Characterization of a Type-N Coaxial Microcalorimeter for Use as Microwave Power Standard at CENAM

This paper describes the metrological characterization of a new type-N coaxial microcalorimeter and its associated thermistor-type transfer standard, which together are used as the primary standard for microwave power from 0.05 to 18 GHz at the Centro Nacional de Metrología, Queretaro, Mexico. The design characteristics, evaluation method, and measurement uncertainties of the primary standard are given and compared to recent realizations found in the open literature. The metrological characterization of the microcalorimeter is based on a comparison method to indirectly obtain its correction factor. The effective efficiency of the transfer standard, directly calibrated in the microcalorimeter by means of calorimetric measurements, is shown and compared against measurements performed by National Institute of Standards and Technology, showing good agreement within the estimated expanded uncertainties (0.21%-0.47%). The developed data acquisition system to automate the effective efficiency measurements and the proposed software leveling loop to keep the microwave power level constant are also discussed. This approach allows to keep an RF power stability of better than 100 ppm over several hours and is a straightforward solution to replace analog leveling systems which require obsolete hardware. Finally, the analysis followed to evaluate the uncertainty for the effective efficiency measurements is thoroughly described.