Crosstalk

Overview and Modes

Crosstalk Overview

2

What is Crosstalk?

Crosstalk Induced Noise

Effect of crosstalk on transmission line parameters

Crosstalk Trends

Design Guidelines and Rules of Thumb

Overview

Crosstalk Overview

3

Crosstalk Induced Noise

Key Topics:

Mutual Inductance and capacitance

Coupled noise

Circuit Model

Transmission line matrices

Crosstalk Overview

4

Crosstalk is the coupling of energy from one line to another via:

Mutual capacitance (electric field)Mutual inductance (magnetic field)

Mutual Inductance and Mutual Inductance and CapacitanceCapacitance

Zs

Zo

Zo

Zo

Mutual Capacitance, Cm Mutual Inductance, Lm

Zs

Zo

Zo

Zo

Cm

Lm

near

far

near

far

Crosstalk Overview

5

The circuit element that represents this transfer of energy are the following familiar equations

Mutual Inductance and Mutual Inductance and Capacitance Capacitance ““Mechanism of coupling”Mechanism of coupling”

dt

dILV mLm

dt

dVCI mCm

The mutual inductance will induce current on the victim line opposite of the driving current (Lenz’s Law)

The mutual capacitance will pass current through the mutual capacitance that flows in both directions on the victim line

Crosstalk Overview

6

The near and far end victim line currents sum to produce the near and the far end crosstalk noise

Crosstalk Induced NoiseCrosstalk Induced Noise““Coupled Currents”Coupled Currents”

Zs

Zo

Zo

Zo

Zs

Zo

Zo

Zo

ICmLm

near

far

near

far

ILm

LmCmfarLmCmnear IIIIII

Crosstalk Overview

7

Near end crosstalk is always positive Currents from Lm and Cm always add and flow into the node

For PCB’s, the far end crosstalk is “usually” negativeCurrent due to Lm larger than current due to CmNote that far and crosstalk can be positive

Crosstalk Induced NoiseCrosstalk Induced Noise““Voltage Profile of Coupled Noise”Voltage Profile of Coupled Noise”

Driven Line

Un-driven Line“victim”

Driver

Zs

Zo

Zo

Zo

Near End

Far End

Crosstalk Overview

8

Graphical ExplanationGraphical Explanation

TD

2TD

~Tr

~Tr

far end crosstalk

Near end crosstalk

Zo

V

Time = 2TD

ZoNear end current terminated at T=2TD

V

Time = 0

Zo

Near end crosstalk pulse at T=0 (Inear)

Far end crosstalk pulse at T=0 (Ifar)

Zo

ZoV

Time= 1/2 TD

ZoV

Time= TD

Zo Far end of current terminated at T=TD

Crosstalk Overview

9

Crosstalk EquationsCrosstalk Equations

Driven Line

Un-driven Line“victim”

Driver

Zs

Zo

Zo

Zo

Near End

Far End

Driven Line

Un-driven Line“victim”

Driver

Zs

Zo

Zo

Near End

Far End

LCXTD

C

C

L

LVA MMinput

4

C

C

L

L

T

LCXVB MM

r

input

2

TD

2TD

Tr ~Tr Tr

AB

TD

2TD

Tr ~Tr ~Tr

AB

C

C

L

LVA MMinput

4

CB2

1

C

C

L

L

T

LCXVC MM

r

input

C

Terminated Victim

Far End Open Victim

Crosstalk Overview

10

Crosstalk EquationsCrosstalk Equations

Driven Line

Un-driven Line“victim”

Driver

Zs

Zo

Zo

Near End

Far End

Near End Open Victim

TD

2TD

Tr Tr Tr

A

B

C

3TD

C

C

L

LVA MMinput

2

C

C

L

L

T

LCXVB MM

r

input

2

C

C

L

LVC MMinput

4

The Crosstalk noise characteristics are dependent on the termination of the victim line

Crosstalk Overview

11Creating a Crosstalk ModelCreating a Crosstalk Model““Equivalent Circuit”Equivalent Circuit”

