Transmitter Jitter Basics: The World of Test

1
Transmitter Jitter Basics The World of Test
PC Bus
Steve Sekel Director of Product
Management SyntheSys Research, Inc
2
Agenda

• So Whats the Problem?
• Jitter Eye Closure Review
• Side Notes
• The Effect of Timing Reference
• Measurement Depth
• Common Methods
• Eye Diagram Jitter
• Mask Testing
• Bathtub/ BERTScan/ Jitter Peak
• Deep Mask Testing
• Jitter Spectrum
• Summary - Tying it All Together

PC Bus
3
Agenda

• So Whats the Problem?
• Jitter Eye Closure Review
• Side Notes
• The Effect of Timing Reference
• Measurement Depth
• Common Methods
• Eye Diagram Jitter
• Mask Testing
• Bathtub/ BERTScan/ Jitter Peak
• Deep Mask Testing
• Jitter Spectrum
• Summary -Tying it All Together

PC Bus
4
Overview

• Serial Data Systems share common design
  challenges, regardless of the transport standard
• PCI-Express
• SATA
• 10GbE
• FibreChannel
• OTN
• .
• Vendor Independent Interoperability is assure
  with compliance standards
• Much of these specifications focuses in two
  areas
• Transmitter (Tx)
• Receiver (Rx)

5
Whats the Problem?

• Communications Systems
• Its all about the Error Performance
• Systems typically specified to work with 1x10-12
  BER or better
• Waveform edges must not interfere with the
  receiver decision point, or bit errors will occur

Good BER Performance Eye
Receiver decision point

• Problem is how to verify that components
  systems can achieve good error performance when
  an error every few billion bits is unacceptable.

BER Level
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Aim of Transmitter Testing.
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Agenda

• So Whats the Problem?
• Jitter Eye Closure Review
• Side Notes
• The Effect of Timing Reference
• Measurement Depth
• Common Methods
• Eye Diagram Jitter
• Mask Testing
• Bathtub/ BERTScan/ Jitter Peak
• Deep Mask Testing
• Jitter Spectrum
• Summary - Tying it All Together

PC Bus
8
Jitter / Eye Closure Overview
Ideal Edge Placement
Jitter Deviation in time from ideal transition
placement
Jitter Amplitude Displacement in time from
ideal transition placement.(Units generally rms)
Number of edges reaching given amplitude
General Distribution Higher jitter amplitudes
occur less frequently
t
Leading
Lagging
9
Simple Jitter Decomposition
Total Jitter (TJ)
Random (RJ)
Deterministic (DJ)
-?
?
-?
?
t
t
Leading
Lagging
Leading
Lagging

• Can be related to data pattern, crosstalk or
  other system events
• Bounded peak magnitude truncated at some level
• Longer measurement, result isnt any bigger

• Typically noise related
• Unbounded
• Longer measurement, bigger answer

10
Jitter Effect on Eye Closure

• Eye diagrams generated from small numbers of
  samples can look clean
• But usually close when more data is taken

• Factors determining amount of closure
• Magnitude of Random Jitter Component
• Magnitude of Deterministic Jitter Components
• Amount of data which is sampled (measurement time)

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Agenda

• So Whats the Problem?
• Jitter Eye Closure Review
• Side Notes
• The Effect of Timing Reference
• Measurement Depth
• Common Methods
• Eye Diagram Jitter
• Mask Testing
• Bathtub/ BERTScan/ Jitter Peak
• Deep Mask Testing
• Jitter Spectrum
• Summary - Tying it All Together

PC Bus
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What You Measure Depends Upon Your Timing
Reference
2.
1.
3.
Some tracked Some not
Clock Recovery
Clock Recovery
Clock Recovery
Analyzer
Analyzer
Analyzer
a.

• Example Timing Experiment
• 3 Different Measurement Setups (1,2,3)
• Two Different Test Signals (a,b)

