March 2007Pommerenke, Zhang 1 EMC Consortium University Missouri Rolla

March 2007

Pommerenke, Zhang 1

EMC Consortium

University Missouri Rolla

March 2007

Pommerenke, Zhang 2

UMR EMC Consortium

IntelSony

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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Pommerenke, 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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Pommerenke, 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.

5

UUMMRR EEMMCC LLaabboorraattoorryy

Jitter

Jianmin Zhang, David J. Pommerenke

March 2007

Pommerenke, Zhang 6

References

● Agilent Technologies, numerous application notes, see http://www.home.agilent.com/agilent/facet.jspx?to=80030.k.1&co=153297.i.1&cc=US&lc=eng&sm=g&k=jitter

● LeCroy, Tektronix, Bertscope● J. Hancock, “Jitter—understanding 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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Pommerenke, 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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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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Process of creating an Eye-diagram

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Ideal sampling position Timing skew Jitter

Ideal reference point

Voltage offset

Voltage Noise and required comparatorinput

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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

0 1

Sampling Point

BER

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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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Pommerenke, 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.

S(t) = P 2π f t + φ(t)d

Jitter is closely related to phase modulation.

P is a wave shape function, e.g., sin or square wave.

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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

1

2 d

tf

( )t

t

T

Phase noise can be analyzed from many points of view:

What Is Jitter?

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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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Jitter Tolerance of ADCs at Nyquist frequency (ps rms)

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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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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.

Transmittedwaveform

Receivedwaveform

Interpretedwaveform

Is this error caused by jitter?

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Signal

Ideal clock 1101001

Noisy clock 1101101

Degraded 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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What Causes Jitter

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Main types of Jitter

DeterministicJitter (DJ)

RandomJitter (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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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

Let’s look at some examples

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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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Phase Modulation - Jitter: The same

[sin(2 )]P ft

[0.04 sin(0.2 ) / 3]P ft

Phase modulated clock

Ideal clock

Sine Modulation term

[sin(2 ) 0.04

sin(0.2 ) / 3]

P ft

ft

f=1 GHz

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How does the Spectra look like?

Square wave

Phase modulated

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

0

10

20

30

40

50

60

70

80

90

Frequency (GHz)

0.6 0.8 1 1.2 1.4 1.6

0

20

40

60

80

Frequency (GHz)

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How Does the Eye-Diagram Look Like?

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How Does the Eye-Diagram Look Like?

How about using a square wave as phase modulator?

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How Does the Eye-Diagram Look Like?

How about using a Gaussian noise as phase modulator?

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Effect of data length:

Few hundred bits:

Billion bits:

What is the consequence for the eye opening, expressed in ps?

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Gaussian noise as modulator

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What is the difference in the histograms?

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Jitter Measurement—Bathtub Plot

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TJ Estimation

TJp-p = N x σrms + 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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Real Zero Crossings vs. Ideal Ones

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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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Histogram in A Square Wave Time interval error and jitter trend (integral of time interval error)

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Channel Characteristics

Frequency dependent

lossy channel

Discontinuities within the channel

Mismatched terminations

of the channel

Bandwidth limitation

Multi-reflections

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Channel Characteristics

Loss, reflections, cross talk, added white noise, time variations

frequency

S21

Frequency

Power

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Tx symbol

…000010000000…

In Out

Pulse response

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LTI property: Superposition of symbols

Tx symbol

…000010011100…

In Out

Response to pattern 100111

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Channel loss effect: Eye closure and DCD (single ended)

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Eye of a loss dominated differential channel

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Effect of reflections: Duty cycle distortion

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What Can You Tell?

Which jitter componentsdo you see?

What might cause them?

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Summarize Jitter Components

(TJ)Total Jitter

(DJ)Deterministic Jitter

(RJ)Random Jitter

(BUJ)Bounded

Uncorrelated Jitter

(DDJ)Data-Dependent

Jitter

(PJ)Periodic

Jitter

(ISI)Inter-Symbol Interference

(DCD)Duty-Cycle Distortion

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Jitter Measurement and Analysis

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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 01010101)

- TIE

- etc.

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Instruments to measure jitter

Real time oscilloscope:

Usually 8 bit, absolute clock, clock re-generation possible, bandwidth up to 20 GHz

Sampling oscilloscope:

11-14 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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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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Gaussian RJ RJ convoluted in DDJRF convoluted in PJ

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.

TJp-p = N x σrms + DJ

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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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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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Jitter Separation: Histogram is the convolution of RJ and PJ

● Single Edge– Histogram for RJPJ– RJ, PJ causes σ– DDJ causes

● Separate RJ, PJ by de-convolution0tt

His

togr

am

Jitter

His

togr

am

Jitter

His

togr

am

Jitter

→

Idea

l

t0t

0tt

RJ PJ

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An entire picture from a Jitter Analyzer (Agilent)

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A View of Real-Time Jitter Measurement

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Inter-Symbol Interference (ISI): Caused by loss

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Inter-Symbol Interference (ISI): Caused by reflection

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Spread Spectrum Clock Measurement

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Uncorrelated Periodic Jitter Coupling

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How about A Gaussian Noise Modulating the Phase?

Bounded and non-bounded jitter?

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Bounded Uncorrelated Jitter (BUJ)

Corrupter

Threshold

TIE trend

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Duty-Cycle Distortion (DCD)

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Inter-Symbol Interference (ISI)

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Inter-Symbol Interference (ISI)

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Uncorrelated Periodic Jitter Coupling

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Overview of Jitter in Systems

Transmitter ReceiverSignal linkpath

· Lossy mediaà ISI· Discontinuitiesà ISI· Crosstalkà BUJ

· Device noise (Shot noise, Thermal noise, and pink noise)à RJ

· Power supply noiseà (RJ, PJ)· Duty-Cycle Distortionà DCD· On-chip couplingà BUJ

· Device noise (Shot noise, Thermal noise, and pink noise)à RJ

· Power supply noiseà (RJ, PJ)· Duty-Cycle Distortionà DCD· On-chip couplingà BUJ· Mismatch terminationsà ISI

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Duty-Cycle Distortion (DCD)

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Duty-Cycle Distortion (DCD)