The circuit must be distributed into N segments as shown in chapter 2

K1

L11(1)

L22(1)

C1G(1)

C12(1)K1

L11(2)

L22(2)

C1G(2)

C12(2)

C2G(2)C2G(1)

K1

L11(N)

L22(N)

C1G(N)

C12(n)

C2G(N)

C1G C2G

C12

2211

12

LL

LK

Line 1

Line 2

Line 1 Line 2

Crosstalk Overview

12

The transmission line Matrices are used to represent the electrical characteristics

The Inductance matrix is shown, where:LNN = the self inductance of line N per unit lengthLMN = the mutual inductance between line M and N

Creating a Crosstalk ModelCreating a Crosstalk Model““Transmission Line Matrices”Transmission Line Matrices”

Inductance Matrix =

NNN

N

LL

LL

LLL

1

2221

11211 ...

Crosstalk Overview

13

The Capacitance matrix is shown, where:CNN = the self capacitance of line N per unit length where:

CNG = The capacitance between line N and ground

CMN = Mutual capacitance between lines M and N

Creating a Crosstalk ModelCreating a Crosstalk Model““Transmission Line Matrices”Transmission Line Matrices”

Capacitance Matrix =

NNN

N

CC

CC

CCC

1

2221

11211 ...

mutualsNGNN CCC

12111 CCC G

For example, for the 2 line circuit shown earlier:

Crosstalk Overview

14

Example Calculate near and far end crosstalk-induced noise magnitudes and sketch the waveforms of circuit shown below:

Vsource=2V, (Vinput = 1.0V), Trise = 100ps.Length of line is 2 inches. Assume all terminations are 70 Ohms. Assume the following capacitance and inductance matrix:

L / inch =

C / inch =

The characteristic impedance is:

Therefore the system has matched termination.

The crosstalk noise magnitudes can be calculated as follows:

nHnH

nHnH

869.9103.2

103.2869.9

pFpF

pFpF

051.2239.0

239.0051.2

4.69051.2

869.9

11

11

pF

nH

C

LZO

vR1 R2

Crosstalk Overview

15

Example (cont.)

VpF

pF

nH

nHV

C

C

L

LVV input

near 082.0051.2

239.0

869.9

103.2

4

1

4 11

12

11

12

VpF

pF

nH

nH

ps

pFnHinchV

C

C

L

L

T

LCXVV

rise

inputfar 137.0

051.2

239.0

869.9

103.2

100*2

051.2*869.9*2*1

2

)(

11

12

11

12

Near end crosstalk voltage amplitude (from slide 12):

Far end crosstalk voltage amplitude (slide 12):

Thus,

100ps/div

200m

V/d

iv

The propagation delay of the 2 inch line is:

nsnHnHinchLCXTD 28.0051.2*869.9(*2

Crosstalk Overview

16

Effect of Crosstalk on Transmission line Parameters

Key Topics:

Odd and Even Mode Characteristics

Microstrip vs. Stripline

Modal Termination Techniques

Modal Impedance’s for more than 2 lines

Effect Switching Patterns

Single Line Equivalent Model (SLEM)

Crosstalk Overview

17

Odd and Even Transmission ModesOdd and Even Transmission Modes

Even Mode

Odd Mode

Crosstalk Overview

18

Potential difference between the conductors lead to an increase of the effective Capacitance equal to the mutual capacitance

Odd Mode TransmissionOdd Mode Transmission

Magnetic Field:Odd mode

Electric Field:Odd mode

+1 -1 +1 -1

Because currents are flowing in opposite directions, the total inductance is reduced by the mutual inductance (Lm)

Drive (I)

Drive (-I)

Induced (-ILm)Induced (ILm)

V

-I

Lmdt

dILmL

dt

IdLm

dt

dILV

)(

)(

I

Crosstalk Overview

19Odd Mode TransmissionOdd Mode Transmission ““Derivation of Odd Mode Inductance”Derivation of Odd Mode Inductance”