b.
(Tracked region)
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What You Measure Depends Upon Your Timing
Reference
2.
1.
3.
Some tracked Some not
Clock Recovery
Clock Recovery
Clock Recovery
Analyzer
Analyzer
Analyzer
1a.
2a.
3a.
1b.
2b.
3b.
See SOME jitter, depends on Jitter frequency
See NO jitter
See ALL jitter
(Tracked region)
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What You Measure Depends Upon Your Timing
Reference
3.
Measured jitter results can vary significantly
depending on the method of triggering. This
applies equally to sampling scopes, BERTs,
real-time scopes etc..
Some tracked Some not
A.
Clock Recovery
Analyzer
Most standards require case 3, a golden PLL
(defined response clock recovery trigger) for
transmitter testing the intention is to
emulate the receiver, and to only have jitter
appear in the measurement that the receiver
cannot track out.
3a.
B.
3b.
C.
See SOME jitter, depends on Jitter frequency
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Test Equipment The Limitations of Shallow
Sampling Views
1.
2.
3.
Clean
Marginal
Closed
?
?
Very different openings at depth
Look very similar on a shallow view
4.
5.
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Eye Diagrams Sample Depth
10 seconds Traditional sampling
scope
1.
It is hard to see what is going on at 10-12 from
a shallow eye diagram. Even higher sampling
rate eye diagrams do not reach down to 10-12
levels.
4.
5.
10 seconds Higher sample
rate scope
2.
Increasing Visibility
lt1 minute BER Contour
(measured depth 10-8)
3.
Increasing Depth
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Agenda

• So Whats the Problem?
• Jitter Eye Closure Review
• Side Notes
• The Effect of Timing Reference
• Measurement Depth
• Common Methods
• Eye Diagram Jitter
• Mask Testing
• Bathtub/ BERTScan/ Jitter Peak
• Deep Mask Testing
• Jitter Spectrum
• Summary - Tying it All Together

PC Bus
18
Eye Diagram Jitter
2.
4.
Samples used to form histogram of crossing point.
Edges fall in a variety of places
t
Samples
3.
Eye Diagram Jitter
t
Samples
5.
Samples are taken of edge placements at the
crossing point.
V
Measured Example
6.
1.
Examining the eye diagram crossing point.
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Eye Diagram Jitter Issues
Increasing measurement time
Apparent Opening
1.
2.
3.
4.
Beyond traditional sampling scopes in a
reasonable time. About limit of higher sampling
rate eye diagrams
Common for eye diagrams on sampling scopes,
measured in a few seconds or so.
The eye that standards would like to have
measured
5.
As more samples are taken, more of the actual
eye performance is uncovered.
6.
But which is the right jitter value?
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Eye Diagram Jitter Summary
Weaknesses
Strengths

• Answer changes over time
• Shallow - Doesnt have measurement depth to catch
  rare events

• Easy measurement on common equipment.
• Gives quick, intuitive feel for signal quality.

Measurement Speed

• Quick for an answer that only shows gross
  problems 10 seconds

Example Measurement Equipment
Example Standards

• None

• Sampling Scope
• BERTScope

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2. Traditional Mask Testing Basics
Example 10 GbE mask Ref. 802.3ae
Measurements
4.
Overshoot
2.
80
Rise Time
3.
Eye Jitter
1.
130
20
100
80
Generic Mask
Normalized Amplitude ()
50
Keep Out
2a.
Ensures no excessive overshoot
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0
Keep Out
Ensures jitter isnt excessive
-20
1a.
0
.22
.375
.625
.78
1
Keep Out
Ensures rise fall times are fast enough
Normalized Time (Unit Interval)
3a.
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2. Traditional Mask Testing
1.
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2. Traditional Mask Testing Mask Margin
1.
Issue 1 How to ensure eye opening at depth is
good, when a mask test is shallow? Traditional
Answer Either the authors of a given standard
expand the mask size when it is written, or users
do to try to ensure only good transmitter pass.
3.
2.
(top bottom regions omitted)
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2. Traditional Mask Testing Mask Margin
4.
1.
Issue 2 Good transmitters fail poor yield,
high costs. Example ISI dominated closure may
fail mask but not hurt deep eye opening much
Issue 3 Bad transmitters pass unhappy
customers. Example VCSELs can have large amount
of noise unbounded eye closure
2.
5.
X
7.
Unbounded Effect
Bounded Effect
(Could measure for longer, but this hardly dents
the issue)
6.
3.
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2. Mask Testing Summary
Weaknesses
Strengths

• Answer changes over time
• Doesnt have measurement depth to catch rare
  events
• Standards dont specify points per waveform or
  how many waveforms left to the engineer, no
  standardization.
• Need to use margin to try to guess whats going
  on at depth - gamble

• Tests all around the eye diagram, including
• Eye Diagram Jitter
• Rise/Fall Times
• Overshoot
• Very common in standards

Measurement Speed

• Quick for an answer that only shows gross
  problems 10 seconds plus

Example Measurement Equipment
Example Standards

• Sampling Scope
• BERTScope

• SONET/SDH
• Fibre Channel
• 1 10 GbE

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3. Bathtub, BERTScan, Jitter Peak
Bathtub BERTScan (MJSQ)
d.
Jitter Peak
b.

