NIST 的AC JVS System！

Part of the Quantum Electrical Metrology Division, Gaithersburg, MD

GoalsTo provide U.S. industry with the essential link between the dc and corresponding ac electrical standards, by maintaining and improving the U.S. national standards of AC-DC difference that are used to provide calibrations and measurement services for thermal converters and shunts.
Customer NeedsIncreasingly accurate, easier-to-use instruments and devices for precision ac voltage and current measurements are being developed by U.S. electronic instrumentation and test equipment manufacturers for use in a wide variety of industrial and scientific applications throughout the world. These instruments are beginning to press the best available uncertainties, especially for calibration of ac waveforms at high currents and for ac voltages below 100 mV. The need continues for better calibration tools with which to verify accuracy claims, achieve consistency, and to help avoid international trade barriers based on technical difficulties.  Examples of specific customer requirements are cited below.

• Instrument manufacturers have developed new methods for calibrating thermal transfer standards at voltages less than 100 mV. The new methods will result in uncertainties close to what NIST gives for routine calibrations, and is likely to require significant reductions in the uncertainty of NIST calibrations at the very lowest voltage levels to support these instruments.
• US military and industrial laboratories require highly accurate calibrations of thermal converters at frequencies greater than 1 MHz. Potential discontinuation of the RF-dc calibration service because of funding concerns would mean that NIST customers would no longer have ready access to required calibrations.
• The U. S. Army primary standards laboratory uses the same primary standards for ac current metrology as NIST.  New standards are required to replace these aging, failing, devices. In addition, the U. S. Air Force primary standards laboratory is seeking improved thermal converters for measurements at frequencies up to 100 MHz.
• The U.S. Air Force primary standards laboratory calibrates 100 A current amplifiers for their secondary laboratories. Although these amplifiers are specified to 100 kHz at 100 A, NIST presently supports only 100 A measurements to 10 kHz. The Air Force requires an expansion of the frequency range for high-current calibrations. The Air Force also requires improved devices for measuring ac voltages from 100 mV to 2 V with smaller uncertainties than existing devices.
• NIST’s international obligations include assisting other National Metrology Institutes (NMIs) improve their metrology capabilities. For example, the National Institute of Standards in Egypt requires significant improvements in their measurement service for AC-DC difference in order to be recognized in the international arena.
• In 2008, the Quantum Electrical Metrology Division will host the 2008 Conference on Precision Electromagnetic Measurements.  This is an extremely important and visible international conference on electrical metrology.  Members of the staff will serve as guest editors for the IEEE Transactions on Instrumentation and Measurement special issue for CPEM 2008.  This task is vital to the success of the conference.

Research and development are required to maintain and to expand NIST calibration and special test services for thermal converters and shunts, especially for calibrating high current shunts and high voltage converters. In addition, research and development of an intrinsic standard for ac voltage is required to successfully address the problem of maintaining extremely accurate calibrations at voltages below 100 mV.

呵呵，看着费劲啊。不过热转换确实是比较可靠的方法，虽然有8508A、3458A之类的东西，但是我们的低频电压的主标准仍是20余年前的热偶式标准，始终舍不得扔掉，关键时候还要用它验证一下，特别是交流波形比较差的信号。此外，超高频电压标准是热电阻式，本质也是热变换；最高等级的电能标准器也是热转换原理的德国参考标准。

20多年前的东西，是否是Fluke 540B？现在计量单位用792A比较多吧？NIST应该有更高级的。

不是540B啊 lymex 老大，540B早已经坏了，是Ballantine的1600A自动平衡真有效值
交直流转换标准，比540B准确度好一些，最高可达0.01%，0.25V~1000V，频率为5Hz~30MHz，操作也容易许多，是自动的。这个公司现在好象还有，前两年还邮寄过产品介绍呢。