HSDD-30-40

Slide -1Week 11: Cross talk

ECEN 5224 High Speed Digital Design Prof Eric Bogatin www.beTheSignal.com

Week 11:

Cross Talk in Uniform Transmission Lines

March 2015

Prof Eric Bogatin, Dean, Signal Integrity Academy

Teledyne LeCroy www.beTheSignal.com, [email protected]

Adjunct Prof, ECEE Dept CU, Boulder

And

Prof Melinda Piket-May

Assoc. Prof, ECEE Dept, CU, Boulder

Spring 2015



Joke of the day



Pop Quiz

(3 points)



Topic Schedule

1. Jan 12: Welcome and intro

2. Jan 19 holiday (chpt 1, 2, 3)

3. Jan 26: Transmission lines and signals (chpt 7)

4. Feb 2: Reflections and circuit simulations (chpt 8)

5. Feb 9: Discontinuities (chpt 5, 6)

6. Feb 16: Differential pairs (chpt 11)

7. Feb 23: S-parameters and the TDR (chpt 2, 12)

8. Mar 2: Attenuation and loss (chpt 9)

9. Mar 9: High speed serial links (chpt 9, 11, 12) (mid term take home)

10. Mar 16: High speed scope measurements and jitter analysis

11. Mar 23: spring break

12. Mar 30 Cross talk (chpt 10)

13. April 6: Ground bounce (chpt 6)

14. April 13: PDN-1 (chpt 4, 6, 13)

15. April 20: PDN-2

16. April 27 last class- final

Textbook for the class



Week 11 Assignments due April 6, 2015

Next lecture: week 12 Ground bounce

Read chapter 160

View online lectures:

EPSI section 5: Ground bounce

EPSI section 6: Return plane transitions

Lab report: HOL HyperLynx cross talk in uniform transmission lines

Term Projects due April 27



Review of the Mid Term



The Problem:

Measured Near and Far End XTK in Two Uniform Microstrips: 5 mil wide

line and space, 4 inches long

Very different signatures

Very different magnitudes

RT

1 v incident

signal

Near endsignal

near

V

VNEXT =

Far endsignal

far

V

VFEXT =

Incident signal

Transmitted signal

NEXT: Measured near end cross talk

FEXT: Measured far end cross talk

200 psec/div

10% noise/div

RT = 100 psec



Fundamental Root Cause of Cross Talk:

Fringe Electric and Magnetic Fields

mutual magnetic field lines

mutual electric field lines Induced cross talk noise:

Changing mutual electric field

Changing mutual magnetic fields

Two general ways of reducing mutual field lines

Move the traces farther apart

Bring return plane closer to the signal lines

Move traces apart Bring return plane closer to signals



Why Do we Use Planes in Boards?

Forget the word shielding!

To control the impedance

To keep the signal-return electric, magnetic fringe fields from spreading to other signals.

Cross talk DRAMTICALLY increases without an adjacent plane!



Two Most Important Principles of Cross Talk in

Busses

1. The only way noise is induced on quiet line is:

Electric field coupling = capacitive changing voltage, dV/dt

Magnetic field coupling = inductive changing current, dI/dt

2. Signals propagates and induced noise propagates

1

2

V

v = 6 inches/nsec

x = RT x v

1

2CW signal current

CCW induced current

@ Near end: IC + IL @ Far end: IC - IL

dI

dt

dV

dt



Dynamics of Cross Talk

Using the XTK SimSIA.exe



Induced Voltage Between Two Current Loops

dt

dI1}Loop 1

}Loop 2

L21

dt

dI

Z

L

Z

VI 1

2

21

2

22 ==

CW

CW CCW

What direction is the induced current?

dt

dILV 1212 =

dt

dI1}Loop 1

}Loop 2

L21

CW

CCW

Lentzs: Direction of induced

current is opposite direction

of inducing current



Most important Ways of Reducing NEXT,

FEXT: Control the Fringe Fields

Move traces farther apart (keeping Z0 = 50 )

Nominal: s = w

Apart: s = 2 x w

Bring return plane closer to signal lines (lower impedance)

Nominal: Z0 = 50 Ohms

Closer plane: Z0 = 35 Ohms

s = w

s = 2 x w

s = w

s = 2 x w

Z0 = 50

Z0 = 35

Z0 = 50

Z0 = 35



How Else to Reduce NEXT?

Arent any!

Independent of rise time

Independent of coupling length

Which has lower NEXT?:

Microstrip

Stripline

How much cross talk is too much?

Mit depends!

Roughly 5%

1 2 3 4 5 6 7 8 90 10

1E-4

1E-3

1E-2

1E-1

1E-5

1

Ratio of Separation to w

Sa

tura

ted

NE

XT

Co

eff

icie

nt

For 50 lines in FR4

Microstrip

Stripline

For worst case NEXT

in a bus, keep NEXT < 2%

Design separation > 2 x w, MS or SL



Other Ways of Controlling FEXT

Reduce FEXT:

Lower Z0

Larger spacing between signal traces

Shorter coupling length

Longer rise time

Example 1: Len = 10 inches, RT = 0.3 nsec, separation = w

k = -0.005, FEXT = 10 inches / 0.3 nsec x 0.005 = 17% !!!!!!

