Crosstalk

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

3
Crosstalk Induced Noise

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

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 Inductance, Lm
Mutual Capacitance, Cm
Zo
Zo
Zo
Zo
far
far
Cm
Lm
near
Zs
near
Zs
Zo
Zo
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
  (Lenzs Law)
• The mutual capacitance will pass current through
  the mutual capacitance that flows in both
  directions on the victim line

6
Crosstalk Induced NoiseCoupled 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
7
Crosstalk Induced NoiseVoltage Profile of
Coupled Noise

• Near end crosstalk is always positive
• Currents from Lm and Cm always add and flow into
  the node
• For PCBs, 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
8
Graphical Explanation
9
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
Zo
Tr
Tr
Tr
2TD
10
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

11
Creating a Crosstalk ModelEquivalent Circuit
12
Creating a Crosstalk ModelTransmission 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
13
Creating a Crosstalk ModelTransmission 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

14
Example
Calculate near and far end crosstalk-induced
noise magnitudes and sketch the waveforms of
circuit shown below Vsource2V, (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
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
Thus,
16
Effect of Crosstalk on Transmission line
Parameters

• Key Topics
• Odd and Even Mode Characteristics
• Microstrip vs. Stripline
• Modal Termination Techniques
• Modal Impedances for more than 2 lines
• Effect Switching Patterns
• Single Line Equivalent Model (SLEM)

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

18
Odd Mode Transmission

• 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
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.
20
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.
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
22
Even Mode Transmission

• 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
23
Even Mode Transmission Derivation 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.
24
Even Mode Transmission Derivation 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.
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
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
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

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
1 -1
1 1
Er1.0
Er1.0
Er4.2
Er4.2

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

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

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

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

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

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