1. Crosstalk Measurements for Signal Integrity
Applications

2. Outline
ı What is crosstalk?
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A brief history of crosstalk
Definition of crosstalk
Why is crosstalk important?
Types of crosstalk
Impact of crosstalk on signal integrity
ı Measurement Methods for crosstalk


Time domain measurements
Frequency domain measurements
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3. What is Crosstalk?
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4. A Brief History of Crosstalk
ı Crosstalk terminology form telephone lines
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Communication crosses line from the intended user to
a victim
Crosstalk if frequency dependent
Significant contribution to crosstalk identified to be
telephone circuit unbalances
“Crosstalk set” measures near-end crosstalk of
telephone line at audio frequencies
Source: L.P. Ferris, R. G. McCurdy: “Telephone Circuit
Unbalances. Determination of Magnitude and
Location”, Pacific Coast Convention of the
A.I.E.E., 1924
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5. Signal Integrity Problems
Transmission line effects
ı Delay
ı Rise time degradation
ı Attenuation
ı Skin effect
ı Overshoots, undershoots,
ı Ringing
ı Reflections
ı Crosstalk
Other effects
ı Skin losses, via stubs, connectors
ı Proximity effects
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6. Signal Integrity Problems
ı Effects that are hard (impossible?) to model
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Inherent process variations
Metal roughness
Non-ideal skin effects
Component dielectrics
How connectors are soldered to the board
Broadside coupling of signals
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7. A Systems View
BER = 0.5
BER
Slope of bathtub
curve
Tj
Dj
Dj
0
1
RjBER
RjBER
TjBER = Dj + alphaBER * Rj
Where alpha is related to the slope of the bathtub curve
LeCroy 2008
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8. Jitter
Eye Opening
Eye Diagram
Some authors believe they can identify
crosstalk by analyzing the eye diagram
Source: Jung CICC 2012
Source: Centric Technologies’ Wireless Cable
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9. Definition of Crosstalk
ı Crosstalk is the interference between signals that are propagating on various lines in the
system.
ı Crosstalk results from the interaction of electromagnetic fields generated by neighboring
data signals as they propagate through transmission lines and connectors.
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10. Why is Crosstalk Becoming Important?
ı A thought experiment
Ideal Transmission Line
Serial Bus
Two serial busses in parallel
Add Capacitive Coupling
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11. Why Crosstalk is Becoming Important
ı Two views of the world:

Time Domain: Send step into transmission line, see what comes out at the other end
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12. Why Crosstalk is Becoming Important
ı Two views of the world:

Frequency Domain: Send step into transmission line, see what comes out at the other
end
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13. Types of Crosstalk
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14. Types of Crosstalk
ı Near-end Crosstalk (NEXT)

The noise induced in the receiving pair due to the signal on the transmitting pair on the
same port. (Source: IEEE1394)
Source: Jung CICC 2012
ı Far-end Crosstalk (FEXT)


The noise induced in the receiving pair due to the signal on the transmitting pair on the
same port.
Source: Jung CICC 2012
Slow transitions  less FEXT
ı Crosstalk Induced Jitter (CIJ)
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Odd mode and even mode have different propagation velocity
Independent of rise/fall times and signal amplitude
Source: Buckwalter SSC 2006
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15. Near End Crosstalk (NEXT)
ı Single-Ended Coupled Microstrip
ı NEXT coefficient Kb
V
K V (t ) V (t 2t t : propagation time through the trace
ı
f
NEXT
ı
Kb
b
1
4
(
in
CM
C Total
in
LM
)
L Total
f
CM, LM: Mutual Capacitance, Inductance per unit length
CTotal, LTotal: Total Capacitance, Inductance per unit length
(Source: Sohn, Advanced Packaging V24(4), 2001)
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16. Far End Crosstalk (FEXT)
ı Single-Ended Coupled Microstrip
ı FEXT coefficient Kf
d
ı
V
Kt
V (t t ) tf: propagation time through the trace
FEXT
ı
Kf
f f
1
2
(
dt
CM
C Total
in
f
LM
)
L Total
CM, LM: Mutual Capacitance, Inductance per unit length
CTotal, LTotal: Total Capacitance, Inductance per unit length
(Source: Sohn, Advanced Packaging V24(4), 2001)
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17. Types of Crosstalk
ı Alien Crosstalk (AXT)

Crosstalk within a group or bundle of cables
 Alien Near-End Crosstalk (ANEXT) (IEEE 802.3 terminology)
 Alien Far-End Crosstalk (AFEXT) (IEEE 802.3 terminology)
ı Power sum near end crosstalk (PSNEXT)

power sum of NEXT of all other wire pairs on crosstalk in one pair (in UTP cables)
ı Equal Level Far end crosstalk (ELFEXT)

FEXT minus attenuation of cable
ı Power Sum Equal Level Far end Crosstalk (PSELFEXT)

power sum of ELFEXT of all other wire pairs on crosstalk in one pair (in UTP cables)
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18. Types of Crosstalk(con’ed)
ı ICR: Insertion crosstalk ratio
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ICR = |IL – PSXT|
Similar to PSELFEXT but includes NEXT
ı ICN: Integrated crosstalk noise

