Back end module parameters for a 44 GHz broadband millimetre wave differential radiometer

This paper describes the contributions from a back end module to the behaviour of a 44 GHz differential radiometer. The developed radiometer is for one of three millimetre wave frequency channels (30, 44 and 70 GHz) used in the space mission Planck, to be launched by European Space Agency in 2007. The broadband and high gain characteristic of the radiometer imposes important trade-offs and limitations in the back end design parameters. The overall radiometer effective bandwidth requirement is 20% and the overall gain is about 60 dB in the RF side and 20 dB in the post-detection stages. Moreover output signals are restricted to be inside a small 6 dB window, so nonideal responses of subsystems, such as ripples, play a very important role in the radiometer behaviour. The back end module is the last radiofrequency stage after a cryogenically cooled front end module, both interconnected by rectangular waveguides. Each radiometer module has been manufactured by a different participant in the Planck consortium, so it is very important to calculate the foreseen results of the radiometer chain assembly from experimental results of each part. The novelty of this paper is to show the relationship among back end and front end parameters using real test results, to check the radiometer function. Several straightforward expressions are presented to obtain power level, noise temperature, output voltage and effective bandwidth parameters.