Agilent Measurement Journal - Issue 5 - 2008 - (Page 13)

Characterizing channel bandwidth An ideal channel has inﬁnite bandwidth and does not affect the spectrum of a data signal. In the past, with data rates in the 100 to 200 Mb/s range, this assertion was basically true. With data rates now in the multi-Gb/s range, this assertion is no longer valid. The bandwidth of common circuit-board materials, such as FR4, is limited in relation to these new, higher data rates. This bandwidth can be displayed in the frequency domain as S (scattering) parameters. The S-parameter for transmission is S21. It displays the gain or loss (as in this case) of the signal propagating through the channel. The ideal channel has zero gain (loss) across the signal’s frequency range, although a real channel will have increasing loss at higher frequencies. A sample channel derived using the 20-inch trace on a demo board was measured and is shown in Figure 5. The ﬁgure depicts a –3 dB point (e.g., 3 dB below the level at the minimum frequency) of 945 MHz. The channel bandwidth measurement was made up to 20 GHz. As will be seen, multi-Gb/s data signals have spectral energy at these frequencies. Observing the bandwidth-limited spectrum To observe the bandwidth-limited spectrum, it is necessary to examine the spectrum of the data signal after it has passed through the channel. Figure 6 shows the original data signal spectrum (in yellow) and the spectrum after propagation through the demo board (in blue). The drop in amplitude starts almost at 0 Hz and is particularly large above 5 GHz. Figure 6. Data signal spectrum before (yellow) and after (blue) propagating through the demo board The same data signals can be viewed in the more familiar time domain (Figure 7). Notice that the limited bandwidth of the channel has slowed the transition time of the signal such that the edges appear more rounded and the amplitude of the isolated bits is reduced (lower trace in Figure 7). Figure 5. S21 of the 20-inch trace on a demo board Figure 7. Limited channel bandwidth effect displayed in the time domain (lower trace) Agilent Measurement Journal 13

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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