An All-Digital PLL for Cellular Mobile Phones in 28-nm CMOS with -55 dBc Fractional and -91 dBc Reference Spurs

We propose a time-predictive architecture of an all-digital PLL (ADPLL) for cellular radios, which is optimized for advanced CMOS. It is based on a 1/8-length time-to-digital converter (TDC) of stabilized 7-ps resolution, as well as wide tuning range, and fine-resolution class-F digitally controlled oscillator (DCO) with only switchable metal capacitors. The 0.4-mW TDC clocked at 40 MHz maintains 7-ps resolution for <-107 dBc/Hz in-band phase noise while the 7.3-mW DCO emits -157 dBc/Hz at 20 MHz offset at 2 GHz. Reference spurs are <-91 dBc, while fractional spurs are <-55 dBc. The ADPLL supports a 2-point modulation and consumes 11.5-mW while occupying 0.22 mm$^2$