The importance of oscillators in science and technology can be outlined by two milestones. The pendulum, discovered by Galileo Galilei in the sixteenth century, persisted as “the” time-measurement instrument (in conjunction with the Earth's rotation period) until the piezoelectric quartz resonator. Then, it was not by chance that the first integrated circuit, built in September 1958 by Jack Kilby at the Bell Laboratories, was a radio-frequency oscillator.

Time, and equivalently frequency, is the most precisely measured physical quantity. The wrist watch, for example, is probably the only cheap artifact whose accuracy exceeds 10−5, while in primary laboratories frequency attains the incredible accuracy of a few parts in 10−5. It is therefore inevitable that virtually all domains of engineering and physics rely on time-and-frequency metrology and thus need reference oscillators. Oscillators are of major importance in a number of applications such as wireless communications, high-speed digital electronics, radars, and space research. An oscillator's random fluctuations, referred to as noise, can be decomposed into amplitude noise and phase noise. The latter, far more important, is related to the precision and accuracy of time-and-frequency measurements, and is of course a limiting factor in applications.

The main fact underlying this book is that an oscillator turns the phase noise of its internal parts into frequency noise. This is a necessary consequence of the Barkhausen condition for stationary oscillation, which states that the loop gain of a feedback oscillator must be unity, with zero phase.