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March 2007Pommerenke, Zhang 1 EMC Consortium University Missouri Rolla

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March 2007Pommerenke, Zhang 2 UMR EMC Consortium Intel Sony LG-Electronics Apple, Altera HuaWei IBM NEC NCR Hitachi Zuken TI 5 faculty 20+ grad. students UMR EMC Consortium - Structure CISCO Focused research areas Methodology results are shared Freescale GTL

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March 2007Pommerenke, Zhang 3 UMR EMC Consortium - Research Classical EMC (Shielding, gaskets, etc.) EMC Expert system EMC test methods IC related EMC (SSN, Current paths, Immunity) Power Distribution - PCB level Power Distribution - IC level Signal Integrity: Link path analysis ESD Application of numerical methods Design of numerical methods Design of test instruments

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March 2007Pommerenke, Zhang 4 Immunity scanning results, identifying sensitive traces and comparing two identical ICs. Performed for ESD like pulses coupling into a PC motherboard. Color indicates sensitivity.

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5 Jitter Jianmin Zhang, David J. Pommerenke

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March 2007Pommerenke, Zhang 6 References Agilent Technologies, numerous application notes, see LeCroy, Tektronix, Bertscope J. Hancock, Jitterunderstanding it, measuring it, eliminating it Part 1-3, From 2004 High Frequency Electronics A. Kuo etc. Jitter models and measurement methods for high-speed serial interconnects S. Tabatabaei etc, Jitter generation and measurement for test of multi-GBPS serial IO Altera Web Site, Le Croy Web site

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March 2007Pommerenke, Zhang 7 Introduction Eye diagram What is jitter? Phase noise, spectrum vs. jitter Why is jitter important? What causes jitter? Jitter fundamentals Jitter components Jitter measurement views Jitter measurement and analysis Real-time jitter analysis Techniques to isolate jitter components Questions and answers Outline

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March 2007Pommerenke, Zhang 8 Eye pattern A very effective method of measuring time distortion thru a data transmission system is based on the eye pattern, displayed on an oscilloscope. The eye pattern is simply the superposition - over one unit interval – of all the Zero–to–One and One–to-Zero transitions, each preceded and followed by various combinations of One and Zero, and also constant One and Zero levels. The data sequence can be generated by a pseudo-random sequence generator (PRSG), which is a digital shift register with feedback connected to produce a max length sequence.

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March 2007Pommerenke, Zhang 9 Process of creating an Eye-diagram

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March 2007Pommerenke, Zhang 10

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March 2007Pommerenke, Zhang 11 Ideal sampling position Timing skewJitter Ideal reference point Voltage offset Voltage Noise and required comparator input

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March 2007Pommerenke, Zhang 12 Logic-Based Measurement E.g., BERT Bit errors caused by jitter Data measured at sampling point has BER Sweeping the sampling point creates bathtub curve 01 Sampling Point BER

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March 2007Pommerenke, Zhang 13 Time BER Time BER Eye diagram Bit error curve as a function of sampling moment How long would it take if we like to get down to 10e-12 Bit Error Rate?

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March 2007Pommerenke, Zhang 14 What Is Jitter? Jitter: The deviation of the significant instances of a signal from their ideal location in time. Or simply, Jitter is how early or late a signal transition is with reference to when it should transition. The significant instances are the transition (crossover) points in a digital signal. Jitter is closely related to phase modulation. P is a wave shape function, e.g., sin or square wave.

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March 2007Pommerenke, Zhang 15 It can be understood as - phase variation (phase noise) - timing variation (time shift of edges) [seconds] mostly used, intuitive system description, independent of data rate. - fractions of the period (unit less) [%] Good for directly seeing how many percent of the eye is open Phase noise can be analyzed from many points of view: What Is Jitter?

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March 2007Pommerenke, Zhang 16 Why Jitter Is Important? Jitter causes transmission errors or in another wording: Jitter limits the transmission speed. Satisfy jitter budget BER (bit error rate) target Identify jitter components diminish/decrease deleterious effects on circuit performance from jitter Jitter limits the ability of A/D converters! Note: Bit errors can also be caused by voltage noise: If the momentary noise voltage exceeds the noise margin, a wrong value can be sampled even if the sampling takes place at the correct moment in time.

