时钟抖动测量

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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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Jitter HistogramsJitter Histograms

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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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设置TIE参数设置TIE参数

查找TIE的参考时钟

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设置TIE直方图的参数设置TIE直方图的参数

设置TIE直方图的参数

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测量直方图测量直方图

Step1：选择P2=hsdev(F8)

直方图的range为抖动峰峰值

直方图的sdev为抖动RMS值

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TIE的测量TIE的测量

TIE jitter的峰峰值与RMS值

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Sinusoidal JitterSinusoidal Jitter

周期性抖动

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Random JitterRandom Jitter

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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 TrackJitter Track

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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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时钟的抖动追踪时钟的抖动追踪

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抖动追踪案例2抖动追踪案例2

通过对track(freq(c1))的波形做FFT，可以在频谱中观察到该时钟受到

2.275MHz的干扰

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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？

Any Question?

The end!