1 Introduction

PCIe, or Peripheral Component Interconnect Express, is a high-speed data transfer standard commonly used in computers and enterprise systems to interface between CPUs and various types of endpoints such as GPUs and SSDs. It is also attracting interest in new areas such as automotive applications. PCIe is an ubiquitous protocol with many advantages: it uses a simple NRZ (non-return-to-zero) differential signaling scheme and the structure based on multiple individual lanes allows it to support both small and large applications as needed.

Industry trends have led to greater and greater PCIe data rate demands with successive generations. PCIe Gen 1 from 2003 allowed for 2.5 GT/s of data transfer per lane. PCIe Gen 5 from 2017 provides for up to 32 GT/s, an increase of nearly 13x. No modifications were made to the original NRZ signaling scheme from Gen 1 to Gen 5 and such data rate increases were accomplished by increasing the symbol rate, leading to new challenges in signal integrity. Signal conditioning devices such as redrivers and retimers are designed to address these problems by maintaining signal quality over higher data rates and longer reaches. Though many methods can be used to condition signals, equalization is one of the most fundamental and is intimately tied to the properties of the materials that signals propagate through.