Phase Noise Analysis in CMOS LC Qudrature VCOver the years, several phase noise analyses have been developed using the linear time-invariant or the
time-variant models [1]-[5], all these studies of phase noise are based on a single LC oscillator. However, in quadrature LC oscillators, in addition to the phase noise of single LC oscillator, some extra phase noise which caused by the coupling transistor is introduced. Because in QVCO, two symmetric LC-tank oscillators are combined together by the coupling transistors to force two oscillators to oscillate in quadrature [6], in certaindurations within one period, the coupling and switching transistors are all on and thus contribute to the phase noise. So compared with the single oscillator, the phase noise of quadrature oscillator is expected worse.In this paper, we focus on phase noise analysis caused by the coupling transistors. One simple model is introduced to analyze the processes that convert thermal noise of the coupling transistors into phase noise of QVCO. The relationship between phase noise and the ratio of the width of coupling transistor to the width of switching transistor is identified, and a simple quantitative expression accurately specifies thermally induced phase noise in QVCO.