Injection Locking

What is "injection locking" phenomenon? In 17th century Dutch scientist, Christian Huygens invented pendulum clock. When he hung two pendulum clocks close to each other on the wall, he noticed that the pendulums moved in unison [1]. It was the first observation of a phenomenon which called injection locking. Injection locking occurs when two oscillatory systems have proper coupling and close frequencies. In such a situation one of the oscillators tends to lock to another one. After lock, frequency of the oscillator under injection will be the same as that of injection source. As another example, biological clock of human being is injection locked to the frequency of earth's cycle.

There are many research papers about injection locking in different fields but for the first time Adler formulated this phenomenon mathematically [2]. Injection locking also has applications in communication circuits and lasers. From integrated circuit point of view, injection locking can be used for phase noise and jitter reduction in oscillatory systems. Frequency division is another application for this phenomenon in electronics.

References:

[1] A. E. Siegman, Lasers, Mill Valley, CA: University Science Books, 1986

[2] R. Adler, "A study of locking phenomena in oscillators," Proc. IEEE, Vol. 61, pp. 1380-1385, Oct. 1973