A Wide-Tracking Range Clock and Data Recovery Circuit

A hybrid analog-digital quarter-rate clock and data recovery circuit (CDR) that achieves a wide-tracking range and excellent frequency and phase tracking resolution is presented in this paper. A split-tuned analog phase-locked loop (PLL) provides eight equally spaced phases needed for quarter-rate data recovery and the digital CDR loop adjusts the phase of the PLL output clocks in a precise manner to facilitate plesiochronous clocking. The CDR employs a second-order digital loop filter and combines delta-sigma modulation with the analog PLL to achieve sub-picosecond phase resolution and better than 2 ppm frequency resolution. A test chip fabricated in a 0.18 mum CMOS process achieves BER <10^-12 and consumes 14 mW power while operating at 2 Gb/s. The tracking range is greater than plusmn5000 ppm and plusmn2500 ppm at 10 kHz and 20 kHz modulation frequencies, respectively, making this CDR suitable for systems employing spread-spectrum clocking.