Low Noise mm-Wave Receivers

In this review paper coherent low noise receivers for frequencies from about 20 GHz up to a few hundred GHz are discussed. Mixers using Schottky barrier diodes, the working horses in mm-wave receivers, are described in some detail. The noise sources are identified and methods to minimize the noise of room temperature and cooled single ended mixers, as well as subharmonically pumped mixers, are related. Liquid helium-cooled ~ 4 K superconducting mixers i.e. Josephson effect mixers and superinsulator-superconductor (SIS) mixers are described. It is pointed out that SIS mixers are quite promising for future ultra low noise receivers for frequencies up to a few hundred GHz. For long mm-waves (λ ≲ 5 mm) the liquid helium cooled maser still offers the lowest noise and some recent results are mentioned.