1. Crosstalk
Overview and Modes

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

3. Crosstalk Induced Noise
Key Topics:
Mutual Inductance and capacitance
Coupled noise
Circuit Model
Transmission line matrices
Crosstalk Overview

4. 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 Capacitance, Cm
Mutual Inductance, Lm Zo Zo Zo Zo
far
far Cm Lm Zs
near
near Zs Zo Zo
Crosstalk Overview

5. 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

6. 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 Lm
ILm
near Zs
near Zs Zo Zo
Crosstalk Overview

7. 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

8. Graphical ExplanationTD
2TD
~Tr
far end
crosstalk
Near end V
Time = 0 Zo
Near end crosstalk pulse at T=0 (Inear)
Far end crosstalk pulse at T=0 (Ifar) Zo V
Time= 1/2 TD Zo V
Time= TD
Far end of current
terminated at T=TD Zo V
Time = 2TD
Near end current
terminated at T=2TD
Crosstalk Overview

9. Crosstalk Equations Terminated Victim Far End Open VictimTD
Driven Line
Un-driven Line
“victim”
Driver Zs Zo
Near End
Far End
Terminated Victim A B Tr
~Tr Tr TD
Far End
Open Victim
2TD Zo
Far End
Driven Line
Un-driven Line
“victim” A B C Zs
Near End
Driver Zo Tr
~Tr
~Tr
Crosstalk Overview
2TD

10. Crosstalk Equations TD
Near End Open Victim Zo Zo
Far End A C
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

11. Creating a Crosstalk Model “Equivalent Circuit”
The circuit must be distributed into N segments as shown in chapter 2 K1
L11(1)
L22(1)
C1G(1)
C12(1)
L11(2)
L22(2)
C1G(2)
C12(2)
C2G(2)
C2G(1)
L11(N)
L22(N)
C1G(N)
C12(n)
C2G(N)
C1G
C2G
C12
Line 1
Line 2
Crosstalk Overview

12. 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

13. 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

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: v R1 R2
Crosstalk Overview

15. 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:
100ps/div
200mV/div
Thus,
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 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
Even Mode
Odd Mode
Crosstalk Overview

18. Odd Mode Transmission +1 -1 +1 -1
Potential difference between the conductors lead to an increase of the effective Capacitance equal to the mutual capacitance
Electric Field:
Odd mode
Magnetic Field:
Odd mode
Because currents are flowing in opposite directions, the total inductance is reduced by the mutual inductance (Lm) V
Drive (I)
Induced (-ILm) I Lm
Induced (ILm)
Drive (-I) -I
Crosstalk Overview

19. 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

20. Odd Mode Transmission “Derivation of Odd Mode Capacitance”
Mutual Capacitance:
Consider the circuit:
C1g Cm
C1g = C2g = CO = C11 – C12 V2
C2g
So,
And again, I1 = -I2 and V1 = -V2, so that:
Thus,
Meaning that the equivalent capacitance for odd mode switching increases.
Crosstalk Overview

21. 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

22. Even Mode Transmission
Since the conductors are always at a equal potential, the effective capacitance is reduced by the mutual capacitance
Electric Field:
Even mode
Magnetic Field:
Because currents are flowing in the same direction, the total inductance is increased by the mutual inductance (Lm) V
Drive (I)
Induced (ILm) I Lm
Induced (ILm)
Drive (I) I
Crosstalk Overview

23. Even Mode Transmission Derivation of even Mode Effective Inductance
Mutual Inductance:
Again, consider the circuit:
L11 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

24. Even Mode Transmission Derivation of even Mode Effective Capacitance
Mutual Capacitance:
Again, consider the circuit: V2
C1g Cm V2
C2g
Thus,
Meaning that the equivalent capacitance during even mode behavior
decreases.
Crosstalk Overview

25. 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

26. 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

27. Microstrip vs. Stripline Crosstalk Crosstalk 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

28. Microstrip vs. Stripline Crosstalk Crosstalk 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
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

29. Microstrip vs. Stripline Crosstalk Crosstalk 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
Er=4.2
Subsequently, if the transmission line is implemented in a homogeneous dielectric, the velocity must stay constant between even and odd mode patterns
Crosstalk Overview

30. Microstrip vs. Stripline Crosstalk Crosstalk 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

31. Termination Techniques Pi 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
Odd Mode
Equivalent -1 R1 R2
Virtual Ground
in center -1 R1 R2 R3 +1
Even Mode
Equivalent R1 R2
2R3
Crosstalk Overview

32. Termination Techniques Pi 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