A servo drive for heterodyne oscillators

HETERODYNE oscillators are used frequently in a measuring system in which the oscillator is swept over its frequency range by means of a motor while the characteristic being measured is recorded on a strip of perforated chart paper linked to the frequency control through a toothed chart drive mechanism. The linkage may be purely mechanical or it may be obtained by the use of two synchronous motors, one driving the frequency control of the oscillator and the other the chart advance mechanism on the recorder. One of the advantages of the latter arrangement is that the recording may take place far from the oscillator—say, at the end of a long cable whose transmission is being measured—provided the motors are driven from a power source of accurate frequency.