Advanced Jitter Analysis with Real-Time Oscilloscopes

Transcription

1 with Real-Time Oscilloscopes August 10, 2016 Min-Jie Chong Product Manager

2 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 2

3 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 3

4 Jitter and Time Interval Error (TIE) On an oscilloscope we monitor the waveform transitions and note the jitter at each transition point. This is called the Time Interval Error (TIE) record. Slide 4

5 Jitter Components Total Jitter (TJ) Bounded UnBounded Deterministic Jitter (DJ) Random Jitter (RJ) Correlated with Data (DDJ) Uncorrelated with Data (BUJ) DutyCycle Distortion (DCD) Tr, Tf D InterSymbol Interference (ISI) Settling Time Reflections Non Flat Freq Response Non Periodic (ABUJ) Crosstalk Non Linear Clock Recovery One-Time Event Acronyms: DDJ: Data Dependent Jitter BUJ: Bounded Uncorrelated Jitter Periodic Jitter (PJ) Clocks Crosstalk Gaussians (s, RJ RMS ) Thermal ABUJ: Aperiodic Bounded Uncorrected Jitter Slide 5 Shot 1/f Burst

6 Jitter Decomposition Overview Waveform Acquisition Clock Reference Evaluate TIE Complete TIE Record DDJ Analysis DDJ: TIE per Bit RJ Extraction RJ/PJ TIE Record Dual Dirac Analysis Reported Values of TJ, RJ, DJ DD Slide 6

7 Jitter Decomposition with Dual Dirac Assumption Fit the RJ Gaussian curve to both tails of the TIE histogram or Jitter Probability Density Function (PDF) DJ DD s L Total Jitter pp (BER) = DJ DD + ns n = f(target BER) For instance for BER = n ~ 14 s = RJrms The jitter that composes DJ DD comes from the deterministic components 7s for BER. s R L R Slide 7

8 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 8

9 Instruments for Jitter Analysis Descriptions Real-Time Scope Sampling Scope Bit Error Ratio Tester (BERT) Measurement Estimates jitter Estimates jitter Measures jitter Analysis mode Clock reference Based on Dual-Dirac Model Software clock recovery and accepts explicit clock Based on Dual-Dirac Model Hardware clock recovery Accumulate and compare 3 times the BER level bits for 95% confidence level E.g. 3x10 12 bits are received without error to meet the BER target Hardware clock recovery Speed Fast (Seconds) Fast (Seconds) Slow (Minutes or Hours) - depends on time to accumulate the bits Report jitter components Yes Yes No Slide 9

10 Jitter Components Reported by Scope and the Caveats TJ pp (BER) Bounded UnBounded DJ DD RJ rms DDJ pp BUJ DD DCD mean ISI PJ dd pp ABUJ rms PJ rms DDPWS mean F/2 mean (Even/Odd) Caveats of jitter decomposition: 1. Jitter decomposition does not follow a linear bottom-up flow. 2. Algorithm is based on Dual-Dirac model and is an approximation, not the exact value. 3. Each jitter component may have a different unit value (rms, dual-dirac (dd), peak-to-peak (pp), mean, etc.) 4. Not every component has a result. Some are convolved with other components and not separable. E.g. DJ pp, PJ pp, ABUJ dd and ABUJ pp (crosstalk) are not separable and reported. 5. Other jitter components can be calculated separately from the jitter decomposition algorithm (in purple). Slide 10

11 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 11

12 time error Spectral vs. Tail Fit Jitter Decomposition Random Jitter (RJ) Extraction Methods RJ Extraction Methods Rationale Spectral Speed/Consistency to Past Measurements Accuracy in low Crosstalk or Aperiodic Bounded Uncorrelated Jitter (ABUJ) conditions 0 0 likely to contain PJ PJ threshold freq Tail Fit General Purpose Accuracy in high Crosstalk or ABUJ conditions Histogram Object Slide 12

13 time error time error Spectral Method PJ Threshold Measurement Details likely to contain PJ Periodic Jitter (PJ) threshold is chosen by experimentation. PJ threshold 0 0 freq likely to contain PJ Integrate PSD to derive RJ RMS Sum the PJ components for PJ RMS PJ threshold 0 0 freq Slide 13

14 Spectral Method PJ Threshold Handling of Different RJ, PJ Spectral Content Separation occurs as described Is it RJ or PJ? What do you do in this case? Slide 14

