Injection locking in self-oscillating magnetometers

Injection locking (IL) is a well-known phenomenon that occurs in nonlinear oscillators subject to external periodic or non-periodic signals. It is a phenomenon of induced synchronization that occurs when an external (injection) signal locks the frequency of the oscillator to the frequency of the external signal. This form of synchronization is relatively straightforward to implement because it does not require specially organized feedback as is the case with phase locked loop. Circuits that exploit IL can have very simple designs and be applied to a broad range of applications, such as to synchronize frames and lines in early television sets, to synchronize lasers, and to function as ac voltmeters, field-intensity meters, amplifier-limiters and AM and FM detectors. However, the focus of this article is the recent application of IL to magnetic field sensors. This novel application highlights the potential benefits of the IL approach but also some of the complexities and opportunities for further development. As with all measurement systems, the consideration of noise is paramount in the design of magnetic sensors. Noise reduction and mitigation strategies are essential. IL can be employed as a noise mitigation strategy in magnetometers that utilize self-oscillations as part of their detection paradigm; it can stabilize the oscillation frequency, potentially simplifying the measurement circuitry, and in some circumstances improve the signal-to-noise ratio. Here we review some magnetometers that have successfully exploited IL principles and highlight design options. We also propose a new circuit that is simple to implement and more straightforward to analyze.