jitter measurement v0bfiles.chinaaet.com/justlxy/blog/20180227/1000019445... · 2018-02-27 ·...
TRANSCRIPT
Page 22009-12-16
简介简介
抖动的峰峰值和有效值
• Pk-pk jitter, RMS jitter
JTA2抖动分析软件包的功能
• Jitter Track
• Jitter Histogram
• Jitter FFT
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抖动定义抖动定义
抖动的定义为“信号的定时事件与其理想位置之间的偏差”。
• SONET SPEC: Jitter is defined as the short-term variations of a digital signal’s significant instants from their ideal positions in time.
按待测试信号的种类来划分
• 时钟抖动
• 数据抖动
按计算方法来划分
• TIE抖动:信号周期相对于一个已知或恢复时钟的误差。
• Period抖动:是在多个周期内对信号周期的变化进行的测量。
• Cycle to Cycle抖动:是对信号相邻周期的变化进行测量的结果。
• Half Period抖动:半个周期的抖动变化(与Period jitter的计算类似)
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抖动定义(续)抖动定义(续)
Pk-pk jitter:峰峰值抖动随着测量时间的增加,测量值变大,测试结果不具备重复性,不能将jitter值与误码率(BER)直接联系起来
• RMS jitter:抖动测量值的有效值
Total jitter:在某BER下总的抖动值。测量Tj(BER)的目的是在某BER下确保互操作性
• 通常TJ是针对串行数据的
• 一般测量Tj(BER=10e-12)
• 一般是数据信号测试Tj和Dj
• Tj(BER<10e-8)只能用BERT测量到,但是示波器可以精确的估算出该值
• Tj可以分解为Rj, Dj, DCD, ISI, Pj
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三种抖动计算举例三种抖动计算举例
某100MHz时钟,第一个到第四个周期分别为9.9ns, 10.1ns, 9.9ns, 10.0ns,假设其理想时钟固定在10ns
• TIE Jitter:• T1 = 10-9.9 = 0.1, T2 = 10-10.1 = -0.1, T3 = 10-9.9 = 0.1 T4 = 10-10 = 0• TIE pk-pk jitter = 0.1 – (-0.1) = 0.2 ns• TIE RMS jitter = 参数T1..T4 的标准偏差
• Period Jitter• P1 = 9.9 P2 = 10.1 P3 = 9.9 P4 = 10• Period Jitter pk-pk value = 10.1 - 9.9 = 0.2 ns• Period Jitter RMS value =参数P1..P4 的标准偏差
• Cycle to Cycle jitter• C1 = P2-P1 = 10.1-9.9 = 0.2 C2 = P3-P2 = 9.9-10.1 = -0.2 C3 = P4-P3 =
10-9.9 = 0.1• Cycle to cycle jitter PK-PK value = 0.4 ns• Cycle to cycle jitter RMS value =参数C1..C4 的标准偏差
标准偏差的计算公式1
)( 2
−−∑n
XmeanXi
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传统的抖动测试方法:余辉显示传统的抖动测试方法:余辉显示
设置非常简单,使用cursor测量波形边沿的宽度
像素或屏幕分辨率(量化误差)会降低精度
只有单个波形,引入了触发抖动
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时钟常见的三种抖动测量时钟常见的三种抖动测量
Cycle to cycle jitter
period jitter
TIE jitter
必须配备JTA2软件包才可以测量三种抖动
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快速设置TIE直方图快速设置TIE直方图
Step1:TIE直方图的快速设置方法
Step2:测量参数选择TIE
Step3:图形选择“Histogram”
Step4:设置相关参数
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几种抖动理论上的关系几种抖动理论上的关系
ReferenceFrequency
WaveformWith Jitter
Distributions OfEdge Locations
TIEMeasurement
Single Edge, Jitter = Jn
WidthMeasurement
Two Edges, Jitter = SQRT(Jn2+Jn+1
2) = 1.41 Jn
PeriodMeasurement
Two Edges, Jitter = SQRT(Jn2+Jn+2
2) = 1.41 Jn
Cycle to CycleMeasurement
Three Edges, Jitter = SQRT(Jn2 + 4Jn+1
2 + Jn+22 )= SQRT(6) Jn
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Rule of ThumbRule of Thumb
TIE=Jn
Width= √2 Jn
Period= √2 Jn
Cycle-Cycle = √6 Jn
Period/TIE = √2
Cyc-Cyc/Period = √3
All the above can vary depending on specific measurement setup
The relationship between Cycle to cycle and period is the most repeatable.
These relationships don’t always show up!
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Jitter Track抖动追踪Jitter Track抖动追踪
Jitter Track波形:X轴为时间,Y轴为某时间的抖
动测量值
• 反映了抖动测量值随时间变化的趋势
• 测量jitter track波形的峰峰值即可得到该抖动的峰峰值
• 测量jitter track波形的标准偏差即可得到抖动的有效值
• 通过jitter track功能可
以找到调制(或干扰)到时钟的信号
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The JitterTrack™ Function
Period Jitter Function
P1 P2 P3 PN-1 PN
P1 P2 P3 PN-1 PN
Source Trace
...
