Characterisation of dry cold load for ALMA front-end verification

Abstract

Several techniques for measuring the radiometric temperature (brightness) of a refrigerated dry calibration load for ALMA frontend verification are presented. The brightness of the load including the effects of the cryostat window is estimated using different techniques and compared at frequencies up to 1 THz. Measured results are compared with those obtained by a conventional calibration technique using a cooled receiver. The brightness estimated by the aforementioned methods show good agreement at lower ALMA bands (below 400 GHz) with increased deviation at higher frequencies.Introduction: Noise temperature is one of the crucial parameters that describe the performance of a high sensitivity cooled receiver. The noise performance of a receiver can be characterised by performing a relative measurement using thermal calibration loads with the radiated noise power or radiometric temperature known a priori. For a reference radiometric temperature, thermal calibration loads commonly use high emissivity absorbing materials immersed in liquid cryogen (wet load), which provides stable radiometric temperatures at stationary state. However, owing to the difficulties in handling of the wet load, measurement could be cumbersome when the receiver under test needs to be measured repeatedly at different orientations. Verification of the Atacama large millimetre/submillimetre array (ALMA) front-end [1] requires such a task, for which use of a closed cycle refrigerator cooled load inside a vacuum container, i.e. a dry cold load shown in Fig. 1, is proposed. This will reduce the need for a liquid nitrogen (LN 2 ) load and will ensure stable load temperature during measurement. Key requirements of the dry cold load for the characterisation of the ALMA receivers include a constant stable brightness temperature over a wide bandwidth up to 1 THz, polarisation insensitiveness, high emissivity, mechanical stability, etc. [2].