抖動和眼圖的視覺分析

1、抖動和眼圖的視覺化分析什么是抖動？TIE為信號相對于標準時鐘或者標準信號的定時誤差TIE在高速數(shù)字系統(tǒng)中即為抖動 抖動為實際數(shù)據(jù)與其理想位置的時間偏差0.0ns 0.990ns 2.000ns 2.980ns 4.000ns什么是眼圖？眼圖是怎么形成的?1 / 392 / 39Random Jtter幗機抖動碰機抖動符合高斯型分布2ft方圖（估計？ pdf（數(shù)學(xué)模型 洲動峰峰值=無窮 大無界！枕部熱能現(xiàn)象？Flicker Noise, Shot Noise熱能的原子與分子振動 份子的 解體力卜部的宇宙射線3 / 39Deterministic Jtter（確定性抖動？確定性抖動是非高斯分布并且

2、有界Periodic Jitter（周期性抖動？TIE隨時間的變化是重復(fù)的、周期性的 ？Periodic jitter和相位調(diào)制（PM是等效的？系統(tǒng)時鐘（抖動頻率在 MHz量級）川關(guān)電源（抖 動頻率在KHz量級）Duty Cycle distortion（占空比失真?±升時間和下降時間不對稱域者測試時參考電平選擇不當0.0vInter-Symbol Interference問干擾抖動?DDJ或PDJ嗷據(jù)相關(guān)性抖動或碼型相關(guān)性抖動，和ISI的術(shù)語是等價的.4 / 39劑型是如何影響隨后的比特位的?由于傳輸鏈路的效應(yīng)、反射等5 / 39換個角度看抖動，時域看看我們有了什么視角?6 / 3

3、97/7/20167 / 3911抖動視覺化出寸間趨勢圖2ft方圖告訴了我們分布，但是只有統(tǒng)計特性，缺少了時間信息？寸間趨勢圖可以直觀告訴我們波形里是否有特定頻率的調(diào)制”圖為5個周期SSC 30khz抖動視覺化8 / 399 / 39Gaussian Random Noise Sinusoidal Jitter7/7/2016127/7/201613抖動視覺化須譜圖陽頻域上觀測抖動10 / 39抖動中決定性的頻率成分會在譜線上明顯超出噪底哪個眼圖好？哪個直方圖好？視覺化眼圖和抖動的問題？11 / 3912 / 39浴盆曲線誤碼率是關(guān)鍵vs. UI張開程度-“BEF?For a given po

4、sition in the time there' s a given probability of errorError Ratio?1 UI基于示波器分析的浴盆曲線Rj 6 6 /Djf6Tj BERAssume bi-modal distribution (dual-Dirac, measure Tj at two BER13 / 39抖動類型分析抖動分離為誤碼產(chǎn)生的根本原因提供了更精確的定位和分析方法洲動分析方法，參照T11 MJSQ,已經(jīng)被工業(yè)界廣泛接受 Constituent Components of Jitter=Unbounded=Bounded7/7/201618J

5、itter Visualization -Bathtub Plot?Rote the eye closure of System 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 II14 / 39Tektronix -

6、Innovators of Jitter Analysis?1998First RealTime Scope Based Jitter 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 (Spectr

7、al jitter methods?2005Invented measurements with Jitter and Noise reconciliation?2011-First scope vendor with BUJ support?2015-RT Noise Analysis and Sampling BER and PDF Mask Testing抖動和眼圖的視覺化眼圖怎么切割的？時鐘決定!15 / 3916 / 39TIE抖動需要參考時鐘?參考時鐘提取的過程就是時鐘恢復(fù)修考時鐘有幾種確定的方式：Constant Clock with Minimum Mean Squared E

8、rrorThis is the mathematically“ideal " clockBut, only applicable when post-processing 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 PLL17 / 39Tracks low-frequency jitter (e.g

9、. clock driftModels “ real world " clock recovery circuits very wellExplicit ClockThe clock is not recovered, 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

10、 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 slope7/7/201622JTF vs PLL Loop Bandwidth18 / 39?Co

11、nfiguring the correct PLL settings is key to correctmeasurements?Most standards have a reference/defined CR setup?zor example, USB 3.0 uses a Type II with JTF of 4.9Mhz?Type I PLL?Type I PLL has 20dB of roll off per decade?JTF and PLL Loop Bandwidth are Equal?Type 2 PLL?Type II PLL has 40dB of roll

12、off per decade?JTF and PLL Loop Bandwidth are not Equal?zor example, USB 3.0 uses a Type 2 PLL with a JTF of 4.9Mhz.The corresponding 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 Tra

