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). A low-noise dc SQUID is used as a null detector. The bridge is the main tool to trace resistance to 10 k/spl Omega/, 100 /spl Omega/ and 1 /spl Omega/ levels from the quantum Hall resistance (QHR). The ratio of the resistors ranging from 1 /spl Omega/ to 10 k/spl Omega/ can be measured with an uncertainty of the order of 1 ppb. The bridge is typically operated with a PC generating a sinusoidal ac signal ranging from 0.1 Hz-2 Hz. Using a vector analyzer as a signal source the bridge is used up to 1 kHz. Both sinusoidal and pseudorandom signals have been used to verify the frequency dependence of the standard resistors against the QHR. At present, the bridge can be used to perform accurate calibrations only up to 10 Hz.