Thermal Noise and Noise Measurements—A 2010 Update

We have presented a close look at effects of order hf/kT in noise measurements and in the definition of noise quantities-noise temperature, noise figure, etc. Given the perennial push to higher frequency, lower noise, and smaller uncertainty, such effects are becoming significant in an increasing number of applications, and especially in radio astronomy where the noise temperature of a modern receiver can be within a factor of a few of the quantum limit. In particular, we have discussed issues arising from the definition of noise temperature and the treatment of contributions from vacuum fluctuations. There is more than one correct way to deal with these effects, but it is important to be consistent in one's approach and not to mix methods.