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Crosstalk

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Crosstalk

Overview and Modes

- What is Crosstalk?
- Crosstalk Induced Noise
- Effect of crosstalk on transmission line parameters
- Crosstalk Trends
- Design Guidelines and Rules of Thumb

Crosstalk Overview

- Key Topics:
- Mutual Inductance and capacitance
- Coupled noise
- Circuit Model
- Transmission line matrices

Crosstalk Overview

Mutual Inductance and Capacitance

- Crosstalk is the coupling of energy from one line to another via:
- Mutual capacitance (electric field)
- Mutual inductance (magnetic field)

Mutual Inductance, Lm

Mutual Capacitance, Cm

Zo

Zo

Zo

Zo

far

far

Cm

Lm

near

Zs

near

Zs

Zo

Zo

Crosstalk Overview

Mutual Inductance and Capacitance “Mechanism of coupling”

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

- 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

Crosstalk Induced Noise“Coupled Currents”

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

Zo

Zo

Zo

Zo

far

far

ICm

ILm

Lm

near

near

Zs

Zs

Zo

Zo

Crosstalk Overview

Crosstalk Induced Noise“Voltage Profile of Coupled Noise”

- 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” negative
- Current due to Lm larger than current due to Cm
- Note that far and crosstalk can be positive

Zo

Zo

Far End

Driven Line

Un-driven Line

“victim”

Zs

Near End

Driver

Zo

Crosstalk Overview

Time = 0

Near end crosstalk pulse at T=0 (Inear)

~Tr

V

Near end

crosstalk

Zo

TD

Far end crosstalk pulse at T=0 (Ifar)

~Tr

Time= 1/2 TD

2TD

V

Zo

far end

crosstalk

Zo

Time= TD

Far end of current

terminated at T=TD

V

Zo

Zo

Time = 2TD

Near end current

terminated at T=2TD

V

Zo

Zo

Graphical Explanation

Crosstalk Overview

Zo

Zo

Far End

Driven Line

Un-driven Line

“victim”

Zs

Near End

Driver

Zo

Crosstalk Equations

TD

Terminated Victim

A

B

Tr

~Tr

Tr

TD

2TD

Far End

Open Victim

Zo

Far End

Driven Line

Un-driven Line

“victim”

A

B

C

Zs

Near End

Driver

~Tr

Zo

~Tr

Tr

Crosstalk Overview

2TD

Crosstalk Equations

TD

Near End Open Victim

Zo

Zo

Far End

C

A

Driven Line

B

Un-driven Line

“victim”

Tr

Tr

Tr

Zs

Near End

2TD

Driver

3TD

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

Crosstalk Overview

Creating a Crosstalk Model“Equivalent Circuit”

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

C12

Line 2

Line 1

C1G

C2G

L11(N)

L11(2)

L11(1)

Line 1

C1G(1)

C1G(N)

C1G(2)

K1

K1

K1

C12(n)

C12(2)

C12(1)

Line 2

L22(N)

L22(2)

L22(1)

C2G(1)

C2G(N)

C2G(2)

Crosstalk Overview

Creating a Crosstalk Model“Transmission Line Matrices”

- 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 length
- LMN = the mutual inductance between line M and N

Inductance Matrix =

Crosstalk Overview

Creating a Crosstalk Model“Transmission Line Matrices”

- 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

Capacitance Matrix =

- For example, for the 2 line circuit shown earlier:

Crosstalk Overview

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:

v

R1

R2

Crosstalk Overview

200mV/div

100ps/div

Example (cont.)

