A theoretical framework for discussing the effects of gain stability in radiometers is developed, clarifying the relationship between frequency domain and time domain measurements. The available literature on experimental results is reviewed with respect to the intrinsic gain fluctuations of HFET amplifiers and the temperature sensitivity of various other components used in radio telescope receivers. The results are applied to ALMA, where it is found that the sensitivity of broad-band total power radiometry is likely to be limited by gain fluctuations rather than thermal noise unless the present state-of-the-art is improved. This is true for both 10 Hz beam switching (via subreflector nutation) and rapid on-the-fly scanning. However, it is also found that at frequencies above 410 GHz fluctuations in atmospheric opacity due to turbulence will be worse than both thermal noise and receiver gain fluctuations a large fraction of the time; further study of the site testing data will allow this to be better quantified.
View a pdf version of ALMA Memo #466.
Last modified: 2003-05-23