Agilent Measurement Journal - Issue 5 - 2008 - (Page 33)

T Total jitter (TJ) is deﬁned by many high-speed, serial-data standards to ensure relatively low bit error rates (BERs) in the presence of timing misalignment. As the date rates are increased, this parameter becomes more critical. For a highspeed serial data link operating at rates beyond 2.5 Gb/s, the required error rate cannot exceed 10-12. Timing misalignment, or jitter, contributes more to the overall BER than any other effect in a typical high-speed serial link and is caused by limited-channel bandwidth, system noise, phase noise, crosstalk, and power supply leakage — among other things (see sidebar, Taxonomy of Jitter). Total jitter, the sum of all the timing errors of the signal, can be measured with either an oscilloscope or bit error rate tester (BERT) by observing the timing error of the crossover point or the BER. The BER measurement is based on the statistical properties of the underlying physical mechanisms. As the sample size is increased, the estimate of the parameter is improved. A conﬁdence level can be assigned to this estimate, especially when the underlying mechanism is stochastic. Total jitter is derived by making multiple BER measurements while adjusting the sampling point. By deﬁning the upper and lower limits on the conﬁdence level, it is possible to identify the range of values for TJ. This article explains this bracketing approach in detail and outlines the search algorithms used to ﬁnd the range and the conﬁdence level for the estimate. Representing jitter Jitter quantiﬁes the allowed uncertainty of the sampling instance within the timing window of the bit. It is formally deﬁned as the expected deviation of a signal’s timing event from its intended or ideal occurrence in time. Ideal occurrences are marked by reference-clock sources that specify when the ideal events should occur. In general, this ideal clock is realized by either a master network clock or a clock that is recovered from the data stream, depending on the communication scheme. Mathematically, jitter can be represented in either the analog or digital domain. In analog communications, jitter is also known as phase noise and is deﬁned as a phase offset that continuously changes the timing of a signal: Equation 1. S(t) = P[t + f(t)] Where S(t) is the jittered signal waveform, P[t] is the undistorted waveform, and f(t) is the phase offset, or phase noise. This deﬁnition is most useful in the analysis of analog waveforms such as clock signals and is frequently used to express the quality of oscillators. Agilent Measurement Journal 33

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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