...
Period Jitter
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The JitterTrack™ Function
Time Interval Error (TIE)
Interval Error Jitter Function
I1 I2 I3IN-1 IN
I1 I2 I3 IN-1 IN
Source Trace
...
Reference Clock Edge
...
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The JitterTrack™ Function
Frequency Jitter Function
P1 P2 P3 PN-1 PN
1/P1 1/P2 1/P3 1/PN-1 1/PN
Source Trace
...
...
Frequency Jitter
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抖动追踪案例3-SATA的扩频时钟测试
Serial ATA Waveform
Unfiltered Track
Filtered Track
Conventional Track Display
Unique LeCroy Filtered Track
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抖动追踪案例4:电源不稳定造成的抖动抖动追踪案例4:电源不稳定造成的抖动:电源不稳定造成的抖动
Clock signal
Track(period(c2))
Track(TIE(c2))
Power Ripple
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JTA2软件包括哪些功能?JTA2软件包括哪些功能?
TIE、cycle to cycle、period等的抖动测试
抖动的直方图分析
Jitter track抖动追踪
其他水平测量参数
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JTA2软件包中包括了以下测量参数JTA2软件包中包括了以下测量参数
Symbol Parameter Name Parameter DefinitionΔp@lv delta period at level Difference of adjacent periods at specified slope and
level
freq@lv frequency at level Cycle frequency at specified slope and level
duty@lv duty at level Percent of period for which data is above or below a specified level
edge@lv edge at level The number of transitions or edges with a specified slope and level
p@lv period at level Cycle duration at a specified slope and level
tie@lv time interval error at level Fractional interval time error for a specified slope and level
wid@lv width at level Duration of signal excursion above or below a specified level
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抖动测量精度
VERTICAL UNCERTAINTYDUE TO:
NOISEQUANTIZATIONNON-LINEARITY
AM
PLIT
UD
E
TIME
HORIZONTAL UNCERTAINTYDUE TO:
SIGNAL JITTERAPERTURE UNCERTAINTYTRIGGER JITTERTIMEBASE INSTABILITY
VERTICAL UNCERTAINTY, ΔvMAPS INTOTIME UNCERTAINTY, Δt
Δt = (Δv * tR(20-80) ) / 0.6 vAMPL
UNCERTAINTIES AREUNCORRELATED ANDADD IN QUADRATURE
Δv
v AMPL
Δt Δt
通道的噪声、频响的非线性、ADC的量化误差导致了垂直方向测量的不确定性
垂直方向的不确定性投影到水平方向导致时间的不确定性
信号的上升时间越快,抖动的测量精度越高
使用高精度的ADC可以提高抖动测量精度
通道噪声越小,抖动测量精度越高
对于频率较低、抖动很小的时钟测试,可以适当使用ERES降噪后测量抖动
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Gaussian DistributionGaussian Distribution
Random jitter is distributed as gaussian.
68.28% of population is in +/-1σ range
95.45% in +/-2σ range
99.73% in +/-3σ range
99.994% in +/-4σ range
99.99994% in +/-5σ range
99.9999998% in +/-6σ range
99.9999999997% in in +/-7σ range68.27%
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Gaussian DistributionGaussian Distribution
68.28% of population is in +/-1σ range means 31.73% of population is out of +/-1σ .
Recognize the events out of the specified range as Error, then +/-1σ range could be related with 31.73% Error rate.
4.55% out of +/-2σ
0.27% out of +/-3σ
0.006% out of +/-4σ
0.00006% out of +/-5σ
0.0000002% out of +/-6σ
0.0000000003% out of +/-7σ 31.73%
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Gaussian DistributionGaussian Distribution
Range as multiple of σ refer to Error rate
Error Rate Peak Pk-Pk
10-3 +/-3.09σ 6.180σ10-4 +/-3.719σ 7.438σ10-5 +/-4.265σ 8.530σ10-6 +/-4.754σ 9.507σ10-7 +/-5.199σ 10.399σ10-8 +/-5.612σ 11.224σ10-9 +/-5.998σ 11.996σ10-10 +/-6.361σ 12.723σ10-11 +/-6.706σ 13.412σ10-12 +/-7.034σ 14.069σ10-13 +/-7.349σ 14.698σ10-14 +/-7.650σ 15.301σ10-15 +/-7.941σ 15.883σ10-16 +/-8.222σ 16.444σ
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抖动分解抖动分解
绝大多数情况下,时钟的固有抖动只有Pj,不包括DDJ
总体抖动 (Tj)、随机抖动 (Rj)、固有抖动 (Dj)Tj = N(BER)*Rj + Dj
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FAQFAQ
三种时钟抖动通常用在什么场合?
怎样测量某PLL或晶振的相噪?
某时钟芯片的手册要求80k-20MHz的RMS抖动小于1ps,怎样测试这类时钟?
怎样测量时钟的Tj?