抖动和眼图的视觉分析

1、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 抖动为实际数据与其理想位置的时间偏差0.0ns 0.990ns 2.000ns 2.980ns 4.000ns-0.010ns 0.000ns -0.020ns TIE 0.000ns什么是眼图?眼图是怎么形成的?Random Jitter帼机抖动碰机抖动符合高斯型分布2ft方图（估计？ pdf（数学模型 洲动峰峰值=无穷 大无界！枕部热能现象？Flicker Noise, Shot Noise热能的原子与分子振动 份子的 解体力卜部的宇宙射线Deterministic Jitte

2、r（确定性抖动？确定性抖动是非高斯分布并且有界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？系统时钟（抖动频率在 MHz量级）川关电源（抖 动频率在KHz量级）Duty Cycle distortion（占空比失真?升时间和下降时间不对称域者测试时参考电平选择不当0.0vInter-Symbol Interference何间干扰抖动?DDJ或PDJ嗷据相关性抖动或码型相关性抖动，和ISI的术语是等价的.剂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?7

3、/7/201611抖动视觉化出寸间趋势图?ft方图告诉了我们分布，但是只有统计特性，缺少了时间信息？寸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”图为5个周期SSC 30khz抖动视觉化Gaussian Random Noise Sinusoidal Jitter7/7/2016127/7/201613抖动视觉化须谱图阳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 theres a given probability

4、of error-“BEFError Ratio?1 UI基于示波器分析的浴盆曲线Rj /Dj与否 BERAssume bi-modal distribution (dual-Dirac, measure Tj at two BER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洲动分析方法，参照T11 MJSQ,已经被工业界广泛接受 Constituent Components of Jitter=Unbounded=Bounded7/7/201618Jitter Visualization -Bathtub Plot?Rote the eye closure of S

5、ystem I vs. System II due to the RJ-RJ is unbounded so the closure increases as BER level increases?stem I has .053UI of RJ with no PJ?System II has .018UI of RJ and .14UI of PJ 5 and 10MhzSystem ISystem ISystem IISystem IITektronix -Innovators of Jitter Analysis?1998First Re用Time Scope Based Jitter

6、 Analysis Software?2002 Invented SW Baed PLL Clock Recovery and the Spectral Approach for Jitter Separation?2004 Invented RT Eye rendering on a Real Time Scope?2004First vendor to support both modeled (Dual-Dirac and measured (Spectral jitter methods?2005Invented measurements with Jitter and Noise r

7、econciliation?2011First scope vendor with BUJ support?2015-RT Noise Analysis and Sampling BER and PDF Mask Testing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修考时钟有几种确定的方式：?Constant Clock with Minimum Mean Squared ErrorThis is the mathematically“ideal clockBut, only applicable when post-processi

8、ng a finite-length waveformBest for showing very-low-frequency effectsAlso shows very-low-frequency effects of scope s timebase?hase Locked Loop (e.g. Golden PLLTracks low-frequency jitter (e.g. clock driftModels “ real world clock recovery circuits very wellExplicit ClockThe clock is not recovered,

9、 but is directly probedExplicit Clock (SubrateThe clock is directly probed, but must be multiplied up by some integral factor7/7/201621Importance 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 i

10、nput clock(unfiltered?Use 1100b or 0011b pattern (proper 50% transition density?Check 1 LF attenuation, 2 -3 dB corner frequency, and 3 slope 7/7/201622JTF vs PLL Loop Bandwidth?Configuring the correct PLL settings is key to correctmeasurements?Most standards have a reference/defined CR setup?zor ex

11、ample, USB 3.0 uses a Type II with JTF of 4.9Mhz?Type I PLL?Type I PLL has 20dB of roll off per decadeJTF and PLL Loop Bandwidth are Equal?Type2 PLL?Type II PLL has 40dB of roll off per decadeJTF and PLL Loop Bandwidth are not Equal?zor example, USB 3.0 uses a Type 2 PLL with a JTF of 4.9Mhz.The cor