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March 2007Pommerenke, Zhang 17 Jitter Tolerance of ADCs at Nyquist frequency (ps rms)

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March 2007Pommerenke, Zhang 18 Jitter applications Signal jitter: Timing of a signal (topic of this presentation) Jitter tolerance: How much jitter, as a function of the jitter frequency, can be tolerated by a system Jitter transfer: How strong, as a function of the jitter frequency, a jitter at an input is transmitted to an output in e.g., by a clock recovery circuit

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March 2007Pommerenke, Zhang 19 Signal jitter: Why Jitter Is Used? Jitter is not a main system property, but Bit Error Rate (BER) is. Why not measuring BER directly? – BER measurement might take hours or days. – BER gives little information about the mechanism that cause errors, but jitter does. Is this error caused by jitter?

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March 2007Pommerenke, Zhang 20 Signal Ideal clock Noisy clock Degraded Bit errors can also be caused by voltage noise: If the momentary noise voltage exceeds the noise margin, a wrong value can be sampled even if the sampling takes place at the correct moment in time.

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March 2007Pommerenke, Zhang 21 What Causes Jitter

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March 2007Pommerenke, Zhang 22 Main types of Jitter Deterministic Jitter (DJ) Random Jitter (RJ) Data Dependent Jitter (DDJ) Inter-symbol Interference (ISI) Duty Cycle Distortion (DCD) Periodic Jitter PJ Data-Correlated Data-Uncorrelated Total Jitter (TJ) Note shown: Sub rate Jitter (SRJ)

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March 2007Pommerenke, Zhang 23 Types of Jitter Jitter can be random or deterministic. In most cases, both types occur. PJ: Periodic Jitter (deterministic). Is a periodic variation in the phase. Causes: External coupling into the circuit, power supply noise, PLL comparator frequency feed-through RJ: Random Jitter Random changes in the phase. It is often assumed to be of Gaussian distribution. Causes: Thermal Noise, Shot Noise Lets look at some examples

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March 2007Pommerenke, Zhang 24 Types of Jitter Jitter can be random or deterministic. In most cases, both types occur. DCD: Duty Cycle Distortion (deterministic). Is the difference in the mean pulse width between positive and negative pulses in a clock. Causes: Amplitude offset, turn-on delay, saturation. ISI: Inter-Symbol Interference (deterministic) Previous signals have not rang down, before new data arrives. Causes: Impulse response is longer than a data bit.

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March 2007Pommerenke, Zhang 25 Phase Modulation - Jitter: The same Phase modulated clock Ideal clock Sine Modulation term f=1 GHz

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March 2007Pommerenke, Zhang 26 How does the Spectra look like? Square wave Phase modulated

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March 2007Pommerenke, Zhang 27 How Does the Eye-Diagram Look Like?

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March 2007Pommerenke, Zhang 28 How Does the Eye-Diagram Look Like? How about using a square wave as phase modulator?

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March 2007Pommerenke, Zhang 29 How Does the Eye-Diagram Look Like? How about using a Gaussian noise as phase modulator?

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March 2007Pommerenke, Zhang 30 Effect of data length: Few hundred bits: Billion bits: What is the consequence for the eye opening, expressed in ps?

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March 2007Pommerenke, Zhang 31 Gaussian noise as modulator

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March 2007Pommerenke, Zhang 32 What is the difference in the histograms?

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March 2007Pommerenke, Zhang 33 Jitter MeasurementBathtub Plot

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March 2007Pommerenke, Zhang 34 TJ Estimation TJ p-p = N x σ rms + DJ If the trigger point is 7 sigma away from the mean event, only 1 in 10e12 crossings will occur even beyond the trigger point.

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March 2007Pommerenke, Zhang 35 Real Zero Crossings vs. Ideal Ones

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March 2007Pommerenke, Zhang 36 Time interval error and jitter trend (integral of time interval error) What does it mean if the jitter trend is continuously increasing over time?

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March 2007Pommerenke, Zhang 37 Histogram in A Square Wave Time interval error and jitter trend (integral of time interval error)

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March 2007Pommerenke, Zhang 38 Channel Characteristics

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March 2007Pommerenke, Zhang 39 Channel Characteristics Loss, reflections, cross talk, added white noise, time variations frequency S21 Frequency Power

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March 2007Pommerenke, Zhang 40 Tx symbol … … In Out Pulse response

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March 2007Pommerenke, Zhang 41 LTI property: Superposition of symbols Tx symbol … … In Out Response to pattern

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March 2007Pommerenke, Zhang 42 Channel loss effect: Eye closure and DCD (single ended)

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March 2007Pommerenke, Zhang 43 Eye of a loss dominated differential channel

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March 2007Pommerenke, Zhang 44 Effect of reflections: Duty cycle distortion

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March 2007Pommerenke, Zhang 45 What Can You Tell? Which jitter components do you see? What might cause them?

