Home>Field Service Test Center>Test How-To

AC metrology at the top of the chain

By Martin Rowe, Senior Technical Editor- February 1, 2011

A 10-V programmable Josephson voltage standard will let metrologists calibrate transfer standards using AC voltages. Courtesy of NIST. Read more from the February 2011 issue.

Josephson-junction arrays have long been used as national standards for DC voltage, making them the top of the calibration chain (Ref. 1). But their outputs are fixed. In October 2010, NIST (National Institute of Standards and Technology) announced that it had begun shipping a 10-V programmable Josephson voltage standard, or PJVS (Ref. 2). The PJVS could replace thermal voltage converters that are the primary instruments for calibrating AC sources and meters. Although its frequency is limited to a few hundred hertz, the PJVS is still fast enough for calibrating transfer standards at the critical 50-Hz and 60-Hz power-line frequencies.

A Josephson voltage standard (whether programmable or DC) consists of an array of Josephson junctions that, at superconducting temperatures (4 K), produce stable and predictable voltages when subjected to microwave energy. The figure shows the PJVS array that NIST engineers developed. The voltage of each junction is exactly proportional to the applied frequency, although it is typically just 30 μV to 150 μV (Ref. 3). Thus, a few hundred thousand junctions are needed to realize a practical voltage such as 10 V.

Researchers designed electronics that, under software control, let the PJVS system select a subset of the junctions to achieve different output voltages. The PJVS is a kind of DAC that's accurate enough to be a DC or AC voltage standard. The system can produce voltages of +12 V to –12 V, although NIST researchers operate it at ±10 V (Ref. 4). The system can produce 1 mA of current.

---

References
1. Rowe, Martin, "Follow the Chain to NIST-Traceable Calibrations," Test & Measurement World, December 1999. www.tmworld.com/article/317180-Follow_the_Chain_to_NIST_Traceable_Calibrations.php.
2. "NIST Ships First Programmable AC/DC 10-Volt Standard," NIST TechBeat, October 26, 2010. www.nist.gov/public_affairs/tech-beat/tb20101026.cfm#volt.
3. Benz, Sam, "Synthesizing Accurate Voltages with Superconducting Quantum-Based Standards," IEEE Instrumentation & Measurement Magazine, June 2010. pp. 8–13.
www.ieee.org.
4. Burroughs, C.J., P.D. Dresselhaus, A. Rüfenacht, D. Olaya, M.M. Elsbury, Y. Tang, and S.P. Benz, "NIST 10 V Programmable Josephson Junction Standard System," Conference on Precision Electromagnetic Measurements, Daejeon, Korea, June 2010. This paper will appear in the April 2011 issue of IEEE Transactions on Instrumentation and Measurement. www.ieee.org.