121111 LLLLL modd

Mutual Inductance:Consider the circuit:

dt

dIL

dt

dILV

dt

dIL

dt

dILV

mO

mO

122

211

2211LL

Lk m

L11

L22

I2

I1

+ V2 -

+ V1 -

Since the signals for odd-mode switching are always opposite, I1 = -I2 andV1 = -V2, so that:

dt

dILL

dt

IdL

dt

dILV

dt

dILL

dt

IdL

dt

dILV

mOmO

mOmO

2222

1111

)()(

)()(

Thus, since LO = L11 = L22,

Meaning that the equivalent inductance seen in an odd-mode environmentis reduced by the mutual inductance.

Crosstalk Overview

20Odd Mode TransmissionOdd Mode Transmission ““Derivation of Odd Mode Capacitance”Derivation of Odd Mode Capacitance”

mmgodd CCCCC 111 2

Mutual Capacitance:Consider the circuit:

C2g

C1g Cm

V2

V2C1g = C2g = CO = C11 – C12

So,

dt

dVC

dt

dVCC

dt

VVdC

dt

dVCI

dt

dVC

dt

dVCC

dt

VVdC

dt

dVCI

mmOmO

mmOmO

121222

212111

)()(

)()(

And again, I1 = -I2 and V1 = -V2, so that:

dt

dVCC

dt

VVdC

dt

dVCI

dt

dVCC

dt

VVdC

dt

dVCI

mOmO

mgmO

22222

11

1111

)2())((

)2())((

Thus,

Meaning that the equivalent capacitance for odd mode switching increases.

Crosstalk Overview

21Odd Mode TransmissionOdd Mode Transmission ““Odd Mode Transmission Characteristics”Odd Mode Transmission Characteristics”

Impedance:

Thus the impedance for odd mode behavior is:

)2:(1211

1211

oddaldifferenti

odd

oddodd

ZZNote

CC

LL

C

LZ

and the propagation delay for odd mode behavior is:

))(( 12111211 CCLLCLTD oddoddodd

Propagation Delay:

Explain why.

Crosstalk Overview

22

Since the conductors are always at a equal potential, the effective capacitance is reduced by the mutual capacitance

Even Mode TransmissionEven Mode Transmission

Because currents are flowing in the same direction, the total inductance is increased by the mutual inductance (Lm)

Drive (I)

Drive (I)

Induced (ILm)Induced (ILm)

V

I

Lmdt

dILmL

dt

IdLm

dt

dILV

)(

)(

I

Electric Field:Even mode

Magnetic Field:Even mode

+1 +1+1 +1

Crosstalk Overview

23Even Mode TransmissionEven Mode Transmission Derivation of even Mode Effective InductanceDerivation of even Mode Effective Inductance

121111 LLLLL meven

2211LL

Lk m

L11

L22

I2

I1

+ V2 -

+ V1 -

Mutual Inductance:Again, consider the circuit:

Since the signals for even-mode switching are always equal and in the samedirection so that I1 = I2 and V1 = V2, so that:

dt

dIL

dt

dILV

dt

dIL

dt

dILV

mO

mO

122

211

dt

dILL

dt

IdL

dt

dILV

dt

dILL

dt

IdL

dt

dILV

mOmO

mOmO

2222

1111

)()(

)()(

Thus,

Meaning that the equivalent inductance of even mode behavior increasesby the mutual inductance.

Crosstalk Overview

24Even Mode TransmissionEven Mode Transmission Derivation of even Mode Effective CapacitanceDerivation of even Mode Effective Capacitance

meven CCCC 110

Mutual Capacitance:Again, consider the circuit:

C2g

C1g Cm

V2

V2

dt

dVC

dt

VVdC

dt

dVCI

dt

dVC

dt

VVdC

dt

dVCI

OmO

OmO

22222

11111

)(

)(

Thus,

Meaning that the equivalent capacitance during even mode behavior decreases.

