12 September 2006

Overcoming interference

Signal conditioning aids high speed interface design.

High speed signalling is tricky business. As we climb beyond Mbit/s to Gbit/s data rates, the signal path behaves as a transmission line. At these speeds, reactive parasitics, pcb materials and layout geometries play a major role. Even minor impedance discontinuities will cause reflections, degrading the transmitted signal.
Signal degradation can be split into two broad categories: transmission loss due to the characteristics of the signalling medium; and distortion due to impedance discontinuities.
As a simple example, let’s consider an ideal homogenous 30in FR4 trace whose termination matches its characteristic impedance. Here, we have a consistent impedance and can concern ourselves only with the effect of the transmission media. Signal attenuation increases with frequency, due to skin effect and dielectric loss, with the net result that the media responds as a low pass filter.
Although we may be transmitting at a set data rate, the signal is composed of a continuous (Fourier) frequency spectrum dependent on the edge rate and the data pattern. The higher frequency components of the signal will have greater attenuation, with this effect increasing per unit length of the media. High frequency signal loss causes intersymbol interference – known as ISI – which closes the ‘eye pattern’, increases deterministic jitter and can eventually cause bit error problems.
An effective method of combating ISI distortion is through the use of signal conditioning. Pre emphasis is a technique where the transmitter signal is modified to increase the high frequency content such that, after traveling down the lossy transmission line, the received signal closely resembles the original unmodified signal. Some transmitters have configurable pre emphasis, allowing the user to optimise the driver signal depending on the amount of loss.

Author
Ken Filliter

Supporting Information

Downloads
7473\Overcoming-interference.pdf

Websites
http://www.natsemi.com

Companies
National Semiconductor UK Ltd

This material is protected by Findlay Media copyright
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.

Do you have any comments about this article?


Add your comments

Name
 
Email
 
Comments
 

Your comments/feedback may be edited prior to publishing. Not all entries will be published.
Please view our Terms and Conditions before leaving a comment.

Related Articles

Toshiba Bluetooth IC

Toshiba has introduced a low power Bluetooth IC that supports Bluetooth Low ...

Bendable optical interconnect

Researchers from Ghent University have created what they believe may be the ...

Power IC for next gen EVs

A single channel power IC, designed to bring the sound of a conventional ...

Interconnect: Polymer fibres

Fibre optics are good for transmitting high speed data across the Atlantic, but ...

Making the standard start

Connectors are not the glamorous end of the market. They are expected to do ...

Connecting with compliance

When the Reduction of Hazardous Substances (RoHS) Directive came into force in ...

Connecting to Android Platform

This white paper from FTDI Chip will describe some of the options for ...

XMOS XS1 Architecture

An XS1 combines a number of XCore processors, each with its own memory, on a ...

Synchronous buck regulator IC

The A8672 from Allegro MicroSystems Europe is a synchronous buck regulator IC ...

Current sensing IC

International Rectifier has launched the IR25750, a general purpose current ...

Power management ICs

A new family of power management ICs, aimed at RF agile radio and FPGA based ...

RFIC simplifies design process

Nikolaus Klemmer, High Speed Products, has been developing communcations ...

Home automation system

Manage your home with TI technology and control various home applications ...

SN65LVDS324 overview

Ross demonstrates an easy and low cost way to build a video camera system that ...

Connectivity, the new essential service

One of the legacies of having an engineering background is that, inevitability, ...

Patrick Zammit, Avnet

Patrick Zammit, president, Avnet Electronics Marketing EMEA, talks with Chris ...

Claire Jeffreys, NEW

Claire Jeffreys, events director, National Electronics Week, talks with Chris ...