Effect of capacitor nonlinearity on the oscillation frequency of a digitally-controlled oscillator using oppositely-coupled PMOS capacitor pairs

Contrary to conventional digitally controlled oscillators, frequency inversion is observed in a digitally controlled oscillator (DCO) using oppositely-coupled pMOS capacitor pairs. This is due to the nonlinearity of the capacitor. In this work, the effect of the capacitor nonlinearity on the oscillation frequency is analyzed and applied to the DCO to explain the frequency inversion. A 4.8 GHz DCO for verifying the method is fabricated with a 0.18-µm CMOS process and measured. Measurements reveal the frequency inversion clearly and show very fine frequency resolution of 76 kHz, corresponding to a capacitance change of 26 aF.