Near end crosstalk voltage amplitude (from slide 12):

Far end crosstalk voltage amplitude (slide 12):

The propagation delay of the 2 inch line is:

Thus,

Crosstalk Overview

- 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

Even Mode

Odd Mode

Odd and Even Transmission Modes

- Electromagnetic Fields between two driven coupled lines will interact with each other
- These interactions will effect the impedance and delay of the transmission line
- A 2-conductor system will have 2 propagation modes
- Even Mode (Both lines driven in phase)
- Odd Mode (Lines driven 180o out of phase)

- The interaction of the fields will cause the system electrical characteristics to be directly dependent on patterns

Crosstalk Overview

Odd Mode Transmission

+1 -1

Electric Field:

Odd mode

Magnetic Field:

Odd mode

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

+1 -1

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

V

Drive (I)

I

Induced (-ILm)

Lm

Induced (ILm)

Drive (-I)

-I

Crosstalk Overview

Odd Mode Transmission“Derivation of Odd Mode Inductance”

L11

I1

Mutual Inductance:

Consider the circuit:

+ V1 -

I2

+ V2 -

L22

Since the signals for odd-mode switching are always opposite, I1 = -I2 and

V1 = -V2, so that:

Thus, since LO= L11 = L22,

Meaning that the equivalent inductance seen in an odd-mode environment

is reduced by the mutual inductance.

Crosstalk Overview

Odd Mode Transmission“Derivation of Odd Mode Capacitance”

V2

Mutual Capacitance:

Consider the circuit:

C1g

Cm

V2

C1g = C2g= CO = C11– C12

C2g

So,

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

Thus,

Meaning that the equivalent capacitance for odd mode switching increases.

Crosstalk Overview

Odd Mode Transmission“Odd Mode Transmission Characteristics”

Impedance:

Thus the impedance for odd mode behavior is:

Explain why.

Propagation Delay:

and the propagation delay for odd mode behavior is:

Crosstalk Overview

Even Mode Transmission

Electric Field:

Even mode

+1 +1

+1 +1

Magnetic Field:

Even mode

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

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

V

Drive (I)

I

Induced (ILm)

Lm

Induced (ILm)

Drive (I)

I

Crosstalk Overview

Even Mode TransmissionDerivation of even Mode Effective Inductance

L11

Mutual Inductance:

Again, consider the circuit:

I1

+ V1 -

I2

+ V2 -

L22

Since the signals for even-mode switching are always equal and in the same

direction so that I1 = I2 and V1 = V2, so that:

Thus,

Meaning that the equivalent inductance of even mode behavior increases

by the mutual inductance.

Crosstalk Overview

Even Mode TransmissionDerivation of even Mode Effective Capacitance

V2

Mutual Capacitance:

Again, consider the circuit:

C1g

Cm

V2

C2g

Thus,

Meaning that the equivalent capacitance during even mode behavior

decreases.

Crosstalk Overview

Even Mode Transmission“Even Mode Transmission Characteristics”

Impedance:

Thus the impedance for even mode behavior is:

Propagation Delay:

and the propagation delay for even mode behavior is:

Crosstalk Overview

Odd and Even Mode Comparison for Coupled Microstrips

Even mode (as seen on line 1)

Input waveforms

Impedance difference

V1

Odd mode (Line 1)

Probe point

Line 1

Line2

v1

v2

V2

Delay difference due to modal velocity differences

Crosstalk Overview

Microstrip vs. Stripline CrosstalkCrosstalk 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

Crosstalk Overview

Microstrip vs. Stripline CrosstalkCrosstalk 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

Microstrip E field patterns

+1 -1

+1 +1

Er=1.0

Er=1.0

Er=4.2

Er=4.2

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

Crosstalk Overview

+1 -1

+1 +1

Er=4.2

Er=4.2

Microstrip vs. Stripline CrosstalkCrosstalk Induced Velocity Changes

- 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

Stripline E field patterns

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

Crosstalk Overview

Microstrip vs. Stripline CrosstalkCrosstalk Induced Noise

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

- Since far end crosstalk takes the following form:
- Far end crosstalk is zero for a homogeneous Er

Crosstalk Overview

+1

R1

Odd Mode

Equivalent

-1

R2

R1

R3

Virtual Ground

in center

R2

2R3

+1

Even Mode

Equivalent

R1

-1

+1

2R3

R2

Termination TechniquesPi 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

Crosstalk Overview

Termination TechniquesPi and T networks

- The alternative is a PI termination
PI Termination

R1

R1

+1

½ R3

Odd Mode

Equivalent

R3

-1

½ R3

R2

R2

-1

+1

R1

Even Mode

Equivalent

+1

R2

Crosstalk Overview