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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 90nm CMOS implemented using this new structure achieves a frequency resolution of 1.6kHz without dithering and a phase noise of −152dBc/Hz at 20MHz offset when oscillating at 3.1GHz. Compared with previously published DCOs in the same process, better frequency resolution and FOM (−187) are achieved.