12、responding loop bandwidth is 10.126 Mhz?Setting the Loop Bandwidth as opposed to JTF will lead to incorrect jitter measurement results7/7/201623PLL Loop Bandwidth vs. Jitter Transfer Function(JTFA: Constant Clock Recovery B: PLL Clock Recovery Ratio of B/A7/7/201624JTF Filtering Effects based on dif

13、ferent PLL bandwidths7/7/201627f 3dB = 30 kHzf 3dB = 300 kHzf 3dB = 3 MHzJitter for Busy PeopleHints, Tips and Common ErrorsUsing the Jitter Analysis Tools?Issues manifested in different layers of theprotocol stack?Crosstalk, jitter, reflections, skew?Disparity, encoding or CRC errors?Where do I sta

14、rt debugging?Jitter and Eye Diagram Tools?Oscilloscope-based for quick results?zast jitter measurements with? One Button Jitter Wizard?Compare timing, jitter, eye, amplitude measurements?User-definable clock recovery, filters, pass/fail limits, and reference levelsMore Hints for Successful Jitter An

15、alysis?Clock Recovery hasa great deal of influence on jitter results. Think about what you re trying to accomplish.?Constant-Clock is the most “ unbiased ”Often best if you re trying to seewveryquency effectsBut it can also show wander in the scope s timebaseto the?LL recovery can model what a real

16、data receiver will seeIt can track and remove lowfrequency effects, allowing you to“see throughjitter that really contributes to eye closure?Explicit-Clock is appropriate if your design uses a forwarded clockMake sure your probes are deskewedHints for looking at Spread-Spectrum Clock?If you want to

17、see the SSC effects, use TIE and PLL clock recovery with a bandwidth of at least 1 MHz. A Type-II (2nd -order PLL will track out the SSC more effectively than a Type-I PLL.?want to observe the SSC profile:?Use a Period measurement and turn on a 3rd -order low-pass filter(in DPOJET with a bandwidth o

18、f 200 kHzBecause Period trends accentuate high frequency noise, the low-frequency SSCtrend will be obscured if you don t use a filter You can t use a Frequency measuremedirectly. The combination of filtering and the reciprocal operation (Freq = 1/Per causedistortion in the resulting waveshape. (This

19、 is a mathematical fact, not a DPOJET defect.?f you use a TIE measuremnt, you ll see modulation that looks like a sine wave. This is normal. It s because TIE measures phase modulation, which is the integral of frequency. It turns out that the integral of a triangle wave looks very much like a sine w

20、ave.误码率与噪声分析Anatomy of a Serial Data LinkComplete LinkChannelAspirational goal: 0 errorsPractical Goal: Bit Error Rate Target BER?Since BER is the ultimate goal, why not measure it directly?Serial Data Link Integrity = Bit Error Rate?Bit Error Ratio Testers (BERTs are the tools for measuring BER dir

21、ectly?Why not use ONLY BERTs for Serial Data Link Analysis?Difficult to model/emulate equalizer?Measurements could take a very long timeInstruments are very expensive and not all that flexible?Does not analyze the root causes of the impairments of the links?Alternative approach: use a scope and adva

22、nced analysis tools?Easily move from Compliance to Debug?Better equipped to identify root causes of eye closure?Equalizer can easily be modeled?More cost effective?Faster throughputWhy Measure Jitter and Noise?Link Model: Transmitter + Channel + Receiver?Transmitter generates a stream of symbols?Rec

23、eiver uses a slicer to make a decision on the transmitted symbol?The Bit Decision is made at a certain time (t of the symbol interval and a comparison of the sliced data to a threshold (v is performed?Jitter impairs the time slicing position?Noise impairs the decision threshold?Jitter combined with

24、Noise Analysis is a better predictor of BER performance!A Quick Look at Jitter and Noise Duality?Jitter analysis evaluates a waveform in the horizontal dimension based on when the waveform crosses ahorizontal reference line.?Jitter decomposition is based on spectral analysis of Time Interval Error v

