In order to realize a quantum capacitance standard, a vacuum-gap cryogenic capacitor based on a coaxial design has been developed. A tuning mechanism allows adjustment of the room temperature value of the capacitance to obtain the value of 1 pF at cryogenic temperature. At 10 mK, the difference between the measured value and the target value is less than 30/spl times/10/sup -6/. In this paper, the capacitor and its tuning mechanism are described in detail. The variation of the capacitance during the vacuum and temperature cycles and its stability at 10 mK are also investigated.
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