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Presented is a study of the optimal injection frequency using a timedomain model as a technique to improve the noise suppression in injection-locked ring oscillators. It is found that offsetting the injection signal frequency by roughly half of the one-sided lock range relative to the free-running oscillator frequency can lead to higher noise-suppression bandwidth and lower phase noise. Applied to a five-stage ring oscillator, the model shows good agreement with simulations, which indicate that the optimal injection frequency increases the noise-suppression bandwidth by up to 5 × , and lowers the phase noise by 15 dB.
(ProQuest: ... denotes formulae omitted.)
Introduction: Compared to their LC counterparts, inductorless ring oscillators provide wider tuning range and more compact layout without requiring a thick top metal for high-Q inductors. However, the noise performance of a ring oscillator tends to be much worse than an LC oscillator with the same power budget [1]. It has been shown that injection-locking a ring oscillator to a clean reference can dramatically improve its noise performance because periodical correction by the injected clock prevents jitter from accumulating indefinitely [2].
This Letter presents a study of noise suppression in injection-locked ring oscillators using a time-domain model. It is found that, for maximum noise suppression, the free-running and the injection frequencies should be set apart by roughly half of the one-sided lock range. This finding enables significant power reduction without degrading the oscillator noise performance.
Time-domain model: Injection-locking has been modelled in both frequency [3] and time [4] domains, with the latter being more appropriate for ring oscillators given the rich harmonic content of their outputs....