抖动和眼图的视觉分析.doc

文档介绍：第 1 页
抖动和眼图的视觉化分析
什么是抖动？
TIE 为信号相对于标准时钟或者标准信号的定时误差
TIE 在高速数字系统中即为抖动… 2011-First scope vendor with BUJ support
•2015–
RT Noise Analysis and Sampling BER and PDF Mask Testing
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抖动和眼图的视觉化
眼图怎么切割的？时钟决定！
TIE 抖动需要参考时钟
参考时钟提取的过程就是时钟恢复•参考时钟有几种确定的方式:
◦Constant Clock with Minimum Mean Squared Error
This is the mathematically “ideal” clock
But, only applicable when post-processing a finite-length waveform
Best for showing very-low-frequency effects
Also shows very-low-frequency effects of scope’s timebase
◦Phase Locked Loop (. Golden PLL
Tracks low-frequency jitter (. clock drift
Models “real world” clock recovery circuits very well
◦Explicit Clock
The clock is not recovered, but is directly probed
◦Explicit Clock (Subrate
The clock is directly probed, but must be multiplied up by some integral factor
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7/7/2016
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Importance of Clock Recovery
•From spec, “The jitter measurement device shall comply with the JTF”.
•How do I verify JTF?
◦JTF is difference between input clock (ref and input clock
(unfiltered
◦Use 1100b or 0011b pattern (proper 50% transition density
◦Check 1 LF attenuation, 2 -3 dB corner frequency, and 3 slope
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JTF vs PLL Loop Bandwidth
•Configuring the correct PLL settings is key to correct
measurements
•Most standards have a reference/defined CR setup
◦For example, USB uses a Type II with JTF of
•Type I PLL
◦Type I PLL has 20dB of roll off per decade
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◦JTF and PLL Loop Bandwidth are Equal
•Type 2 PLL
◦Type II PLL has 40dB of roll off per decade
◦JTF and PLL Loop Bandwidth are not Equal
▪For example, USB uses a Type 2 PLL with a JTF of .
The corresponding loop bandwidth is Mhz
▪Setting the Loop Bandwidth as opposed to JTF will lead to incorrect jitter measurement results
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PLL Loop Bandwidth vs. Jitter Transfer Function
(JTF
A: Constant Clock Recovery B: PLL Clock Recovery Ratio of B/A
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