25、s. timeIndividual jitter components can be separated (i.e. PJ, RJ, DDJ, etc.?TJ can then be estimated at a target BER level38?Noise evaluates along a vertical dimension on the basis ofcrossings of a vertical reference line at some percentage of the unit interval (usually 50%.?Noise decomposition is

26、based on spectral analysis of voltage error vs. time-Individual noise components can be separated (i.e. PN, RN, DDN, etc.-TN can then be estimated at a target BER level抖动和噪声的解析Jitter and Noise Decomposition provide deep insight into BERFull Jitter Analysis vs. Mask Testingstatistical eye closure at

27、any other voltage.?Conventional mask testing considers both time and voltage , but cannot extrapolate eye closure at low BER.Can we combine the best of both?4Statistical Jitter + Noise Analysis?By jointly analyzing Jitter and Noise, behavior at allpoints in the eye can be extrapolated at low BER?The

28、 methodology is analogous to current jitter analysis, but is performed across both dimensions of the eyeJitter and noise are separated into components (Random, Periodic, Data- Dependent,？The components are reassembled into a model that allows accurate extrapolation.42Timing-Induced Jitter?Since jitt

29、er is defined as a shift in an edge ime relative to itsasxpected position, it iseasy to think of jitter as being causedby horizontal (chronological displacement.?Note that the displaced edge (green has not moved vertically in this example.43Noise-Induced Jitter?Consider a burst of voltage noise (rig

30、ht that displaces a waveform vertically.?In this case, the displaced edge (green has not moved horizontally.?The jitter as measured at the chosen reference voltage is identical in these cases!?So, why should we care?Two fundamentally different effects have caused the same amount of jitter, and44Nois

31、e-to-Jitter (AM-to- PM Conversion ?Since waveform transitions are never instantaneous, the slope (slew rate of the edge acts as a gain constant that controls how effectively noise is converted toobserved jitter45Horizontal and Vertical Components of Random Jitter?We can think of RJ as being composed

32、 of two components.?Horizontally induced: RJ(h?Vertically induced: RJ(v?Since these two components are uncorrelated with eachher, they add in theRSS sense:RJ =RJ(h2+RJ(v2?Similarly, PJ can be decomposed into PJ(h and PJ(v based on root cause46Horizontal and Vertical Components of Random Noise?We mea

33、sure noise at a reference point in the bit interval (usually 50%?Ifew rate isn t zero, jitter (horizontal displacement causes observed noise?So as with RJ, RN can be decomposed into components:?Horizontally induced: RN(hVertically induced: RN(vSimilarly, PN can be decomposed into PN(h and PN(v based

34、 on root causeNoise to Jitter and Jitter to Noise ConversionConsider: an ideal “ edge in a patternactually has two impairments:?Jitter(h (see the blue traceand Noise(note that both of Jitter and Noise result in jitter on edgeThe Combined response (bottomright includes the jittercaused bynoiseNon-imp

35、aired bit edgeWe can separate the noisecontribution of jitter for diagnosticpurposes by breaking RJ intoRJ(v and RJ(hDPOJET and 80SJNB are the only tool that will show you this separation, and thus give youan important troubleshooting hint: e.g. is it crosstalk causing trouble, or the clocks?48Theor

36、y: Construction of the BER Eye?Consider a very simple pattern: 7 bit repeating?Overlay multiple segments of the 7bit pattern. Each one has noise and jitter, so although the bit pattern is clear, they follow many slightly different paths:.?This wavefc?Average many pattern repeats together. Everything

37、 that is uncorrelated with the pattern averages out. What remains is called the correlated waveformfully characterizes DDJ, DCD, DDN, ISI -all data dependent effects49Theory: Construction of the BER Eye Part 2?The correlated waveform can be snipped into individual bits and overlaid to form an eye diagram, using the recovered clock as the alignment reference. This forms the correlated eye50Theory: Construction of the BER Eye -Part 3?Spectral jitter separation is used to find PDFs of the random and periodic jitter.?The RJ and PJ PDFs are convolved to find the uncorrelated jitter P