Flicker noise upconversion mechanisms in K-band CMOS VCOs

This paper analyzes the flicker noise upconversion mechanisms in CMOS LC-VCOs, focusing on the bias circuitry and core cross-coupled pair. In contrary to what is widely understood, a current-biased VCO does not necessarily have more upconverted flicker noise compared to a resistor-biased VCO. Resistor biasing obviously does not generate 1/f noise itself, but it allows for a transfer of the core transistors flicker noise to phase noise through AM-PM conversion and the Groszkowski effect. The high impedance of a tail current source improves the phase noise because the amplitude noise is controlled by the bias current source which can be designed with low flicker noise. On the other hand, flicker noise from the cross-coupled pair becomes a main contributor of phase noise at low offset frequency. An analytical expression is proposed to describe such mechanism. To verify the theory, three 25GHz VCOs are taped out in a CMOS 28nm process. The measurement results show that all VCOs have averaged FoM of -187dB at 10MHz offset while the current-biased VCO has 1~2dB better phase noise/FoM at 1MHz offset due to the better amplitude noise control.

Publication year:2015

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