AC resistance bridge based on the cryogenic current comparator

This paper describes a low-frequency ac resistance bridge based on a Cryogenic Current Comparator (CCC). The bridge is a main tool for resistance traceability of 10 kO, 100 O, and 1 O to the quantum Hall resistance (QHR). The bridge is typically operated with a PC generating a sinusoidal ac signal ranging from (0.1 to 2) Hz. Using a vector analyzer as a signal source the bridge is used to 1 kHz. Both sinusoidal and pseudo random signals have been used to compare the frequency dependence of standard resistors against the QHR. At present the bridge can be used to perform accurate calibrations to 10 Hz, at higher frequencies the self-resonance of the CCC increases the uncertainty.