an introduction to crosstalk measurements

Crosstalk Measurements for Signal Integrity Applications

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Outline

ı What is crosstalk? 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


What is Crosstalk?


A Brief History of Crosstalk

ı Crosstalk terminology form telephone lines 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


Signal Integrity Problems

Transmission line effectsı Delayı Rise time degradation ı Attenuationı Skin effectı Overshoots, undershoots, ı Ringingı Reflectionsı Crosstalk

Other effectsı Skin losses, via stubs, connectorsı Proximity effects

Given enough resources all these effects can be

Modeled and accounted for


Signal Integrity Problems

ı Effects that are hard (impossible?) to model Inherent process variations Metal roughness Non-ideal skin effects Component dielectrics How connectors are soldered to the board Broadside coupling of signals

LeCroy 2008 7

A Systems View

BE

R

0 1

Dj Dj

Slope of bathtub curve

BER = 0.5

TjBER = Dj + alphaBER * RjWhere alpha is related to the slope of the bathtub curve

Tj

RjBER RjBER


Eye Diagram

Jitter

Eye

Ope

ning

Source: Centric Technologies’ Wireless Cable

Some authors believe they can identify crosstalk by analyzing the eye diagramSource: Jung CICC 2012


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.

But why is there suddenly so much interest in Crosstalk?


Why is Crosstalk Becoming Important?

ı A thought experiment

Ideal Transmission LineSerial Bus

Two serial busses in parallel

Add Capacitive CouplingNow let’s look at the behavior of these two coupled serial busses


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


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

At higher bus speeds Crosstalk becomes Increasingly more important


Types of Crosstalk


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)

ı Far-end Crosstalk (FEXT) The noise induced in the receiving pair due to the signal on the transmitting pair on the

same port. Slow transitions less FEXT

ı Crosstalk Induced Jitter (CIJ) Odd mode and even mode have different propagation velocity Independent of rise/fall times and signal amplitude

Source: Jung CICC 2012

Source: Jung CICC 2012

Source: Buckwalter SSC 2006


Near End Crosstalk (NEXT)

ı Single-Ended Coupled Microstrip

ı NEXT coefficient Kb

ı tf: propagation time through the trace

ı CM, LM: Mutual Capacitance, Inductance per unit length

CTotal, LTotal: Total Capacitance, Inductance per unit length

)L

L

C

C(K

Total

M

Total

M41

b

fininbNEXT 2(V)(VKV ttt

(Source: Sohn, Advanced Packaging V24(4), 2001)


Far End Crosstalk (FEXT)

ı Single-Ended Coupled Microstrip

ı FEXT coefficient Kf

ı tf: propagation time through the trace

ı CM, LM: Mutual Capacitance, Inductance per unit length

CTotal, LTotal: Total Capacitance, Inductance per unit length

)L

L

C

C(K

Total

M

Total

M21

f

)(Vd

dKV inffFEXT ftt

tt

(Source: Sohn, Advanced Packaging V24(4), 2001)


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)


Types of Crosstalk(con’ed)

ı ICR: Insertion crosstalk ratio 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)


Sources of Crosstalk

ı Crosstalk happens even in ideal transmission linesı Crosstalk on TSVs (through silicon vias)ı Crosstalk in packagesı Launch pattern for BGAsı Crosstalk on vias in PCB,ı Crosstalk through difference in propagations velocity of different modes in coupled

stripline/microstrip

Source: Wu, EMC V55(4), 2013Source: Mukherjee, ECTC, 2013

Source: Hsu, ECTC, 2012

Source: Lim, EMC V55(4), 2013


Sources of Crosstalk

ı Increased number of features in computers systemsı Data rates increase

ı Board size decreases (or stays the same)

ı Need for “right the first time”


Crosstalk Measurements


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


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


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


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


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


How does a VNA work?

ı VNA consists of Generator, directional element and receiver

ı Generator sends out pure sine-wave ı Incident wave is measured with reference

receiverı Reflected wave is measured with one

measurement receiverı Transmitted wave is measured with another

measurement receiver


Requirements

ı In principle, 2n port device can me measured with a 2-port VNA True differential measurements require at least 4-

ports with 2 coherent sources Modern VNAs provide up to 48 ports via switch matrix

ı Accuracy of models depends on accuracy of S-parameter measurements Stability of setup is crucial Connecting/reconnecting cables is error prone

DUT

PORT 3

Bias Tee

Meas. Receiver

Ref. Receiver

Bias Tee

Meas. Receiver

Ref. Receiver

Bias Tee

Meas. Receiver

Ref. Receiver

PORT 2

PORT 1

PORT 4

Meas. Receiver

Ref. Receiver

Reflectometer 3

Reflectometer 1

Reflectometer 4

Reflectometer 2

Bias Tee


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


How to handle crosstalk


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


Equalization Techniques

ı Various active/passive techniques proposedı Receiver side equalization

Noise enhancementı TX side pre-emphasis

Coupling of energy into adjacent channels


Summary and Outlook


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 metı Design margins are eroding

Trend to higher speed and higher integration

ı Crosstalk is next frontier in conquering high-speed designs 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

Source: Mukherjee, ECTC, 2013

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• Download complete slide presentation via Slideshare

• Access app notes, white papers and other supporting material via our Twitter feed

slideshare.net/rohdeschwarzNA

@RohdeSchwarzNA

an introduction to crosstalk measurements

Technology

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victim crosstalk

crosstalk results

integrated crosstalk

end crosstalk elfext

end crosstalk fext single

ringing reflections