(仅供参考)抖动和眼图的视觉分析
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抖动和眼图的视觉化分析
抖动为实际数据与其理想位置的时间偏差
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…
0.0ns0.990ns 2.000ns 2.980ns 4.000ns
P2P3P4
P
1
TIE0.000ns
-0.010ns0.000ns-0.020ns
眼图是怎么形成的?
Random Jitter(随机抖动)
•随机抖动符合高斯型分布
•直方图(估计) ↔ pdf(数学模型)
•抖动峰峰值=无穷大…无界!
1-sigma or RMS 7-sigma
•内部热能现象
•Flicker Noise, Shot Noise •热能的原子与分子振动
•分子的解体
•外部的宇宙射线
Deterministic Jitter(确定性抖动)•确定性抖动是非高斯分布并且有界
Peak-to-Peak
Periodic Jitter(周期性抖动)•TIE 随时间的变化是重复的、周期性的•Periodic jitter 和相位调制(PM)是等效的Peak-to-Peak
•系统时钟(抖动频率在MHz 量级)•开关电源(抖动频率在KHz 量级)
Duty Cycle distortion(占空比失真)
•上升时间和下降时间不对称
•或者测试时参考电平选择不当
0.0v
-0.1v
Inter-Symbol Interference(码间干扰抖动)
•DDJ 或PDJ –数据相关性抖动或码型相关性抖动,
和ISI的术语是等价的.
•码型是如何影响随后的比特位的?
◦由于传输链路的效应、反射等
换个角度看抖动,时域
看看我们有了什么视角?
抖动视觉化–时间趋势图
▪直方图告诉了我们分布,但是只有统计特性,缺少了时间信息
▪时间趋势图可以直观告诉我们波形里是否有特定频率的调制
▪下图为5个周期SSC @ 30khz
抖动视觉化
Gaussian Random Noise Sinusoidal Jitter
抖动视觉化–频谱图
▪从频域上观测抖动
▪抖动中决定性的频率成分会在谱线上明显超出噪底
哪个眼图好?哪个直方图好?视觉化眼图和抖动的问题?
浴盆曲线
误码率是关键vs. UI 张开程度
•For a given position in the time there’s a given probability of error –
“BER ”, Bit Error Ratio
•For a given position in the time there’s a given probability of signal crossing –PDF , probability density function
1 UI
P r o b a b i l i t y o f ‘h i t ’
P r o b a b i l i t y o f E r r o r –B E R
基于示波器分析的浴盆曲线
Rj δδ/Dj δδ与Tj @ BER
Assume bi-modal distribution (dual-Dirac), measure Tj at two BER Fit curve to points, slope is Rj, Intercept is Dj
Measured
Tj @ 10-7Measured
Tj @ 10-4
½Dj δδ½xRj δδ
Estimated
Tj @ 10-12
x≈7.4σx≈10.4σ
x≈14.1σ双狄拉克模型Conditions: only where Gaussian.
抖动类型分析
•
抖动分离为误码产生的根本原因提供了更精确的定位和分析方法•抖动分析方法,参照T11 MJSQ ,已经被工业界广泛接受Constituent Components of Jitter
= Unbounded
= Bounded Total Jitter
(TJ)
Duty-Cycle Jitter (DCD)
Data Dependent Jitter (DDJ)Periodic Jitter
(PJ)Deterministic Jitter (DJ)
Random Jitter
(RJ)
Jitter Visualization –Bathtub Plot
▪Shows the Eye Opening at a Specified BER Level
▪Note the eye closure of System I vs. System II due to the RJ-RJ is unbounded so the closure increases as BER level increases
▪System I has .053UI of RJ with no PJ
▪System II has .018UI of RJ and .14UI of PJ @ 5 and 10Mhz
System I System I
System II System II