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A 3.125-Gb/s clock and data recovery (CDR) circuit using a half-rate digital quadricorrelator frequency detector and a shifted-averaging voltage-controlled oscillator is presented for 10-Gbase-LX4 Ethernet. It can achieve low-jitter operation and improve pull-in range without a reference clock. This CDR circuit has been fabricated in a standard 0.18-μm CMOS technology. It occupies an active area of 0.6 × 0.8 mm2 and consumes 83 mW from a single 1.8-V supply. The measured bit-error rate is less than 10-12 for 27 - 1 PRBS 3.125-Gb/s data. It can meet the jitter tolerance specifications for the 10-Gbase-LX4 Ethernet application.