Injection locking

Injection locking and injection pulling are the frequency effects that can occur when a harmonic oscillator is disturbed by a second oscillator operating at a nearby frequency. When the coupling is strong enough and the frequencies near enough, the second oscillator can capture the first oscillator, causing it to have essentially identical frequency as the second. This is injection locking. When the second oscillator merely disturbs the first but does not capture it, the effect is called injection pulling. Injection locking and pulling effects are observed in numerous types of physical systems, however the terms are most often associated with electronic oscillators or laser resonators. Injection locking has been used in beneficial and clever ways in the design of early television sets and oscilloscopes, allowing the equipment to be synchronized to external signals at a relatively low cost. Injection locking has also been used in high performance frequency doubling circuits. However, injection locking and pulling, when unintended, can degrade the performance of phase-locked loops and RF integrated circuits. Injection pulling and injection locking can be observed in numerous physical systems where pairs of oscillators are coupled together. Perhaps the first to document these effects was Christiaan Huygens, the inventor of the pendulum clock, who was surprised to note that two pendulum clocks which normally would keep slightly different time nonetheless became perfectly synchronized when hung from a common beam. Modern researchers have confirmed his suspicion that the pendulums were coupled by tiny back-and-forth vibrations in the wooden beam. The two clocks became injection locked to a common frequency. In a modern-day voltage-controlled oscillator an injection-locking signal may override its low-frequency control voltage, resulting in loss of control. When intentionally employed, injection locking provides a means to significantly reduce power consumption and possibly reduce phase noise in comparison to other frequency synthesizer and PLL design techniques.