An Introduction to Preemphasis and Equalization in Maxim GMS

Transmit preemphasis and receive equalization can allow serializer/deserializer (SerDes) devices to operate over inexpensive cables or over extended distances. This application note describes how signals are degraded over cables and how to compensate for that degradation. Additionally, this document explains how to achieve a robust link with Maxim gigabit multimedia serial link (GMSL) products when using lossy cables. The article also provides an overview on line equalization. 

Introduction

Recent advances in video applications, along with the exponential expansion of data traffic volume, have raised the demand for higher data rates. As a result, low-cost twisted-pair (TP) cables have gained special interest. However, frequency-dependent attenuation over long runs of these TP cables is a major limiting factor to their optimal use. This frequency-dependent attenuation causes significantintersymbol interference (ISI) in the received signal, which, in turn, creates difficulty for clock and data recovery and causes a higher bit-error rate (BER). Figure 1 shows the representation of a transmitted signal being degraded by the cable before the signal arrives at the receiver. By significantly reducing ISI and recovering the severely degraded data, the transmitter and the receiver can employ some form of line equalization to enable reliable operation.

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Figure 1. ISI at the receiver end.

); margin-bottom: 15px; "> The high-speed 3.125Gbps transceivers in Maxim GMSL parts provide a robust link, by allowing the system designer to dynamically program the equalization level for a specific cable. The transmitter and receiver both have equalization adjustments that can be programmed either separately or together to extend the transmission distance. This flexible equalization adjustment allows the use of a wide range of low-cost lossy cables.

); margin-bottom: 25px; "> This application note explains how to design a robust link with Maxim GMSL products and lossy cables. It also provides an overview of line equalization.

GMSL Transmitter Preemphasis and Receiver Equalization

); margin-bottom: 25px; "> The GMSL link employs transmitter preemphasis and receiver equalization to compensate the losses of the transmission.