Takes into account spectrum of excitation signal (Source: Sercu, DesignCon 2010)
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19. Sources of Crosstalk
Source: Mukherjee, ECTC, 2013
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ı
ı
ı
ı
Source: Wu, EMC V55(4), 2013
Crosstalk happens even in ideal transmission lines
Crosstalk on TSVs (through silicon vias)
Source: Lim, EMC V55(4), 2013
Crosstalk in packages
Launch pattern for BGAs
Crosstalk on vias in PCB,
Crosstalk through difference in propagations velocity of different modes in coupled
stripline/microstrip
Source: Hsu, ECTC, 2012
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20. Sources of Crosstalk
ı Increased number of features in computers systems
ı Data rates increase
ı Board size decreases (or stays the same)
ı Need for “right the first time”
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21. Crosstalk Measurements
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22. Typical VNA measurements for SI Engineers
ı Insertion loss
Common mode and differential
ı Return loss
 Common mode and differential
ı Crosstalk

ı Within Channel measurements
insertion loss, return loss, pair-to-pair near-end crosstalk loss (NEXT), power sum
NEXT loss, pair-to-pair attenuation to crosstalk ratio, far-end (ACRF), power sum
ACRF, return loss, and delay
ı Between Channel measurements
 alien crosstalk parameters, power sum alien attenuation to crosstalk ratio, far-end
(PSAACRF) and power sum alien NEXT

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23. Measurement Techniques
ı Bit error rate test-set
Not suitable to evaluate the amount of Crosstalk
ı Real-time Oscillscopes
 Some information on crosstalk via eye diagram
 Statistical correlation between different source are conceptually possible
ı Time domain Reflectometry (TDR)
ı Frequency domain measurements
 Using Vector Network Analyzer

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24. Time Domain 1/2
ı Eye diagram
Eye height
 Eye width
ı Advantages:
 Related to system performance
ı Disadvantages
 Not easy to figure out what part of eye closure is due to crosstalk
 In presence of ISI, crosstalk can not be identified
 Requites large data sets (can’t do PRBS31)
 No information on how to fix the problem

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25. Time Domain 2/2
ı TDR
Measure reflected/transmitted energy and frequency content
 Based on equivalent time oscillosopes
ı Advantages:
 Intuitive Measure
 Information on impedance as function of electrical length
ı Disadvantages
 Not very accurate at high frequency
 Calibration is questionable
 Low dynamic range
 Repeatability
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26. Frequency Domain Measurements
ı S-parameters
Frequency/phase response of a channel
ı Advantages:
 Highly accurate
 Large dynamic range
 Well-known calibration procedures/embedding/de-embedding
 Spatial information via IFT
 Up to 500+ GHz
ı Disadvantages
 Has reputation of being complicated
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27. How does a VNA work?
ı VNA consists of Generator, directional
ı
ı
ı
ı
element and receiver
Generator sends out pure sine-wave
Incident wave is measured with reference
receiver
Reflected wave is measured with one
measurement receiver
Transmitted wave is measured with another
measurement receiver
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28. Requirements
Reflectometer 2
Meas. Receiver
Ref. Receiver
ı In principle, 2n port device can me measured with a 2port VNA
 True differential measurements require at least 4ports with 2 coherent sources
 Modern VNAs provide up to 48 ports via switch matrix
ı Accuracy of models depends on accuracy of Sparameter measurements
 Stability of setup is crucial
 Connecting/reconnecting cables is error prone
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PORT 2
Bias Tee
Reflectometer 4
Meas. Receiver
Ref. Receiver
PORT 4
Bias Tee
Reflectometer 1
Meas. Receiver
Ref. Receiver
PORT 1
Bias Tee
Reflectometer 3
Meas. Receiver
Ref. Receiver
PORT 3
Bias Tee
28
DUT

29. Are TDR and VNA measurements equivalent?
ı TDR instruments are a lot easier to set up, why bother with a VNA?
ı Arguments for TDR
Easier to set up/use
 Cheaper
ı Arguments for VNA
 At high speed much lower uncertainty (TDR @50 GHz: 12 dB uncertainty)
 TDR dynamic range: 35 dB, VNA: 100+ dB
 Sources of VNAs are much cleaner than for TDRs
 Can not adjust step amplitude of TDRs
 No bias-T option for TDRs
 TrueDifferential
 Sophisticated calibration procedures
 Easy to de-embed probes, cables, fixtures
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30. How to handle crosstalk
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31. How to Reduce Crosstalk
ı Design:
Increase spacing between traces
 Guard traces, serpentine microstrip lines, spiral layout
 Segmented transmission lines using Genetic Algorithms (Seki, EDAPS 2012)
 High quality connectors
 Backdrilled VIAS
ı Compensation
 Active X-talk cancellation
 Amplitude (Pelard, JSSC, 2004)
 Timing
 TX side
 RX side

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32. Equalization Techniques
ı Various active/passive techniques proposed
ı Receiver side equalization
Noise enhancement
ı TX side pre-emphasis
 Coupling of energy into adjacent channels
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33. Summary and Outlook
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34. System Level Approach
ı Much of today’s design flow is driven by systems specs
PCB/component/package/device specs are not always well
defined
 Specs can be traded off against each other as long as
system requirements are met
ı Design margins are eroding
 Trend to higher speed and higher integration
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ı Crosstalk is next frontier in conquering high-speed designs
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Nearly impossible to spec crosstalk on a systems level
Successful designs will require integrated
modeling/characterization cycles that integrate crosstalk
mitigation on device/package and PCB level
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Source: Mukherjee, ECTC, 2013
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35. For More Information
ı Download complete slide presentation via Slideshare
slideshare.net/rohdeschwarzNA
ı Access app notes, white papers and other supporting material via our
Twitter feed
@RohdeSchwarzNA

36. •
Download complete slide presentation via Slideshare
slideshare.net/rohdeschwarzNA
•
Access app notes, white papers and other supporting material via our Twitter
feed
@RohdeSchwarzNA