Example 2: Len = 2 inches, RT = 0.5 nsec, separation = 2 x w

k = -0.003, FEXT = 2 inches / 0.5 nsec x 0.003 = 1%

Most important way of eliminating FEXT

Move to stripline! (IC = IL)

FEXT

2 3 4 5 6 71 8

-0.005

-0.004

-0.003

-0.002

-0.001

-0.006

0.000

Separation/w

Valu

e o

f couplin

g c

oeffic

ient

[ ]

[ ]

Len inFEXT k

RT nsec=

For 50 lines in FR4

RT Len



Surface traces are on every board

Think Far End Cross Talk



Eliminate FEXT with StriplineMeasured at Far end, Near end terminated, TDT end Open

Differences:

Far end cross talk

in microstrip:

IL > IC

No far end cross

talk in stripline:

IL = IC

MicrostripStripline

TDR open50 Ohms Far end noise

TDR response

Noise response



Diff Pair Near End Cross Talk:

Design Guidelines

For the same line width and diff impedance, which has more channel to channel differential cross talk?

Tightly coupled stripline differential pairs

Loosely coupled stripline differential pairs



Cross Talk and Coupling in Stripline for Constant

Line Width, 100 Ohm diff impedance

Specs:

Z0_diff = 100 Ohms

W = 7 mils

oz copper

Dk = 4

top

bottom

top

bottom

top

bottom

Spacing = 1 x w

H1 = H2 = 13.1 mils

Spacing = 2 x w

H1 = H2 = 8.6 mils

Spacing = 3 x w

H1 = H2 = 8.2 mils

Tightly coupled diff pair requires planes farther away more fringe field coupling to neighbors

Loosely coupled diff pairs enables planes closer confines the fringe fields to neighbors

For the same line width and 100 Ohm diff impedance:



Near End Cross Talk:

Design SpaceTight coupling: line to line s = w

Loose coupling:

line to line s = 2 x w

Uncoupled:

Line to line s = 3 x w

-45 dB (0.5%)

-30 dB (3%)

For -45 dB xtk:

Tightly coupled diff pair, pair to pair spacing = 3.5 x w

Loosely coupled diff pair, pair to pair spacing = 2.5 x w

For -30 dB xtk:

Tightly coupled diff pair, pair to pair spacing = 1.5 x w

Loosely coupled diff pair, pair to pair spacing = 1 x w



A Common Problem with Dual Stripline: Two

Routing Layers Between Return Planes

Normally, route the layers orthogonal as x-y

But what if there is

inadvertent overlap? h

h

h

w

For 50 Ohm lines, h ~ w

Broad side

coupled

stripline



Broadside Coupling Can be Huge!

Signal is 1 vWhen there is perfect

overlap, broad side coupling,

NEXT ~ 18%

When signals are offset by w,

NEXT ~ 12%

When signals are edge

coupled, NEXT ~ 5%

If dielectric thickness

between signal layers is

thinner, NEXT is larger

Broadside coupling can be HUGE!

Always try to add dielectric fill layers between signal layers



Inadvertent Broad Side Coupled Stripline: Unintentional Co-parallel

Overlaps in BGA Breakout Region

Courtesy of Altera

Keep inadvertent broad side coupling < 1/3 Lensat for the given rise time

Inadvertent broadside near end cross talk in

BGA break region can be > 15%!

This is a major sneaky failure mode



Summary

Cross talk is generated by capacitive and inductive coupling: fringe electric and magnetic fields

Reduce NEXT, FEXT by:

Moving traces farther apart

Bringing return plane closer- lower impedance

Propagation direction creates different noise generated in the forward or backward directions

For most digital applications, when s ~ w, watch out for NEXT, FEXT. Try to keep s > 2 x w

Watch out for inadvertent broad side coupling

FEXT can be much higher than NEXT

See microstrip think FEXT

Far end noise is eliminated in stripline, always estimate FEXT in microstrip



Additional Important Topics Covered in

the Hands on Labs using HyperLynx

HSDD-30-41

What is RT and output impedance of driver?

Scaling of length, rise time, spacing for NEXT, FEXT, MS, SL

Saturation length and decreasing NEXT by decreasing coupling length

HSDD-30-42 diff pair cross talk

Cross talk in differential pairs: NEXT, FEXT, MS, SL

HSDD-30-43 broadside coupling

How much larger broadside coupling is than edge to edge coupling

Using time domain S31 to get spatial information about where coupling happens

Saturation length and decreasing NEXT by decreasing coupling length



HSDD-30-40

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