Crosstalk Overview

25Even Mode TransmissionEven Mode Transmission ““Even Mode Transmission Characteristics”Even Mode Transmission Characteristics”

Impedance:

Thus the impedance for even mode behavior is:

1211

1211

CC

LL

C

LZ

even

eveneven

and the propagation delay for even mode behavior is:

))(( 12111211 CCLLCLTD eveneveneven

Propagation Delay:

Crosstalk Overview

26

Odd and Even Mode Comparison Odd and Even Mode Comparison

for Coupled Microstripsfor Coupled Microstrips

Input waveformsEven mode (as seen on line 1)

Odd mode (Line 1)

v2

v1

Probe point

Delay difference due to modal velocity differences

Impedance difference

V1

V2

Line 1

Line2

Crosstalk Overview

27Microstrip vs. Stripline CrosstalkMicrostrip vs. Stripline Crosstalk Crosstalk Induced Velocity ChangesCrosstalk Induced Velocity Changes

Chapter 2 defined propagation delay as

Chapter 2 also defined an effective dielectric constant that is used to calculate the delay for a microstrip that accounted for a portion of the fields fringing through the air and a portion through the PCB material

This shows that the propagation delay is dependent on the effective dielectric constant

In a pure dielectric (homogeneous), fields will not fringe through the air, subsequently, the delay is dependent on the dielectric constant of the material

cT r

pd

Crosstalk Overview

28Microstrip vs. Stripline CrosstalkMicrostrip vs. Stripline Crosstalk Crosstalk Induced Velocity ChangesCrosstalk Induced Velocity Changes

Odd and Even mode electric fields in a microstrip will have different percentages of the total field fringing through the air which will change the effective Er

Leads to velocity variations between even and odd

+1 +1+1 -1

The effective dielectric constant, and subsequently the propagation velocity depends on the electric field patterns

Er=4.2

Er=1.0

Er=4.2

Er=1.0

Microstrip E field patterns

Crosstalk Overview

29Microstrip vs. Stripline CrosstalkMicrostrip vs. Stripline Crosstalk Crosstalk Induced Velocity ChangesCrosstalk Induced Velocity Changes

Subsequently, if the transmission line is implemented in a homogeneous dielectric, the velocity must stay constant between even and odd mode patterns

If the dielectric is homogeneous (I.e., buried microstrip or stripline) , the effective dielectric constant will not change because the electric fields will never fringe through air

+1 +1 +1 -1

Er=4.2Er=4.2

Stripline E field patterns

Crosstalk Overview

30Microstrip vs. Stripline CrosstalkMicrostrip vs. Stripline Crosstalk Crosstalk Induced NoiseCrosstalk Induced Noise

The constant velocity in a homogeneous media (such as a stripline) forces far end crosstalk noise to be zero

11

12

11

12

1112121112111112

1211121112111211 ))(())((

C

C

L

L

CLCLCLCL

CCLLCCLL

TDTD evenodd

02

)_(11

12

11

12

C

C

L

L

T

LCXVstriplinefarCrosstalk

r

input

Since far end crosstalk takes the following form:

Far end crosstalk is zero for a homogeneous Er

Crosstalk Overview

31Termination TechniquesTermination Techniques Pi and T networksPi and T networks

Single resistor terminations described in chapter 2 do not work for coupled lines 3 resistor networks can be designed to terminate both odd and even modes

T Termination

-1

R1

R2

R3

+1Odd Mode

Equivalent-1

R1

R2

Virtual Ground

in center

+1Even Mode

Equivalent+1

R1

R2

2R3

2R3

oddZRR 21

oddeven ZZR 2

13

Crosstalk Overview

32Termination TechniquesTermination Techniques Pi and T networksPi and T networks

The alternative is a PI termination

PI Termination

+1Odd Mode

Equivalent

-1

R1

R2

R3

-1

½ R3

½ R3

+1Even Mode

Equivalent +1

R1

R2

evenZRR 21

oddeven

oddeven

ZZ

ZZR

23

R1

R2