In this section selected methods of measurement are described which are applicable to the electrical characteristics of terrestrial radio-relay system transmitters of the heterodyne type excluding any r.f. branching and switching networks. The block diagram of Figure 1, page 18, is a functional representation of such a transmitter but in practice the actual diagram may differ in detail. Local oscillator noise, although an important parameter of a transmitter, is not usually measured on the transmitter itself, but rather between baseband terminals in the radio-relay system. This parameter, which results in path-loss-independent baseband noise, is not therefore considered in this section