Agilent Measurement Journal - Issue 5 - 2008 - (Page 12)

The technique used to obtain an overall view of the digital signal is to display an eye diagram, which is an overlay of the different bit transitions plotted one on top of the other (Figure 2). In the eye diagram, maximum and minimum values of the zero and one levels can be seen along with the extremes of rise and fall times. On a spectrum analyzer, the measured power spectrum shows the absolute value of the sinc2(f) function. If the digital signal is a repeating pulse, the resulting power spectrum will be spectral lines rather than a continuous envelope. The spectral nulls fall at the pulse frequency and its multiples. The spectral lines follow the pulse repetition rate. If the digital signal is more complicated than a repeating pulse such as a data signal, its spectrum is more complex. In this case, the spectral nulls will fall at the data rate and its multiples, similar to the pulse train. For a repeating pattern, such as a PRBS7, the spectral lines fall at the inverse of the pattern length in time. For a 5-Gb/s data rate, PRBS7 data signal, the spectral lines will be spaced at 5e9/127 or 39.4 MHz (Figure 4). The longer the pattern, the closer the spectral lines fall and the more closely the spectrum envelope follows the sinc2(f) function. Figure 2. Data signal eye diagram Viewing data in the frequency domain When digital signals propagate through a bandwidth-limited channel, they are low-pass ﬁltered and the higher frequency content is attenuated. To understand this effect, look at the data signal in the frequency domain (e.g., the signal’s spectrum). In the simplest case of one bit, or impulse, the spectrum takes the form of a sinc function (e.g., sin(x)/x) (Figure 3). Figure 4. Spectral line spacing of the PRBS7 pattern The other signiﬁcant effect on the spectrum is the transition time of the signal (e.g., its rise and fall times). The faster the transition time, the more energy there is at higher frequencies, which is indicated by increases in the power of the higherfrequency spectral lines. With the channel acting as a lowpass ﬁlter, it has a greater effect on these higher-frequency components. Increased data rates translate into a short bit time or period, necessitating faster transition times. The faster transition times mean that more of the signal’s energy is at higher frequencies and therefore, the bandwidth limitation of the channel has a larger effect on the signal spectrum. Figure 3. Spectrum of an impulse, the sinc function 12 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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