3.125Gbps Ethernet transmitter for IEEE 802.3a.e.

Optical communication systems become more and more important in LAN. In this application, 10GBase-LX4 has good performances at low cost and high data rate. It will play an important role in the low cost data communications. The architecture of 10GBase-LX4 is composed of four lanes. One of lanes is required to transmit data with the rate of 3.125-Gb/s. Through 8B/10B encoders, the system generates a 10-Gb/s data stream finally. This thesis will introduce circuit designs in the lane and how to complete a 3.125-Gb/s Ethernet transmitter which achieves the specification of 10GBase-LX4.
A transmitter includes a serializer, a laser driver, and a laser. Through the serializer, parallel data are transformed into a serial data stream. To achieve high data rate and low circuit complexity, the architecture combines shift register and tree-type MUX. In additional, a 20% duty-cycle clock generator provides the clock for switching transmitting paths to realize this architecture. The laser driver optimizes the characteristics of data streams and provides the adequate modulation current to the laser. The pulsewidth of data will be distorted because of the nonlinear channel. The laser driver controls the pulsewidth, and achieves low circuit complexity in high speed communication. For a laser’s characteristic, a waveform with short rising/ falling times causes a damping output of the laser, and it will affect transmitting quality. Hence, the laser keeps the slew-rate within a moderate range. Previous works make the slew-rate proportional to the bias current. However, laser driver needs to provide large modulation current. This methodology enlarges transistors’ size and increase the power consumption. Therefore, the proposed controller makes the slew-rate proportional to the voltage. Finally, an impedance-controlled loop corrects the output resistance automatically to achieve output impedance matching.
The laser driver is realized in a standard 0.18-μm CMOS technology. The tuning ranges of the pulsewidth and the slew-rate are 35%~65% and 1.52V/ns~1.83V/ns individually. Impedance-controlled loop achieves the resistance variation within +/-2.5%. The power consumption is 110-mW power with 1.8-V supply voltage. The measured peak-to-peak jitter is 40ps.