1. University Missouri Rolla
EMC Consortium
University Missouri Rolla
March 2007

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

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
March 2007

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

5. Jianmin Zhang, David J. Pommerenke
Jitter
Jianmin Zhang, David J. Pommerenke

6. References
Agilent Technologies, numerous application notes, see
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
March 2007

7. Outline Introduction Jitter fundamentals
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
March 2007

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.
March 2007

9. Process of creating an Eye-diagram
March 2007

10. March 2007

11. Ideal sampling position Timing skew Jitter
Voltage Noise and
required comparator
input
Voltage offset
Ideal reference point
March 2007

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 1
BER
Sampling Point
March 2007

13. Bit error curve as a function of sampling moment
Eye diagram
Time
BER
Time
BER
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?
March 2007

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.
March 2007

15. What Is Jitter? Phase noise can be analyzed from many points of view:
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
March 2007

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.
March 2007

17. Jitter Tolerance of ADCs at Nyquist frequency (ps rms)
March 2007

18. Jitter applications Signal jitter: Timing of a
signal (topic of this presentation)
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
Jitter tolerance: How much jitter, as a function of the jitter frequency, can be tolerated by a system
March 2007

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?
March 2007

20. 1101001 1101101 Signal Ideal clock Degraded Noisy clock
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.
March 2007

21. What Causes Jitter
March 2007

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

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
March 2007
Let’s look at some examples

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.
March 2007

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

26. How does the Spectra look like?
Square wave
Phase modulated
March 2007

27. How Does the Eye-Diagram Look Like?
March 2007

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

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

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

31. Gaussian noise as modulator
March 2007

32. What is the difference in the histograms?
March 2007

33. Jitter Measurement—Bathtub Plot
March 2007

34. 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.
March 2007

35. Real Zero Crossings vs. Ideal Ones
March 2007

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?
March 2007

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

38. Channel Characteristics
March 2007

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

40. In
Out
Pulse response
Tx symbol
… …
March 2007

41. LTI property: Superposition of symbolsIn
Out
Tx symbol
… …
Response to pattern
March 2007

42. Channel loss effect: Eye closure and DCD (single ended)
March 2007

43. Eye of a loss dominated differential channel
March 2007

44. Effect of reflections: Duty cycle distortion
March 2007

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

46. Summarize Jitter Components
March 2007

47. Jitter Measurement and Analysis
March 2007

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.
March 2007

49. 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.
March 2007

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)
March 2007

51. TJp-p = N x σrms + DJ Isolate Jitter Components for BER Estimation
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.
Probability density Function (PDF) of jittering edge timing
March 2007

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
March 2007

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.
March 2007

54. →  Jitter Separation: Histogram is the convolution of RJ and PJ
Ideal
Single Edge
Histogram for RJPJ
RJ, PJ causes σ
DDJ causes
Separate RJ, PJ by de-convolution
Histogram
Jitter
Histogram
Jitter
Histogram
Jitter RJ PJ → 
March 2007

55. An entire picture from a Jitter Analyzer (Agilent)
March 2007

56. Thank You!
&
Questions?
March 2007

57. A View of Real-Time Jitter Measurement
March 2007

58. Inter-Symbol Interference (ISI): Caused by loss
March 2007

59. Inter-Symbol Interference (ISI): Caused by reflection
March 2007

60. Spread Spectrum Clock Measurement
March 2007

61. Uncorrelated Periodic Jitter Coupling
March 2007

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

63. Bounded Uncorrelated Jitter (BUJ)
Corrupter
Threshold
TIE trend
March 2007

64. Duty-Cycle Distortion (DCD)
March 2007

65. Inter-Symbol Interference (ISI)
March 2007

66. Inter-Symbol Interference (ISI)
March 2007

67. Uncorrelated Periodic Jitter Coupling
March 2007

68. Overview of Jitter in Systems
March 2007

69. Duty-Cycle Distortion (DCD)
March 2007

70. Duty-Cycle Distortion (DCD)
March 2007