A 3.5-6.8-GHz Wide-Bandwidth DTC-Assisted Fractional-N All-Digital PLL With a MASH ΔΣ-TDC for Low In-Band Phase Noise

Abstract

This paper proposes a digital-to-time converter (DTC)-assisted fractional-N wide-bandwidth all-digital phase-locked loop (ADPLL) with a fine-resolution time-to-digital converter (TDC). The TDC employs a two-channel time-interleaved time-domain register with an implicit adder/subtractor realizing an error-feedback topology. Such an error-feedback unit of a first-order Δ Σ -TDC can be cascaded as a multi-stage noise shaping configuration to achieve higher-order noise-shaping and, thereby, low in-band phase noise (PN) of the ADPLL. A digitally controlled oscillator with a transformer and a pair of cross-coupled NMOS amplifiers exploits magnetic and capacitive coupling to achieve nearly an octave frequency coverage, i.e., 1.73-3.38 GHz (after a ÷2 division). Fabricated in 40-nm CMOS, the ADPLL achieves better than -110-dBc/Hz in-band PN and occupies an active area of 0.5 mm². With a 50-MHz reference clock, a 2-GHz output RF clock, and a loop bandwidth of 800 kHz, this prototype achieves 420-fsrms jitter, integrated from 1-kHz to 30-MHz offset, while drawing 10.7 mW.