15 Spectral Method PJ Threshold Non-linear Period Jitter (PJ) threshold can help Wide RJ Bandwidth Analysis Narrow RJ Bandwidth Analysis Non-Linear PJ Threshold Linear and Flat PJ Threshold RJ RMS = 1.06ps PJ DD = 93.17ps RJ RMS = 9.66ps PJ DD = 27.18ps Which PJ Threshold or RJ bandwidth analysis do you choose? Slide 15

16 Spectral Method Wide vs. Narrow Wide RJ Bandwidth Narrow RJ Bandwidth Wide RJ under reports TJ Narrow RJ more accurately reports TJ Smoothness of slope continuity between measured and extrapolated result on the bathtub plot indicates the better PJ threshold (RJ bandwidth) method. Slide 16

17 Spectral Method with Presence of Crosstalk or ABUJ (ABUJ = Aperiodic Bounded Uncorrelated Jitter) Something is wrong here... Using the slope continuity concept we expect the extrapolated curve to look like this. The RJ/PJ spectral extraction does not deal with Crosstalk or ABUJ well. The RJ is overestimated severely. Slide 17

18 ABUJ: Crosstalk or Ground Bounce Amplitude interference uncorrelated with data and not periodic in nature. Victim Gaussian Slope continuity Aggressor Dv No crosstalk Bathtub and RJ,PJ Histogram Victim Out Slope discontinuity Non-Gaussian Dt Dt = Dv/Slope victim With crosstalk Bathtub and RJ,PJ Histogram Slide 18

19 Tail Fit Method Gaussian Extraction Measurement Detail Histogram and Gaussian fit to right tail 1. Fit a Gaussian characteristic to the right and left extremes of the RJ/PJ histogram distribution jitter, ps 2. Actual data is never smooth Find low probability event (crosstalk) at the end of tail that does not fit Gaussian characteristics. Histogram Fits. True RJrms = 2, PJmax = Slide 19

20 What Makes Tail Fit Hard Measurement Detail Histogram Object High Precision Low accuracy Fit Window DJ end Noisy data Low Precision High accuracy Hard to detect Crosstalk events out in the tail. Might take longer time for Tail Fit results to converge Curve fit error Slide 20

21 RJ Extraction with Presence of Crosstalk (ABUJ) Spectral vs. Tail Fit Extraction No Crosstalk With Crosstalk Spectral Extraction Spectral Extraction Slope discontinuity. Over reports RJ. Tail Fit Extraction Tail Fit Extraction Analyze the bathtub plot with both RJ extraction modes to explore the presence of crosstalk or ground bounce. Slide 21

22 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 22

23 Jitter Analysis with Crosstalk Removal Tool Crosstalk Identification Which signals are coupling onto your victim? Crosstalk Quantification How much error and jitter do each aggressor add to your victim? Crosstalk Removal for Jitter Analysis What would your signal look without crosstalk present on victim? How much jitter margin can be recovered without crosstalk? If the signal was failing the jitter spec, can it pass without crosstalk? Assist in making important design decisions: Is it worth reducing crosstalk impact in design? Where to improve? Slide 23

24 Remove Crosstalk from Victim Signal Power supply aggressor signal Original serial data victim signal Serial data victim signal with crosstalk removed Eye diagram with crosstalk Eye diagram without crosstalk Slide 24

25 Features of the N8833A Crosstalk Analysis Application 1. Analyze up to four signals (victim or aggressor) at once. 2. Remove Near-End Crosstalk (NEXT), Far- End Crosstalk (FEXT) and Power Supply Crosstalk from Victim signal. 3. Plot waveform without crosstalk on the scope which can be: Used for eye diagram, jitter decomposition, de-embedding, equalization and mask test Saved as a waveform file Slide 25

26 Crosstalk Analysis Setup 1. Probe up to 4 signals (Aggressors or victims). No simulation models or inputs are required. 2. Setup the victim signal. 4. The app reports the amount of crosstalk from each aggressors and return a waveform without crosstalk for analysis. 3. Set the number of aggressors and configure the aggressor type. Slide 26

27 Power Supply Crosstalk on Victim No Power Supply Aggressor With Power Supply Aggressor on the Transmitter PLL Noise and Data TIE correlates Clean Power Supply Noise Power Supply TIE Trend TIE Trend Slide 27

28 Removing Power Supply Crosstalk from Victim With Power Supply Crosstalk on the Transmitter PLL Power Supply Crosstalk Removed with Improvement on Data TIE Trend TIE Trend with glitch removed Noise Power Supply Noise Power Supply TIE Trend TIE Trend Slide 28

