A low-noise inductor-less fractional-N sub-sampling PLL with multi-ring oscillator

In this paper, a compact inductor-less PLL using multiple coupled rings oscillator is presented. Sub-sampling technique with soft loop gain switching is applied to reduce the in-band phase noise. As a result, the loop bandwidth can be widened, which suppresses the phase noise from ring oscillator as well. Fractional-N mode is implemented by utilizing the multiple phase outputs inherently generated by the ring VCO. Using multiple rings instead of one allows generating more phases for finer frequency resolution without decreasing oscillation frequency. The coupled multi-ring oscillator with proper phase shift also achieves reduced phase noise comparing to their single-ring counterpart. The PLL was implemented in a 0.13um CMOS technology, consuming 19 mW from a 1.3 V power supply. The measured largest in-band fractional spur at 2.08 MHz is -42 dBc. The measured integrated jitters were 571 fs and 690 fs around 1.2GHz output in integer mode and fractional mode respectively, achieving a FoM of -230 dB.