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This project aims to provide the world's best basis for accurate impedance measurements by tying the U.S. legal system of electrical units to the International System of Units (SI) through the realization of the SI unit of capacitance. This work also forms the foundation of NIST's measurement services for electrical impedance, ensuring a sound metrological basis for impedance measurements, both nationally and internationally, and ensuring that the claims of measurement accuracy by U.S. industries are recognized and accepted worldwide. The need continues for better representation of capacitance and also for better calibration tools at NIST with which to verify objectively claims of improved performance specifications, to achieve consistency, and to help avoid technical trade barriers.


setting up an automated system
Setting up an automated system for determining the frequency dependence of fused-silica capacitors.

The primary facility for connecting the U.S. legal system of electrical units to the international system of units is the NIST calculable capacitor, with which the measurement of capacitance is effectively achieved through a measurement of length. Both the calculable capacitor and the chain of high precision measurements that transfers the SI unit to the calibration laboratories must be maintained, improved, and compared with other national metrology laboratories to ensure measurement consistency on an international level.

Over the last few decades, NIST has successfully invested in two key quantum representations of electrical quantities; both the Quantum Hall Resistance (QHR) and Josephson Voltage standards have now achieved measurement uncertainties approaching parts in 109. These quantum standards, however, represent only a few points in a multi-dimensional world of electrical measurements. The crucial link between the fundamental electrical standards and commercial electronic instrumentation is provided by precision AC measurement standards. A combination of transformer techniques and modern Digital Signal Processor (DSP) techniques has the potential to extend our expertise over a wider dynamic range.

Consistency between resistance and impedance measurement services from NIST is expected by the instrumentation industry and DOD laboratories. An improved resistance-capacitance (RC) link is also needed to realize the farad from the QHR standard, if the proposed redefinition of the SI occurs in the near future, and to advance basic research such as closing the quantum metrology triangle.

Development of wideband impedance measurement services requires reference standards that can be characterized over the impedance and frequency ranges of interest. NIST has developed a system to characterize commercial four-terminal-pair capacitance standards from 1 pF to 1 nF over the frequency range from 1 kHz to 10 MHz. A bootstrapping technique using an LCR meter and an inductive voltage divider can extend the characterization to higher-valued capacitance standards up to 10 µF.

Major Accomplishments

  • Developed a DSP based signal generator that can be phase and frequency locked to a Josephson source, enabling the Quantum Watt Project
  • Developed concept for new calculable capacitor based on laser frequency combs
  • Developed a programmable capacitance standard to provide capacitance values from 100 pF to 10 µF with a resolution of 1 pF
  • Served as pilot laboratory for SIM capacitance key comparison

Created November 21, 2008, Updated January 23, 2018