A microwave radiometer with reduced volume, mass, phase noise, and power requirements and increased harmonic rejection, includes a fixed number of frequency banks configured to provide signals within separate, non-overlapping local oscillation frequency bands, a detection circuit configured to detect one or more microwave RF signals, and an RF downconverter configured to mix the signals within the separate, non-overlapping local oscillation frequency bands with the one or more microwave RF signals to provide a continuous range of down converted frequencies.