Broadband Linear Modulator Driver Design for High-data-rate Wireline Communications

Abstract

This thesis presents the design, analysis, and implementation of a broadband optical modulator driver targeting high-speed wireline communication systems in a 22-nm FD-SOI CMOS technology. The driver is designed to meet stringent specifications for 15.7-dB gain, 90-GHz bandwidth, linear output swing of 4 Vppd, and minimum energy per bit, with a focus on compensating for high-frequency losses in optical modulators through the driver's 9-dB gain peaking. The core of the design revolves around a two-stage broadband amplifier, incorporating a novel shunt/double-series interstage network to achieve amplitude peaking at 75 GHz and extended bandwidth. The interstage network is optimized to provide a 4-pole, 1-zero transfer function, enabling efficient inductive peaking and minimizing in-band ripple. The preamp stage features a PFET cascode amplifier with a shunt/double-series input matching network, designed to achieve broadband input matching up to 80 GHz. The postamp stage employs a parallel-path NFET differential cascode amplifier with an output combining network, optimized for more than 15.2-dBm output 1-dB compression point up to 75 GHz. Experimental results demonstrate the driver's performance across a wide range of operating conditions. The prototype achieves a small-signal bandwidth of over 90 GHz, with a peak gain of 20.4 dB at 76 GHz for a 60-Ohm differential load. The driver demonstrates a low-frequency gain of 15 dB and maintains a flat group delay of 17 ps up to 60 GHz, peaking at 30 ps at 76 GHz. The driver's linearity is characterized using the 1-dB compression point (OP-1dB) and total harmonic distortion (THD). The OP-1dB remains above 13 dBm, equivalent to a differential output swing of 4 Vppd, up to 75 GHz across a 100-Ohm differential load. The THD is measured at 1.6% for a 1-GHz input signal with a 4-Vppd output swing. Time-domain measurements demonstrate the driver's ability to transmit 140-GBaud NRZ signals with a 4-Vppd eye amplitude. For 100-GBaud PAM-4 signals, the driver achieves a 4-Vppd outer optical-modulation amplitude, but the top and bottom eyes begin to close due to the driver's frequency response and distortion with 100-Ohm differential loads. The use of a 4-tap feedforward equalizer (FFE) compensates for frequency response limitations, resulting in improved eye opening. The driver's performance is compared with other state-of-the-art broadband drivers, highlighting its competitive bandwidth, linearity, and energy efficiency. The prototype occupies a core area of 0.17 mm2 and a total area of 0.7 mm2, including pads. The results provide valuable insights into the design and optimization of broadband drivers for high-speed communication systems.