2. 2
Crosstalk Overview
What is Crosstalk?
Crosstalk Induced Noise
Effect of crosstalk on transmission line
parameters
Crosstalk Trends
Design Guidelines and Rules of Thumb
Overview
4. 4
Crosstalk Overview
Crosstalk is the coupling of energy from one line
to another via:
Mutual capacitance (electric field)
Mutual inductance (magnetic field)
Mutual Inductance and Capacitance
Zs
Zo
Zo
Zo
Mutual Capacitance, Cm Mutual Inductance, Lm
Zs
Zo
Zo
Zo
Cm
Lm
near
far
near
far
5. 5
Crosstalk Overview
The circuit element that represents this
transfer of energy are the following familiar
equations
Mutual Inductance and Capacitance
“Mechanism of coupling”
dt
dI
L
V m
Lm
dt
dV
C
I m
Cm
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
6. 6
Crosstalk Overview
The near and far end victim line currents sum to
produce the near and the far end crosstalk
noise
Crosstalk Induced Noise
“Coupled Currents”
Zs
Zo
Zo
Zo
Zs
Zo
Zo
Zo
ICm
Lm
near
far
near
far
ILm
Lm
Cm
far
Lm
Cm
near I
I
I
I
I
I
7. 7
Crosstalk Overview
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
Crosstalk Induced Noise
“Voltage Profile of Coupled Noise”
Driven Line
Un-driven Line
“victim”
Driver
Zs
Zo
Zo
Zo
Near End
Far End
8. 8
Crosstalk Overview
Graphical Explanation
TD
2TD
~Tr
~Tr
far end
crosstalk
Near end
crosstalk
Zo
V
Time = 2TD
Zo
Near 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
Zo
V
Time= 1/2 TD
Zo
V
Time= TD
Zo Far end of current
terminated at T=TD
9. 9
Crosstalk Overview
Crosstalk 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
LC
X
TD
C
C
L
L
V
A M
M
input
4
C
C
L
L
T
LC
X
V
B M
M
r
input
2
TD
2TD
Tr ~Tr Tr
A
B
TD
2TD
Tr ~Tr ~Tr
A
B
C
C
L
L
V
A M
M
input
4
C
B
2
1
C
C
L
L
T
LC
X
V
C M
M
r
input
C
Terminated Victim
Far End
Open Victim
10. 10
Crosstalk Overview
Crosstalk 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
L
V
A M
M
input
2
C
C
L
L
T
LC
X
V
B M
M
r
input
2
C
C
L
L
V
C M
M
input
4
The Crosstalk noise characteristics are
dependent on the termination of the victim line
11. 11
Crosstalk Overview
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)
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
22
11
12
L
L
L
K
Line 1
Line 2
Line 1 Line 2
12. 12
Crosstalk Overview
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
Creating a Crosstalk Model
“Transmission Line Matrices”
Inductance Matrix =
NN
N
N
L
L
L
L
L
L
L
1
22
21
1
12
11 ...
13. 13
Crosstalk Overview
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 Model
“Transmission Line Matrices”
Capacitance Matrix =
NN
N
N
C
C
C
C
C
C
C
1
22
21
1
12
11 ...
mutuals
NG
NN C
C
C
12
1
11 C
C
C G
For example, for the 2 line circuit shown earlier:
14. 14
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:
nH
nH
nH
nH
869
.
9
103
.
2
103
.
2
869
.
9
pF
pF
pF
pF
051
.
2
239
.
0
239
.
0
051
.
2
4
.
69
051
.
2
869
.
9
11
11
pF
nH
C
L
ZO
v
R1 R2
15. 15
Crosstalk Overview
Example (cont.)
V
pF
pF
nH
nH
V
C
C
L
L
V
V
input
near 082
.
0
051
.
2
239
.
0
869
.
9
103
.
2
4
1
4 11
12
11
12
V
pF
pF
nH
nH
ps
pF
nH
inch
V
C
C
L
L
T
LC
X
V
V
rise
input
far 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
200mV/div
The propagation delay of the 2 inch line is:
ns
nH
nH
inch
LC
X
TD 28
.
0
051
.
2
*
869
.
