march 2007pommerenke, zhang 1 emc consortium university missouri rolla
TRANSCRIPT
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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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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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
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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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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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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)