NIST establishes reference for measuring frequency dependence of capacitors.

Recent work at NIST could lead to improvements in the capacitance calibration services at NIST by a factor of three or four. The primary maintenance standard for NIST capacitance calibrations consists of a bank of four 10 pF fused-silica standards (referred to as the Farad Bank) which are maintained in an oil bath at 25 [degrees]C. The Farad Bank is calibrated twice a year against the calculable capacitor at a frequency of 1592 Hz. The calculable capacitor provides an absolute determination of capacitance in terms of length only and is the ultimate reference for all impedance measurements in the United States. Until recently, there was a lack of a well-characterized reference capacitor over the audio frequency range, resulting in large assigned uncertainties for capacitance calibrations to account for the uncharacterized frequency dependence of the Farad Bank.

The value of a standard capacitor may vary slightly with frequency because the imperfect medium between its electrodes has varying degrees of dielectric relaxation over the frequency range and the leads and electrodes of the capacitor have residual inductance. Using a combination of a 1 pF cross capacitor that has negligible frequency dependence due to electrode surface films and a 10 pF nitrogen dielectric capacitor with a very small residual inductance as references, NIST staff have measured the frequency dependence of two 10 pF transportable fused-silica capacitors from 50 Hz to 20 kHz. The relative standard uncertainties determined at 400 Hz and 100 Hz (0.15 X 1[0.sup.-6] and 0.32 X 1[0.sup.-6], respectively) are smaller than the uncertainties previously assigned to frequency dependence at these frequencies by a factor of five. This will lead to improvements in the capacitance calibration services at NIST by a factor of three or four.

The results were presented in an invited talk in August at the 2003 National Conference of Standards Laboratories; an archival paper describing the work has been published in the September issue of Review of Scientific Instruments. NIST plans to transfer the frequency dependence data to the Farad Bank and other reference capacitance standards, so that in the near future, improved capacitance calibrations will be available from NIST for the entire audio frequency range.

CONTACT: Jerry Stenbakken, [phone]; [e-mail].

General Developments

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