The purpose of a receiver is to collect efficiently the astronomical signal that has been concentrated by the antenna near its focal point, and to amplify it with a minimum of extra noise to a level suitable for further processing by the spectrometers or continuum detectors. Next figure shows the main subsystems of a receiver, that we will discuss below.

Synoptic diagram of a typical receiver. This diagram is grossly simplified; for instance, the optics involves mirrors, elliptical and planar, and also grids; the LO/PLL system is actually more complicated than shown.