5.1 Introduction

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. This is illustrated by figure 5.1.

Figure 5.1: Orders of magnitude for frequencies and power levels delivered by the antenna to the receiver, and by the receiver to the spectrometer. The main roles of the receiver are to down-convert the signal frequency to a range suitable for analysis, and to amplify it while adding as little noise as possible.

Figure 5.2 shows the main subsystems of a receiver, that we will discuss below.

Figure 5.2: 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.