Effect of Tail Capacitor on Phase Noise in LC Cross-connected Oscillators: An Analytical Investigation

Abstract

This paper investigates the effect of tail capacitance on phase noise of an LC-VCO (LC voltage-controlled-oscillator). First, the analytical relations of the phase noise for different values of tail capacitor (CT) are derived and then for verifying them, simulation and calculated results are compared. For simplicity, three scenarios such as small, medium and large values of CT are considered. In a case study an LC-VCO is designed in a standard 0.18µm CMOS technology, and simulation and numerical results have been presented for different values of CT. In this case study, numerical analysis shows that for CT =200fF (medium CT) and CT =10pF (large CT), the phase noise at 1MHz offset from the 5.2GHz is -96dBc/Hz and -118dBc/Hz, respectively. According to the results, the ISF (Impulse sensitivity function) is improved by increasing the amount of CT. Numerical values also demonstrate that excessive increase of CT has no effect on the phase noise. While choosing bigger CT can effectively reduce the noise contribution of the tail by bypassing the noise of tail transistor, but low impedance path generated by CT may degrade the phase noise by reducing tank quality factor.