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March 2007Pommerenke, Zhang 46 Summarize Jitter Components

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March 2007Pommerenke, Zhang 47 Jitter Measurement and Analysis

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March 2007Pommerenke, Zhang 48 Sequence of Jitter analysis Step 1: Measure - Real time scope - Equivalent time scope - Spectrum Analyzer Step 2: Separate jitter components - Average - Change data pattern (PRBS to ) - TIE - etc.

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March 2007Pommerenke, Zhang 49 Instruments to measure jitter Real time oscilloscope: Usually 8 bit, absolute clock, clock re-generation possible, bandwidth up to 20 GHz Sampling oscilloscope: bit, relative to clock, bandwidth up to 1000 GHz Spectrum Analyzer: No time resolution (real time spectrum analyzers are an exception), very good dynamic range. Bandwidth up to 1000 GHz.

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March 2007Pommerenke, Zhang 50 Real Time Jitter Analysis Timing Measurements: – Data: Time Interval Error (TIE), also called phase jitter. – Clock: Period, Cycle-to-cycle. Views: – Eye diagrams (repetitive volts vs. time) – Trend (time error vs. time) – Histograms (hits vs. time error) – Spectrum (time error vs. frequency) – Phase noise – Bathtub curves (BER vs. eye opening)

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March 2007Pommerenke, Zhang 51 Isolate Jitter Components for BER Estimation Probability density Function (PDF) of jittering edge timing As mentioned on slide 11, RJ measurements must be decomposed from DJ components for total jitter estimation. Separating jitter components individually to diagnose root causes of jitters for further reducing TJ to meet jitter budget in systems. TJ p-p = N x σ rms + DJ

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March 2007Pommerenke, Zhang 52 Advanced Instruments for Jitter Analysis Advanced instruments for jitter analysis such as DCA-J (Digital Communication Analyzer-Jitter ) and BERT (Bit Error Ratio Tester) support: Decomposition of jitter into Total Jitter (TJ), Random Jitter (RJ), Deterministic Jitter (DJ), Periodic Jitter (PJ), Data Dependent Jitter (DDJ), Duty Cycle Distortion (DCD), and Jitter induced by Inter symbol Interference (ISI) Jitter frequency spectrum

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March 2007Pommerenke, Zhang 53 If always the same edge of a bit pattern is observed, then all DDJ is removed. Only non-correlated jitter, RJ and non correlated PJ remains. If averaging is performed over the pattern (not the bits), then all non-correlated jitter: RJ and PJ is removed.

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March 2007Pommerenke, Zhang 54 Jitter Separation: Histogram is the convolution of RJ and PJ Single Edge – Histogram for RJ PJ – RJ, PJ causes σ – DDJ causes Separate RJ, PJ by de-convolution Histogram Jitter Histogram Jitter Histogram Jitter Ideal RJPJ

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March 2007Pommerenke, Zhang 55 An entire picture from a Jitter Analyzer (Agilent)

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March 2007Pommerenke, Zhang 56

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March 2007Pommerenke, Zhang 57 A View of Real-Time Jitter Measurement

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March 2007Pommerenke, Zhang 58 Inter-Symbol Interference (ISI): Caused by loss

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March 2007Pommerenke, Zhang 59 Inter-Symbol Interference (ISI): Caused by reflection

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March 2007Pommerenke, Zhang 60 Spread Spectrum Clock Measurement

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March 2007Pommerenke, Zhang 61 Uncorrelated Periodic Jitter Coupling

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March 2007Pommerenke, Zhang 62 How about A Gaussian Noise Modulating the Phase? Bounded and non-bounded jitter?

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March 2007Pommerenke, Zhang 63 Bounded Uncorrelated Jitter (BUJ) Corrupter Threshold TIE trend

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March 2007Pommerenke, Zhang 64 Duty-Cycle Distortion (DCD)

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March 2007Pommerenke, Zhang 65 Inter-Symbol Interference (ISI)

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March 2007Pommerenke, Zhang 66 Inter-Symbol Interference (ISI)

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March 2007Pommerenke, Zhang 67 Uncorrelated Periodic Jitter Coupling

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March 2007Pommerenke, Zhang 68 Overview of Jitter in Systems

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March 2007Pommerenke, Zhang 69 Duty-Cycle Distortion (DCD)

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March 2007Pommerenke, Zhang 70 Duty-Cycle Distortion (DCD)

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