A 1.25-Gb/s digitally-controlled dual-loop clock and data recovery circuit with enhanced phase resolution

A new 1.25-Gb/s digitally-controlled dual-loop clock and data recovery circuit is realized. To overcome jitter problems caused by the phase resolution limit, the CDR has two phase generation stages: coarse generation by a phase interpolator and fine generation by a variable delay buffer. The performance of the proposed CDR was verified by behavioral and transistor-level simulations. A prototype CDR chip fabricated with 0.18 μm CMOS process shows error-free operation for ±400 ppm frequency offset. The chip occupies 165 × 255 μm² and consumes 17.8 mW.