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 oscillator. 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. In similar fashion, the frequency output of large lasers can be purified by injection locking them with high accuracy reference lasers (see injection seeder).

An injection-locked oscillator (ILO) is usually based on cross-coupled LC oscillator. It has been employed for frequency division or jitter reduction in PLL, with the input of pure sinusoidal waveform. It was employed in continuous mode clock and data recovery (CDR) or clock recovery to perform clock restoration from the aid of either preceding pulse generation circuit to convert non-return-to-zero (NRZ) data to pseudo-return-to-zero (PRZ) format or nonideal retiming circuit residing at the transmitter side to couple the clock signal into the data. In the late 2000s, the ILO was employed for a burst-mode clock-recovery scheme.

The ability to injection-lock is an inherent property of all oscillators (electronic or otherwise). This capability can be fundamentally understood as the combined effect of the oscillator's periodicity with its autonomy. Specifically, consider a periodic injection (i.e., external disturbance) that advances or lags the oscillator's phase by some phase shift every oscillation cycle. Due to the oscillator's periodicity, this phase shift will be the same from cycle to cycle if the oscillator is injection-locked. Moreover, due to the oscillator's autonomy, each phase shift persists indefinitely. Combining these two effects produces a fixed phase shift per oscillation cycle, which results in a constant frequency shift over time. If the resultant, shifted oscillation frequency matches the injection frequency, the oscillator is said to be injection-locked. However, if the maximum frequency shift that the oscillator can experience due to the injection is not enough to cause the oscillation and injection frequencies to coincide (i.e., the injection frequency lies outside the lock range), the oscillator can only be injection pulled (see § Injection pulling).

High-speed logic signals and their harmonics are potential threats to an oscillator. The leakage of these and other high frequency signals into an oscillator through a substrate concomitant with an unintended lock is unwanted injection locking.

Injection locking can also provide a means of gain at a low power cost in certain applications.