29 Removing Power Supply Crosstalk from Victim Measured Victim Without Crosstalk Measured Victim with Power Supply Crosstalk Eye diagram correlates Victim after Power Supply Crosstalk removed Slide 29

30 Jitter Improvement Without Power Supply Crosstalk Compare jitter results before and after crosstalk removal. Jitter with Crosstalk TJ = 158ps PJdd = 58ps DJdd = 68ps Jitter without Crosstalk An Improvement of 20% to Total Jitter without Crosstalk. TJ = 124ps PJdd = 27ps DJdd = 33ps Slide 30

31 Analyze Crosstalk & Noise that Contributes to Jitter The crosstalk application can remove the ideal, ISI and return Unknown Crosstalk + Noise (residual) content. Perform further analysis on this residual waveform with measurements such as FFT, markers, etc. to root cause the source of aggressor. Remove: Ideal + ISI of Victim Show: Only Unknown Crosstalk + Noise Measured waveform = Ideal + ISI + Unknown XT + Noise 150 MHz clock coupled into the serial data signal inside the package Simulated waveform = Unknown XT + Noise FFT on Unknown Crosstalk + Noise Slide 31

32 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 32

33 Influence of Scope Noise to Jitter Performance Random jitter will vary with slew rates. 1. Every scope has intrinsic vertical noise floor. This vertical noise can translate into horizontal jitter. 2. As signal slew rate decreases, vertical noise increases the random jitter. 3. Measured random jitter is a function of signal slew rate, scope noise and scope sample clock jitter. Slide 33

34 Scope Random Jitter Removal Calibrate and remove scope random jitter contribution Scope RJ calibration is available to remove the contribution of scope noise to measured RJ. User is asked to disconnect the signal from Channel to measure the ACV rms noise for the current Vertical setting. Slide 34

35 Other Jitter Measurement Considerations Gain Margin by removal of Scope contribution to RJ Signal with Fast Rise Time DUT TX With no Scope RJ removal DUT TX Lossy Channel After Scope RJ removal Gain margin through scope RJ removal. Slide 35

36 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method Jitter Analysis with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 36

37 Jitter Analysis with BER Eye Contour Estimate Jitter and Eye Opening to various BER level Specify the BER eye contours you want the scope to plot. Specify which BER contour to highlight in red. BER Eye Contours Eye Contour at BER Slide 37

38 BER Eye Contour matches Jitter Decomposition Results matches at various threshold settings 0 V Jitter at 0 V threshold The BER eye contour width agrees with TJ BER result at ~41ps at 0V threshold. 100 mv Jitter at 100 mv threshold The BER eye contour width agrees with TJ BER result at ~53ps at 100mV threshold. Slide 38

39 Analyze Jitter at Various Test Points Jitter Analysis with De-embedding and Equalization + Tx - Txp Txn Connector Channel Connector Rxp Rxn + Rx - EQ After Scope De-embedding to the TX point Measurement Node After Scope Equalization Slide 39

40 Agenda Review of Jitter Decomposition Assumptions and Limitations Spectral vs. Tail Fit Method Jitter Analysis with Crosstalk Removal Tool Scope Random Jitter Removal from Jitter Analysis Other Tools to Consider for Jitter Analysis Summary Slide 40

41 Keysight Real-Time Scope Jitter Analysis Tools N5400A EZJIT Plus for Jitter Analysis and RJ Scope Removal Calibration N8823A EZJIT Complete for Vertical Noise Analysis E2688A High-Speed SDA for Reference Clock Recovery and Eye Analysis N8827A PAM-4 Clock Recovery N8833A Crosstalk Analysis and Removal Application N5461A Serial Data Equalization Software N5465A InfiniiSim De-embedding Software BER Eye Contour Comes standard with E2688A and N8823A Slide 41

42 time error Jitter Analysis Summary 1.2 Histogram Object likely to contain PJ PJ threshold 0.2 Dual Dirac Model for Jitter Decomposition 0 0 Spectral vs. Tail Fit for ABUJ (Crosstalk) Jitter Analysis freq Slope discontinuity Use Smoothness of Slop Continuity on the Bathtub Curve Use Crosstalk Removal Tool to Recover Jitter Margins Scope Random Jitter Removal BER Eye Contour, De-embedding and Equalization for Jitter Analysis Slide 42