A −242dB FOM and −75dBc-reference-spur ring-DCO-based all-digital PLL using a fast phase-error correction technique and a low-power optimal-threshold TDC

Abstract

To improve efficiency in the use of silicon, there have been many efforts to develop ring-oscillator-based clock generators with low jitter. A PLL using a fast phase-error correction (FPEC) technique [1] is one promising architecture. By emulating the phase-realignment mechanism of an injection-locked clock multiplier (ILCM), the FPEC PLL can achieve ultra-low jitter that is almost comparable to that of ILCMs. In addition, since the FPEC PLL has an integrator in its transfer function, it can also achieve a low reference spur and a high multiplication factor (N), which is different from ILCMs. However, the FPEC PLL of an analog implementation in [1] has difficulty maintaining optimal loop characteristics, which vary easily due to PVT variations or a change in the output frequency. To facilitate the calibration of loop characteristics, the FPEC can be implemented in an all-digital PLL (ADPLL), increasing the control word of a DCO, DFCW, in a very short duration, TFPEC, as shown in Fig. 25.4.1. Since the FPEC technique can rapidly remove the accumulated jitter of the DCO from the previous reference period, fREF, the variance of the output jitter, VAR[JOUT](f), becomes saw-tooth-shaped along with the accumulating jitter. In a conventional ADPLL, the accumulated jitter is removed gradually over TREF, so the variance of the jitter is nearly constant [2]. This difference enables the FPEC ADPLL to have much lower RMS jitter, σRMS. However, the FPEC ADPLL is limited in its ability to achieve extremely low jitter, i.e., it cannot reduce σRMS as much as analog FPEC PLLs can. This is because typical ADPLL TDCs provides less precise information regarding the oscillator jitter than a PD does in analog PLLs. When it detects a timing error, τerr, a TDC generates a digitized value, DTDC; thus, the amount of error to be corrected becomes rather than τerr. This results in a quantization error, τq, thereby increasing σrms. To minimize τq (or [τq 2]), the resolution of a TDC must be improved significantly to a level at which the quantity of jitter can be distinguished, but this is difficult when a typical CMOS process is used. Even if the design itself were possible, additional power would be required to generate many evenly spaced time thresholds.