1. Key features

From the very beginning of intercontinental communications in the HF (high frequency: 3 to 30 MHz) band, frequency stability has been a source of difficulty. By 1930, several articles were already reporting on time-consuming and costly methods for obtaining the most stable frequency possible. But the techniques used were more applicable to laboratory equipment than to field use. The first frequency synthesizers, known as frequency standards, were of the direct synthesis type and appeared around 1943. The advent of semiconductors led to the replacement of electronic tubes in the first generation of instruments, reducing their size and power consumption and improving their performance. However, their cost and size still reserve their use for sophisticated military equipment or power transmitters.

The advent of digital...