TABLE OF CONTENTS A browse through the phase-locked loop literature of the past is humbling. Although we often consider phase-locked loops as relatively new structures, historical literature dates the concept as early as 1919 [2]. Vincent [2] and Appleton [3] experimented and analyzed, respectively, the practical synchronization of oscillators.
After these initial papers, research and development continued up until the 1940s. At that time, the initial interest in synchronization was for a) a local oscillator in FM demodulation and b) the exciter for an atomic particle accelerator amplifier [4].
The control theory for phase-locked loops was based on the well-developed theory for feedback amplifiers. Early pioneers in the feedback analysis included the Bell Labs researchers Bode [5] and Nyquist [6]. Their techniques for analyzing the stability of feedback structures are still used by researchers today.
In 1935, Travis published "Automatic Frequency Control", [7] which suggested two reasons for controlling the local oscillator of a receiver. "Oscillator drift, if not corrected by more or less frequency manual readjustment, is capable of mistuning the signal by many channels in the course of a few hours run" [7]. His second reason for oscillator control is more entertaining, "It seems to be quite true that the average listener does not tune his set well enough to obtain the best quality it is capable of giving, partly from negligence, and partly from lack of the necessary skill, in which case the mechanical design of the set is a...
