A 15-Gb/s Sub-Baud-Rate Digital CDR

This paper presents a sub-baud-rate clock and data recovery (CDR) circuit that can recover clock and data using only differential quarter-rate clocks. A combination of eight samplers and an integrator recover four data bits in each clock cycle. Four of the eight samplers are re-used for phase detection as well as for background calibration to improve the robustness of the CDR to process, voltage, and temperature variations. A continuous-time linear equalizer is used to compensate for inter-symbol interference up to 11 dB. The CDR prototype fabricated in a 65-nm CMOS recovers 15.2-Gb/s data using only differential 3.8-GHz clock and achieves bit error rate (BER) 10-MHz jitter tolerance (JTOL) corner, and 548 fsrms recovered clock jitter. The total power consumption is 29 mW, which translates to an energy efficiency of 1.9 pJ/bit.