Digitally controlled CMOS quadrature ring oscillator with improved FoM for GHz range all-digital phase-locked loop applications

This paper presents a 14-bit digitally controlled ring oscillator (DCO) with operating frequency up to 3.4 GHz in 0.18 μm CMOS technology. Digital control is employed to control the transistor operation to obtain higher voltage swing and lower flicker (1/f) noise that in turn results in the lowest phase noise in a ring oscillator. Furthermore, sub-feedback topology is used to obtain the stable quadrature outputs from even number of stages. The DCO has a frequency tuning from 523 MHz to 3.405 GHz. Phase noise at 4 MHz offset is 134.82 dBc/Hz. This results in figure of merit (FoM) to be 169.9 dBc/Hz which is 7.7 dB improvement over its recently published analog counterpart.