Frequency resolution enhancement for digitally controlled oscillators using series switched varactor

A novel way to improve the frequency resolution of digitally-controlled oscillators (DCO) is proposed. A minimum sized MOS capacitor in parallel with a closed transmission gate forms a small fixed capacitor unit. The fine tuning capacitor bank in DCO is then implemented using this unit in series with the switched MOS varactor to achieve better frequency resolution. A DCO in 90 nm CMOS implemented using this new structure achieves a frequency resolution of 1.6 kHz without dithering and a phase noise of -152 dBc/Hz at 20 MHz offset when oscillating at 3.1 GHz. Compared with previously published DCOs in the same process, better frequency resolution and FOM (-187) are achieved.