The performance analysis of microwave radiometers is often simplified by assuming that most or all circuits are perfectly matched and that all are at the same physical temperature. However, the increasingly high performance demanded of future instruments requires assessment of their performance, including the following effects: actual complex S parameters of the various subsystems (nonzero insertion losses, imperfect matching, finite isolation etc.), physical temperature gradients etc. in order to correct them in software, when it is not possible to eliminate these nonidealities by design. A full noise-wave analysis is presented in this paper. Linearity and bias errors are evaluated, as well as their sensitivity to thermal gradients and characterization errors. Finally, the required intervals between calibrations can be estimated based on this analysis.