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This brief presents a differentially terminated CML transmitter with a self-calibration scheme based on time-domain reflectometry for preemphasis strength control. Without any handshaking or receiver mode control, the transmitter measures the time of flight by applying the same step input on the two transmission lines of the differential link. Since the receiver does not change its configuration, the proposed scheme greatly simplifies the preemphasis adaptation. To verify the calibration scheme, the proposed transmitter is fabricated in a 0.18 ??m- CMOS. For various lengths of the microstrip line on the printed circuit board up to 80 cm, the tested transmitter greatly improves signal integrity and shows clear eye diagrams at 5 Gb/s.