Injection locking

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Injection locking and injection pulling refer to 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 from grandfather clocks to lasers[edit]

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.

Cross coupled LC oscillator with output on top

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).

Injection-locked oscillator[edit]

An injection-locked oscillator (ILO) is usually based on cross-coupled LC oscillator. It has been employed for frequency division [1] 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[2] or nonideal retiming circuit residing at the transmitter side to couple the clock signal into the data.[3] Recently, the ILO was employed for burst mode clock recovery scheme.[4]

The operation of ILO is based on the fact that the local oscillation can be locked to the frequency and phase of external injection signal under proper conditions.

Unwanted injection locking[edit]

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.

Gain by injection locking[edit]

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

Injection pulling[edit]

Injection pulling and locking heard alternatively when one oscillator of two sweeps in frequency

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Injection (aka frequency) pulling occurs when an interfering frequency source disturbs an oscillator but is unable to injection lock it. The frequency of the oscillator is pulled towards the frequency source as can be seen in the spectrogram. The failure to lock may be due to insufficient coupling, or because the injection source frequency lies outside the locking window of the oscillator.

Spectrogram of the above audio

See also[edit]


  1. ^ M. Tiebout, "A CMOS direct injection-locked oscillator topology as high-frequency low-power frequency divider," IEEE Journal of Solid-State Circuits, vol. 39, pp. 1170-1174, 2004.
  2. ^ M. De Matos, J. B. Begueret, H. Lapuyade, D. Belot, L. Escotte, and Y. Deval, "A 0.25um SiGe receiver front-end for 5GHz applications," SBMO/IEEE MTT-S International Conference on Microwave and Optoelectronics 2005, pp. 213-217.
  3. ^ [54] T. Gabara, "An 0.25 μm CMOS injection locked 5.6 Gbit/s clock and data recovery cell," in Symposium on Integrated Circuits and Systems Design 1999, pp. 84 - 87.
  4. ^ J. Lee and M. Liu, "A 20Gbit/s burst-mode CDR circuit using injection-locking technique," in IEEE International Solid-State Circuits Conference (ISSCC), pp. 46-586, 2007.

Further reading[edit]

* Wolaver, Dan H. 1991. Phase-Locked Loop Circuit Design, Prentice Hall, ISBN 0-13-662743-9, pages 95-105

  • Adler, Robert (June 1946). "A Study of Locking Phenomena in Oscillators". Proceedings of the IRE 34 (6): 351–357. doi:10.1109/JRPROC.1946.229930. 
  • Kurokawa, K. (October 1973). "Injection locking of microwave solid-state oscillators". Proceedings of the IEEE 61 (10): 1386–1410. doi:10.1109/PROC.1973.9293. 

* Lee, Thomas H. 2004. The Design of CMOS Radio-Frequency Integrated Circuits, Cambridge, ISBN 0-521-83539-9, pages 563-566

External links[edit]