13、nsfer Function19 / 39(JTFA: Constant Clock Recovery B: PLL Clock Recovery Ratio of B/A 7/7/201624JTF Filtering Effects based on different PLL bandwidths20 / 397/7/201627f 3dB = 30 kHzf 3dB = 300 kHzf 3dB = 3 MHzJitter for Busy PeopleHints, Tips and Common ErrorsUsing the Jitter Analysis Tools?Issues

14、 manifested in different layers of theprotocol stack?Crosstalk, jitter, reflections, skew21 / 39?Disparity, encoding or CRC errors?Where do I start debugging?Jitter and Eye Diagram Tools?Oscilloscope-based for quick results?zast jitter measurements with? ' One Button ' Jitter Wizard?Compare

15、timing, jitter, eye, amplitude measurements?User-definable clock recovery, filters, pass/fail limits, and reference levelsMore Hints for Successful Jitter Analysis?Clock Recovery hasa great deal of influence on jitter results. Think about what you' re trying to accomplish.?Constant-Clock is the

16、most “ unbiased ”O(jiān)ften best if you ' re trying to s-iewveryquency effectsBut it can also show wander in the scope' s timebase?°LL recovery can model what a real data receiver will seeIt can track and remove lowfrequency effects, allowing you to“see through “ to thejitter that really con

17、tributes to eye closure?Explicit-Clock is appropriate if your design uses a forwarded clock22 / 39Make sure your probes are deskewedHints for looking at Spread-Spectrum Clock?If you want to see the SSC effects, use TIE and PLL clock recovery with a bandwidth of at least 1 MHz. A Type-II (2nd -order

18、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 of 200 kHzBecause Period trends accentuate high frequency noise, the low-frequency SSCtrend will be obscured if

19、 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 is a mathematical fact, not a DPOJET defect.?f you use a TIE measuremeit, you ' ll see modulat

20、ion 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 wave.誤碼率與噪聲分析Anatomy of a Serial Data Link23 / 39Complete LinkChannelAspirational goal:

21、 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 directly?Why not use ONLY BERTs for Serial Data Link Analysis?Difficult to mo

22、del/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 advanced analysis tools?Easily move from Compliance to Debug?Better equipped t

23、o identify root causes of eye closure?Equalizer can easily be modeled?More cost effective?Faster throughput24 / 39Why Measure Jitter and Noise?Link Model: Transmitter + Channel + Receiver?Transmitter generates a stream of symbols?Receiver uses a slicer to make a decision on the transmitted symbol?Th

24、e 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 Noise Analysis is a better predictor of BER performance!A Quick Loo

25、k 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 vs. timeIndividual jitter components can be separated (i.e. PJ, RJ,

26、DDJ, etc.?TJ can then be estimated at a target BER level25 / 3938券oise 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 based on spectral analysis of voltage error vs. time-Individu

27、al noise components can be separated (i.e. PN, RN, DDN, etc.-TN can then be estimated at a target BER level抖動和噪聲的解析?Jitter and Noise Decomposition provide deep insight into BER26 / 39Full Jitter Analysis vs. Mask Testing27 / 39statistical eye closure at any other voltage.?Conventional mask testing c

28、onsiders 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 BER28 / 39?The methodology is analogous to current ji

29、tter 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 jitter is defined as a shift in an edge ime

30、 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 Jitter29 / 39?Consider a burst of voltage noise (right that displaces a waveform ver

31、tically.?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, and44Noise-to-Jitter (AM-to- PM Conversio

32、n ?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 to"observed jitter ” .30 / 3945Horizontal and Vertical Components of Random Jitter?We can think of RJ as being composed of two compone

33、nts.?Horizontally induced: RJ(h?Vertically induced: RJ(v?Since these two components are uncorrelated with eachher, they add in the31 / 39RSS 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 measure noi

34、se 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(h?Vertically induced: RN(v32 / 39Similarly, PN can be decomposed into PN(h and PN(v

35、 based 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 by

36、noiseNon-impaired 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 you33 / 39an important troubleshooting hint: e.g. is it crosstalk causing trouble, or

37、 the clocks?48Theory: 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 repe

38、ats together. Everything that is uncorrelated with the pattern averages out. What remains is called the' correlated waveformfully characterizes DDJ, DCD, DDN, ISI -all data dependent effects34 / 3949Theory: Construction of the BER Eye Part 2?The correlated waveform can be snipped into individual

39、 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 PDFred ?A similar analysis of the noise yields the uncorrelated noise PDF (blue ?Ca