Resistance bridge based on the cryogenic current comparator in a transport dewar

An optimized resistance bridge based on the Cryogenic Current Comparator (CCC) with SQUID readout has been developed. The bridge can be used for routine secondary resistance calibrations as well as for the traceability of 100 /spl Omega/ to the quantum Hall resistance. The CCC-SQUID, shielding and wiring are mounted in a telescopic insert, fitted in a low-evaporation rate, transportable liquid He dewar, with an autonomy of around 1 month. A difference compensator method is used to balance the bridge. A new, more reliable and self-standing bridge electronics has been fabricated and tested. An uncertainty in the comparison of resistors at least one order of magnitude better than room temperature CCs is expected.

I. Introduction

The ccc with SQUID readout is the most accurate system for the comparison of standard resistors, and traceability to the quantum Hall resistance (QHR). Given the complexity and high cost of these systems, practically all CCC based bridges are owned by National Metrology Institutes (NMIs) [1]–[4]. Officially, the calibration of primary resistors to the QHR has to take place at NMIs. However, the existence of a simple CCC for routine secondary resistance calibrations at industry and other institutes would be very desirable. These are the requirements that such a system should fulfill: i) the cryogenic system should be of small size, transportable, have a low evaporation rate and be easy to refill, ii) different ratios should be available for versatility, with no need to warm up the system, iii) the bridge electronics should be reliable, protected against environmental noise and able to work on batteries for a long time, iv) an accuracy at least one order of magnitude better than that of room temperature CCs (<10⁻⁷), and finally, v) the system should be easy to handle outside of the community of specialists. This paper describes the development and test of such a CCC bridge.