Signal Path

This section describes the path of the signal from the initial reflection off the antenna surface, through to the data recorded onto export media. Some radio-astronomy jargon is also explained. The details of this section are not required by observers, but are included for the sake of curiosity anyway. A knowledge of the system is however essential if you want to design an experiment that uses the array in a new or unconventional way. Familiarity with the names and functions of some critical components can also be helpful if you need to diagnose faults. Many system components are housed in modules, and repairs can be made quickly by replacing a module once a correct diagnosis has been made. For clarity, only the path of a 3-cm band signal is described. The paths of 6-cm, 13-cm and 20-cm band signals are entirely analogous: mm frequencies undergo an extra frequency conversion and the 20cm/13cm signals go through one less. Complete information on each of the subsystems described in this section can found in the documents listed in this appendix.

Radio waves (approximately within a range of 320MHz to 116GHz) are accurately reflected from the primary surface of the main parabolic dish and are re-reflected off a secondary reflector into a feed-horn. The 3cm/6cm band feed-horn operates in the range 4.4 to 10.8GHz. At the base of the feed-horn is a directional, coupling waveguide. In this coupler (the cal coupler) is a diode that injects a noise signal of known amplitude. This noise signal is used to calibrate the system temperature (equation 1.3) and gain of the receiving system.