Agilent Measurement Journal - Issue 5 - 2008 - (Page 11)

D Examining data signals in the time domain Digital designers traditionally work in the time domain. Using the primary tool of choice — the oscilloscope — they look at how a signal’s voltage levels vary with time. Figure 1 offers an example of a captured signal displayed in the form of a singlevalued waveform. For each point in time, or position on the oscilloscope display, the waveform has one value. The rise and fall times of the signal, as well as the zero and one levels, can all be easily identiﬁed on the display; however, due to a typical screen’s limited size and resolution, only a limited duration of the waveform can be seen. Because an overall view of the signal is not provided, it is easy to miss a waveform’s extreme values. The designer can scroll the oscilloscope display to view different sections of the waveform, but still it is impossible to get an overall picture. Data rates in digital systems are on the rise. These increased data rates need to be maintained in deployed systems and in new systems where cost is a concern. These constraints produce systems in which the bandwidth of the channel (e.g., backplane, circuit board) can not be increased as the data rate increases. At the same time, the channel’s ﬁnite bandwidth heavily limits its high-speed performance. Consequently, this limited bandwidth constrains the maximum allowable data rate that can be transmitted through the channel. One of the limiting factors in increasing the data rate is the intersymbol interference (ISI) created by the limited-bandwidth data channel. Limiting factors such as implementation costs and in-place systems hinder a designer’s ability to increase channel bandwidths. In deployed systems, for example, it is not practical to change out the backplane to increase the channel bandwidth. It is therefore necessary to determine how much ISI the existing channels produce at the new higher data rate. For new designs, cost constrains the developer’s ability to create a higherbandwidth channel. Higher-bandwidth materials exist and can be used in place of standard FR4 but these materials come at a greatly increased cost. This article examines the effect of limited-bandwidth channels on digital data signals. It also explains how to overcome these effects through compensation for the bandwidth limitation by emphasis and equalization. Figure 1. Display of a captured signal as a single-valued waveform Agilent Measurement Journal 11

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 5 - 2008

Agilent Measurement Journal Issue 5 Finding a New Path when Assumptions Break Down Contents Testing MIPI D-PHY Protocols Oscilloscope Firmware Update DNA Replication Joining the LTE/SAE Trial Initiative Testing Proteins Praise for DC Power Analyzer Scripting Features for AMDS Testing Hybrid PCBA Systems Improving the Gene Expression System Workflow Turning a “Good Enough” Test Strategy into One That’s Reliable, Repeatable and Traceable Understanding the Effects of Limited-Bandwidth Channels on Digital Data Signals Introducing the 3GPP LTE Downlink Validating the 26 Validating the Physical and Protocol Layers in DDR Memory Interfaces Understanding Total Jitter Measurements of Low Probabilities Using Behavioral-Model Simulation to Accurately Predict First-Order PLL Performance Creating Synchronous High-Frequency Sampling across Multiple Digitizers Overcoming the Challenges of Testing FlexRay Networks Understanding Operating Life and Repeatability of Electromechanical Switches and their Effect on Total Cost of Ownership Implementing Micro and Nano LC/MS Techniques for High Sensitivity Lipidomics Analysis

Agilent Measurement Journal - Issue 5 - 2008

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