• BERT decision point samples center of the eye,
  measures BER. (a.)
• Decision point is moved in time, BER measured
  again.
• Towards crossing point, measured BER increases.
  (b.)
• Jitter is measured as width of crossing point in
  BER.
• Resulting graph is bathtub curve (c. d.)

c.
BERT Decision Point
a.
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3. Bathtub, BERTScan, Jitter Peak Summary
Weaknesses
Strengths

• Only jitter dimension measured
• Older equipment can give slow and inconsistent
  results

• If measured using BER, based on deep sample
  depth, little extrapolation needed to low BERs
• Can give direct measurement of TJ
• One method of obtaining RJ/DJ separation

Measurement Speed
Equipment dependent, can be slower than shallow
eye diagram jitter measurements
Example Measurement Equipment
Example Standards

• BERTs
• BERTScope
• Estimates from some sampling scopes etc on
  shorter patterns.

• MJSQ (and therefore Fibre Channel)

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4. BER Contour A Path to Deep Mask Testing
4.
1.
3.
Can extend the bathtub idea by taking multiple
radial slices through the eye with the decision
point to cover all of the inside of the eye.
2.
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4. Deep Mask Testing
Compare deep compliance mask with measured BER
Contour
2.
Traditional Mask
1.
Deep Mask
3.
4.
Measured XFI Device Example
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4. Deep Mask Testing Summary
Weaknesses
Strengths

• Mandated devices must pass at depth, but not
  specified how to test to get there
• Arent many instruments that can measure this
  directly

• Specifies eye opening down where it is needed, at
  10-12 BER
• Measures all around the inside of the eye like
  conventional mask testing

Measurement Speed

• Not a fast manufacturing test 1 minute plus

Example Measurement Equipment
Example Standards

• BERTScope

• OIF CEI
• XFI
• SFP electrical signal to module input

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5. Jitter Spectrum
Lo F
1.
Hi F
Sampling for 1 minute for transmission
standards. Imagine edge positions recorded, then
FFTd Frequency spectrum gt80 MHz measured for
10Gb/s Many analyzers dont show spectrum, just
show answer.
2.
5.
4.
6.
Jitter (edge movement) frequencies
3.
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5. Jitter Spectrum
Weaknesses
Strengths

• Jitter Test Only
• Traditionally, repeatability issues from
  instruments with higher than ideal intrinsic
  jitter

• Gives idea ofwhat will and will not be tracked
  by Rx PLL
• Can be a good diagnostic test

Measurement Speed

• Not a fast manufacturing test 1 minute
  measurement

Example Measurement Equipment
Example Standards

• SONET Functional Testers
• BERTScope DCRj, CRj

• SONET/ SDH, e.g. 9.95 Gb/s
• OTN, e.g. 10.709, 11.095 Gb/s

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Agenda

• So Whats the Problem?
• Jitter Eye Closure Review
• Side Notes
• The Effect of Timing Reference
• Measurement Depth
• Common Methods
• Eye Diagram Jitter
• Mask Testing
• Bathtub/ BERTScan/ Jitter Peak
• Deep Mask Testing
• Jitter Spectrum
• Summary - Tying it All Together

PC Bus
34
Why Do I See Many Different Jitter Results?
(Other Factors Also at Play, Not Covered Here)
Jitter Frequencies Included in Measurement
Banded to 80 MHz
LBW ? Bit Rate
Broadband, 0 ? Bit Rate
With Clock Recovery

• Eye Diagram Jitter

• Eye Diagram Jitter

Shallow
80M
20k
10G
LBW
80M
20k
10G
4M

• Jitter Peak
• BER Contour

• Jitter Peak
• BER Contour

• OmniBER
• DCRj Jitter Spectrum

Deep
Depth
80M
20k
10G
4M
80M
20k
10G
LBW
80M
20k
10G
4M
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Summary
PC Bus

• Transmitter test is designed to ensure error free
  system operation.
• Test methods vary between standards, partly
  because of the available equipment at the time
  they were written.
• Older standards specified mask tests but not test
  depth.
• Telecom standards also worry about the frequency
  spectrum of jitter.
• In order to assure correct operation at BER
  levels of system operation, deep measurements are
  preferable.
• Many newer standards require testing at deep BER
  levels, either with deep masks or Bathtub Jitter.

Note All the jitter methods discussed here,
including for 11.1Gb/s 10GbE LAN PHY, are
available from the BERTScope family.