The variations of osillation freqeuncy according to the oscillation amplitudes in DCO

Abstract

In DCO, oscillation frequency can be continuously changed by large oscillation amplitudes. The effect of oscillation amplitude is valid for any oscillator with steep C-V characteristic. The effect is critical in DCO because it needs very small capacitance control. For verifying the oscillation effect, an opposite-coupled pair in DCO with oscillation amplitude and a separated opposite-coupled pair are measured, respectively. As the result, the polarity of unit tuning frequency can be even inverted at large oscillation amplitude. Although capacitance is increased by control voltage in the separated pair, the oscillation frequency of DCO is increased, which differs from an expectation. This means that the effective capacitance of DCO is decreased by oscillation amplitude. This paper analyzes this effect and explains it with the effective capacitance.