Agilent Measurement Journal - Issue 5 - 2008 - (Page 14)

This change is easier to see when viewing this signal as an eye diagram (Figure 8). The eye, though still open, has been “pinched” and is more closed (lower trace in Figure 8). For some bit transitions, the amplitude does not rise to the “one” level at all. Reducing the channel bandwidth by having a longer trace, or increasing the data rate would potentially cause the eye to completely close. Figure 9. Transition bits showing emphasis Figure 8. Limited channel bandwidth effect displayed as an eye diagram (lower trace) Dealing with the limited-bandwidth effect To achieve higher data rates through existing or low-cost channels, the designer must compensate for the bandwidth limitation. Because the channel attenuates the higher frequency components, they must be ampliﬁed. Two common techniques exist for addressing this task. One option is to apply emphasis at the transmitter, increasing the transmitted energy in the higher frequencies. This is done by increasing the amplitude of the data signal on the transition bits relative to the amplitude on the non-transition bits (Figure 9). A transition bit occurs where the data signal transitions from a one to a zero or from a zero to a one. Non-transition bits occur where the signal remains at the same level. Because it is the transitions that contain higher frequency energy, emphasis increases the energy in the higher frequencies that are low-pass ﬁltered by the channel. Emphasis can be created by boosting the transition level above the nominal signal level (pre-emphasis) or by reducing the non-transition level below the nominal signal level (de-emphasis). Since most digital standards constrain the maximum signal level, de-emphasis is the more common implementation. The effect that emphasis can have on a bandwidth-limited signal can be dramatic. As an example, compare the bandwidth-limited eye diagram in the lower trace in Figure 8 with the emphasized bandwidth-limited eye diagram in the lower trace of Figure 10. Figure 10. Eye diagrams with emphasis, before and after the bandwidth limited channel 14 Agilent Measurement Journal

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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