Phase noise suppression in LC oscillators: Tutorial
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AbstractThis paper presents a comprehensive study of phase noise (PN) suppression in LC‐tank oscillators. The goal of this study is to provide designers with the latest techniques for reducing PN in cross‐coupled oscillators. To this end, we begin with a discussion of two prevalent PN models in oscillators: Hajimiri and Demir. We prefer the Hajimiri model because it does not involve very complicated math, and it offers engineers better insight into designing low‐PN oscillators in the two‐PN close‐in regions in an oscillator spectrum (1/f2 and 1/f3). In 1/f2 region, we show that a need for a large output‐voltage swing leads to Class D and B oscillators, and a large output‐current swing results in Class C oscillators. Also, reduction of the impulse sensitivity functions (ISFs) of an oscillator core can happen in Class F oscillators. A few solutions are presented for mitigating flicker noise up‐conversion, such as adding resistances, controlling the oscillation amplitude, decreasing the conduction angle, guiding the high‐frequency harmonics of current, and shifting the phase of VGS against VDS. We also provide a comparison of recent state‐of‐the‐art literature to show what constitutes a good PN in both 1/f2 and 1/f3 regions in cross‐coupled oscillators. We conclude that a cross‐coupled oscillator can reach the best performances in 1/f3 and 1/f2 PN regions if the oscillator is designed in Class C with the K block and uses the techniques of narrowing the conduction angle, the tail inductor, and the modified tank simultaneously.
