A PLL Technique: Charge-Steering Sampling

This article introduces a charge-steering sampling (CSS) technique for time-error detection (TD), an equivalent of phase detection (PD), in phase-locked loops (PLLs). The CSS mechanism presets the input capacitors of a successive approximation register (SAR) analog-to-digital converter (ADC) to V_DD and subsequently discharges them during a reference-triggered pulse through a pseudo-differential MOS pair directly driven by the oscillator. The resulting differential-mode (DM) charge residue, proportional to the time error, is digitized by the ADC to support all-digital PLL (ADPLL) operation. The proposed technique simultaneously achieves high-TD gain for low jitter, the excellent oscillator isolation for reduced reference spur, and multi-bit digital TD output for fast locking, fully leveraging the capabilities of advanced CMOS technology. A digital loop filter (DLF) featuring a dead zone (DZ) in the integral path is introduced to mitigate potential conflicts with the proportional path. To accommodate the short-oscillator period T_osc at millimeter-wave (mm-wave) frequencies, we propose extending the CSS pulsewidth to 1.5 T_osc. In addition, a damped-sine waveform model for the CSS current is developed, providing deeper insights into the high-TD gain characteristics. The comprehensive noise analysis of the CSS is conducted using a multirate timestamp model, identifying contributions to the output phase noise (PN). Fabricated in 22-nm CMOS, the 18.8–23.3-GHz CSS-ADPLL prototype achieves 63-fs rms jitter, -52.4-dBc reference spur, and a figure of merit (FoM) of -254 dB, while consuming 9.95-mW total power, with only 1.3 mW allocated to the loop. For an initial frequency error of 200 MHz, the system achieves a locking time of 0.61 μs, benefiting from the combined effects of a counter-based frequency-locked loop (FLL) (0.27 μs) and the multi-bit digital output of the CSS-ADPLL (0.34 μs).