A 5-Gb/s Adaptive Continuous Time Linear Equalizer Using Ferroelectric Capacitor

Abstract

This article presents a 5-Gb/s continuous-time linear equalizer (CTLE) for serial link interface where adaptive nature is achieved using a ferroelectric capacitor. This barium strontium titanate (BST)-based STPTIC-15G2 capacitor model from STMicroelectronics provides a smooth 5:1 tuning range for a voltage range of 0&#x2013;18 V and eliminates the use of blocking capacitor required in case of varactor diode. The tunability of the high-pass filter response of the proposed CTLE provides the flexibility to meet the allocated loss budget of any serial link more precisely than a design with a discrete value capacitor. Also, its design methodology is much simpler than conventional MOSFET-based CTLE. Moreover, it avoids the use of complex algorithms seen in the case of digital equalization. For validation, the design is implemented on an FR4 board. While studying the data rate tolerance and channel tolerance of the printed prototype, the measured results closely match the simulation results. The compensation level of the equalizer is verified for an FR4 channel by varying its length from <inline-formula> <tex-math notation="LaTeX">$7^{\prime \prime }$ </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">$28^{\prime \prime }$ </tex-math></inline-formula> which is equivalent to a change in insertion loss from 1.3 to 5.2 dB at 2.5 GHz, which is the Nyquist frequency for a 5-Gb/s signal. Also, for a <inline-formula> <tex-math notation="LaTeX">$28^{\prime \prime }$ </tex-math></inline-formula> FR4 channel, a channel loss requirement of 8&#x2013;9.5 dB is achievable by simply varying the applied bias to the ferroelectric capacitor from 0 to 10 V.