage and current replicas.
Performance
The IVDs are expected to have in-phase and quadrature errors lower than 0.2 V/V and 0.5 V/V, respectively. Achieving, at power frequencies, IVD calibration uncertainties about ten times lower is demanding. Preliminary testing with AC calibrators and precision voltmeters indicated IVD ratio errors lower than a few V/V. A calibration of the 132 V IVD was performed with the current comparator-based capacitance bridge and low loss gas-dielectric capacitors. It indicated in-phase and quadrature errors lower than 10-6. For lower uncertainty calibrations at power frequencies, using Thompson's method [9]-[11], a 10:-1:1 calibrating transformer, voltage comparator, and special sockets were built.
Calibration of IVDs with low-uncertainties is the basis for verification that in-phase and quadrature errors of the composite voltage amplifier operating together with two IVDs are better than 0.5 V/V and 1.0 V/V, respectively.
Calibrations of the current transformers in the I/V converter were performed using current-comparator-based techniques. The errors of the current transformer CT2, 5 A (1 A)/20 mA, are not critical for the output power as CT2 plays an auxiliary role in the I/V converter.
The performance of the current transformer CT1, 5 A (1 A)/10 mA, is critical for the I/V converter. At a burden Rb as large as 120 ȍ, the CT1 two-stage
design is ineffective. However, when CT2 and RC network provide optimum operating conditions, CT1 errors drop to only a fraction of 1 A/A, as shown in Table I.
Table I Current Transformer CT1 Errors CT1: 5A (1A)/10mA
Rb [ȍ] f [Hz] I [%In] In-Phs. [ȝA/A] Quad. [ȝA/A]
120 100 -570 3620 CT1 10 -200 4440 100 -860 3320 10 -610 4010 120 100 0.1 0.1 CT1 10 0.1 0.1 & 100 0.0 0.1 CT2 10 0.1 0.1 Errors 57 63 57 63
A specially compensated 120 ȍ foil-resistor with the temperature coefficient below 1 ȍ/(ȍ·K) is used as the reference resistor.
Additional results regarding the performance of the system will be available at the conference.
Acknowledgment
The continuing collaboration of the NIST labs in Boulder and Gaithersburg on this NRC project is gratefully acknowledged. The assistance of the Electrical Standards Group of INMS/NRC is also acknowledged.
References
[1] C. A. Hamilton, C. J. Burroughs, and R. L. Kautz, “Josephson D/A converter with fundamental accuracy,” IEEE Trans. Instrum. Meas., vol. 44, no. 2, pp. 223–225, Apr. 1995.
[2] P. Kleinschmidt, P. D. Patel, J. M. Williams, and T. J. B. M. Janssen, “Investigation of binary Josephson arrays for arbitrary waveform synthesis,” Proc. Inst. Electr. Eng.—Sci. Meas. Technol., vol. 149, no. 6, pp. 313–316, Nov. 2002.
[3] R. Behr, J. M. Williams, P. Patel, T. J. B. M. Janssen, T. Funck, and M. Klonz, “Synthesis of precision waveforms using a SINIS Josephson junction array,” IEEE Trans. Instrum. Meas., vol.54, no. 2, pp. 612– 615, Apr. 2005.
[4] L. Palafox, R. Behr, W.G. Kürten Ihlenfeld, F. Müller, E. Mohns, M. Seckelmann, and F. Ahlers, "The Josephson-Effect-Based Primary AC Power Standard at the PTB: Progress Report", IEEE Trans. Instr. Meas., Vol. 58, No. 4, pp. 1049-1053, Apr. 2009. [5] B. C. Waltrip, B. Gong, T. L. Nelson, Y. Wang, C. J.
Burroughs, Jr., A. Rüfenacht, S. P. Benz, and P. D. Dresselhaus, "AC Power Standard Using a Programmable Josephson Voltage Standard", IEEE Trans. Instr. Meas., Vol. 58, No. 4, pp. 1041-1048, Apr. 2009.
[6] A. Rüfenacht, C. J. Burroughs, Jr., S. P. Benz, P. D. Dresselhaus, B. C. Waltrip, and T. L. Nelson, "Precision Differential Sampling Measurements of Low-Frequency Synthesized Sine Waves With an AC Programmable Josephson Voltage Standard", IEEE Trans. Instr. Meas., Vol. 58, No. 4, pp. 809-815, Apr. 2009.
[7] B. Djokic, "Development of a Low-Frequency Quantum-Based AC Power Standard at NRC Canada", Proc. of Conference on Precision Electromagnetic Measurements (CPEM) 2008, Broomfield, Colorado, pp. 194-195, June, 2008.
[8] P. Miljanic: "High precision calibration of ac current-to-voltage converters" Metrologia, Vol. 41, No. 6, Dec. 2004, pp. 365-368.
[9] A. M. Thompson, "Precise Calibration of Ratio Transformers", IEEE Trans. Instrum. Meas., Vol. 32, No. 1, pp. 47-50, Mar. 1983.
[10] Y.Nakamura, A.Fukushima, Y.Sakamoto, “Calibration of a 10:1 ratio transformer using Thompson's method”, Metrologia, Vol. 34, pp. 353-355, 1997.
[11] K. Kochav and B. Wood, "Transformer Calibration at NRC Using Thompson’s Method", Proc. NCSL Intl. Workshop and Symposium, paper 1E-2, Aug. 2008.
