Harmonic content modulation of the oscillator output\ud voltage can contribute to flicker noise up-conversion in LC-tuned\ud oscillators. The paper reports a quantitative analysis of the effect\ud in Van der Pol oscillators using the framework of the impulse sensitivity\ud function (ISF). It is shown that most of the up-conversion efficiency\ud results fromthe first harmonic of the ISF, which is not perfectly\ud in quadrature to the output voltage waveform, and from the\ud first harmonic of the transistor current, which is slightly lagging\ud the voltage waveform. A closed-form expression of flicker-induced phase noise in voltage-limited LC-tuned oscillator is derived that is in\ud good agreement with circuit simulations. The paper also shows that\ud the values of both phase shifts are determined by the non-linearity\ud of the active element and are linked to the relevant oscillator parameters,i.e., excess gain and tank quality factor

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Traditional design of voltage-controlled oscillators (VCOs) can lead to unacceptable degradation of the $1/f^3$ phase-noise component when a broad tuning range is required. In this paper, the flicker noise up-conversion mechanisms in a voltage-biased topology are deeply investigated, providing general design guidelines for its mitigation. An improved VCO architecture is then introduced, where a segmented transconductor tailors the excess gain depending on the operating range to ensure that $1/f$ noise up-conversion remains minimal over the tuning range. The solution covers both fourth-generation and WiMAX 2.5-GHz bands and leads to a 10-dB reduction of the $1/f^3$ phase noise with respect to a traditional design without impairing the $1/f^2$ phase-noise performance