A 2.4-mm Coaxial Microcalorimeter for Use as Millimeter-Wave Power Standard at CENAM

To establish the RF power traceability up to 50 GHz in Mexico by means of a primary standard for microwave power and millimeter-wave power, a new coaxial microcalorimeter system in 2.4-mm line size along with thermoelectric-type transfer standards (TSs) were developed and their metrological characterization was performed. Coaxial microcalorimeters allow both broadband operation and power substitution through its feeding line; however, characterization and fabrication of millimeter-wave coaxial lines are technologically difficult. Thus, the design of the microcalorimeter is described, as well as the RF performance of its adiabatic lines. To overcome the lack of coaxial TSs up to 50 GHz, commercially available 2.4-mm thermoelectric power sensors are investigated to use them as low-frequency (LF) power substitution detectors. The effective efficiency is measured in the microcalorimeter by means of calorimetric measurements and the substitution of LF power; the developed measurement procedure is presented as well as the measurement results and the uncertainty analysis. The theoretical basis for the procedure employed to determine the microcalorimeter correction factor, based on the use of specially constructed open and short standards, is described with the measurement results and uncertainties are shown as well. To avoid the drift of the millimeter-wave power during the calorimetric measurements, a novel software leveling loop method for thermoelectric power sensors was devised. This method allows to keep the RF power level variations less than 100 ppm and being a straightforward solution to recent complex hardware realizations.