Measurements of frequency dependence of fused-silica capacitors

In order to improve the capacitance calibration services at the National Institute of Standards and Technology, we need to determine the frequency dependence of all the reference capacitance standards over the audio frequency range. 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, we have measured the frequency dependence of two 10 pF transportable fused-silica capacitors from 50 Hz to 20 kHz. The results and the associated uncertainty analysis are discussed. Kevwords calibration, capacitance, farad, frequency dependence, fused-silica capacitor, impedance, measurement, uncertainty