8.7 A 4-to-10.5Gb/s 2.2mW/Gb/s continuous-rate digital CDR with automatic frequency acquisition in 65nm CMOS

Continuous-rate clock-and-data recovery (CDR) circuits with automatic frequency acquisition offer flexibility in both optical and electrical communication networks, and minimize cost with a single-chip multi-standard solution. The two major challenges in the design of such a CDR are: (a) extracting the bit-rate from the incoming random data stream, and (b) designing a wide-tuning-range low-noise oscillator. Among all available frequency detectors (FDs), the stochastic divider-based approach has the widest frequency acquisition range and is well suited for sub-rate CDRs [1]. However, its accuracy strongly depends on input transition density (0 ≤ ρ ≤ 1), with any deviation of ρ from 0.5 (50% transition density) causing 2×(ρ-0.5)×10⁶ ppm of frequency error. In this paper, we present an automatic frequency-acquisition scheme that has unlimited range and is immune to variations in transition density. Implemented using a conventional bang-bang phase detector (BBPD), it requires minimum additional hardware and is applicable to sub-rate CDRs as well. Instead of using multiple LC oscillators that are carefully designed to cover a wide frequency range [2,3], a ring-oscillator-based fractional-N PLL is used as a digitally controlled oscillator (DCO) to achieve both wide range and low noise, and to decouple the tradeoff between jitter transfer (JTRAN) bandwidth and ring-oscillator-noise suppression.