9
(
*
2
16. 16
Crosstalk Overview
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)
17. 17
Crosstalk Overview
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
Odd and Even Transmission Modes
Even Mode
Odd Mode
18. 18
Crosstalk Overview
Potential difference between the conductors lead to an
increase of the effective Capacitance equal to the mutual
capacitance
Odd 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
Lm
dt
dI
Lm
L
dt
I
d
Lm
dt
dI
L
V
)
(
)
(
I
19. 19
Crosstalk Overview
Odd Mode Transmission
“Derivation of Odd Mode Inductance”
12
11
11 L
L
L
L
L m
odd
Mutual Inductance:
Consider the circuit:
dt
dI
L
dt
dI
L
V
dt
dI
L
dt
dI
L
V
m
O
m
O
1
2
2
2
1
1
22
11L
L
L
k m
L11
L22
I2
I1
+ V2 -
+ V1 -
Since the signals for odd-mode switching are always opposite, I1 = -I2 and
V1 = -V2, so that:
dt
dI
L
L
dt
I
d
L
dt
dI
L
V
dt
dI
L
L
dt
I
d
L
dt
dI
L
V
m
O
m
O
m
O
m
O
2
2
2
2
1
1
1
1
)
(
)
(
)
(
)
(
Thus, since LO = L11 = L22,
Meaning that the equivalent inductance seen in an odd-mode environment
is reduced by the mutual inductance.
20. 20
Crosstalk Overview
Odd Mode Transmission
“Derivation of Odd Mode Capacitance”
m
m
g
odd C
C
C
C
C
11
1 2
Mutual Capacitance:
Consider the circuit:
C2g
C1g
Cm
V2
V2
C1g = C2g = CO = C11 – C12
So,
dt
dV
C
dt
dV
C
C
dt
V
V
d
C
dt
dV
C
I
dt
dV
C
dt
dV
C
C
dt
V
V
d
C
dt
dV
C
I
m
m
O
m
O
m
m
O
m
O
1
2
1
2
2
2
2
1
2
1
1
1
)
(
)
(
)
(
)
(
And again, I1 = -I2 and V1 = -V2, so that:
dt
dV
C
C
dt
V
V
d
C
dt
dV
C
I
dt
dV
C
C
dt
V
V
d
C
dt
dV
C
I
m
O
m
O
m
g
m
O
2
2
2
2
2
1
1
1
1
1
1
)
2
(
))
(
(
)
2
(
))
(
(
Thus,
Meaning that the equivalent capacitance for odd mode switching increases.
21. 21
Crosstalk Overview
Odd Mode Transmission
“Odd Mode Transmission Characteristics”
Impedance:
Thus the impedance for odd mode behavior is:
)
2
:
(
12
11
12
11
odd
al
differenti
odd
odd
odd
Z
Z
Note
C
C
L
L
C
L
Z
and the propagation delay for odd mode behavior is:
)
)(
( 12
11
12
11 C
C
L
L
C
L
TD odd
odd
odd
Propagation Delay:
Explain why.
22. 22
Crosstalk Overview
Since the conductors are always at a equal potential, the
effective capacitance is reduced by the mutual capacitance
Even 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
Lm
dt
dI
Lm
L
dt
I
d
Lm
dt
dI
L
V
)
(
)
(
I
Electric Field:
Even mode
Magnetic Field:
Even mode
+1 +1
+1 +1
23. 23
Crosstalk Overview
Even Mode Transmission
Derivation of even Mode Effective Inductance
12
11
11 L
L
L
L
L m
even
22
11L
L
L
k 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 same
direction so that I1 = I2 and V1 = V2, so that:
dt
dI
L
dt
dI
L
V
dt
dI
L
dt
dI
L
V
m
O
m
O
1
2
2
2
1
1
dt
dI
L
L
dt
I
d
L
dt
dI
L
V
dt
dI
L
L
dt
I
d
L
dt
dI
L
V
m
O
m
O
m
O
m
O
2
2
2
2
1
1
1
1
)
(
)
(
)
(
)
(
Thus,
Meaning that the equivalent inductance of even mode behavior increases
by the mutual inductance.
24. 24
Crosstalk Overview
Even Mode Transmission
Derivation of even Mode Effective Capacitance
m
even C
C
C
C
11
0
Mutual Capacitance:
Again, consider the circuit:
C2g
C1g
Cm
V2
V2
dt
dV
C
dt
V
V
d
C
dt
dV
C
I
dt
dV
C
dt
V
V
d
C
dt
dV
C
I
O
m
O
O
m
O
2
2
2
2
2
1
1
1
1
1
)
(
)
(
Thus,
Meaning that the equivalent capacitance during even mode behavior
decreases.
25. 25
Crosstalk Overview
Even Mode Transmission
“Even Mode Transmission Characteristics”
Impedance:
Thus the impedance for even mode behavior is:
12
11
12
11
C
C
L
L
C
L
Z
even
even
even
and the propagation delay for even mode behavior is:
)
)(
( 12
11
12
11 C
C
L
L
C
L
TD even
even
even
Propagation Delay:
26. 26
Crosstalk Overview
Odd and Even Mode Comparison for
Coupled Microstrips
Input waveforms
Even 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
27. 27
Crosstalk Overview
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
c
T r
pd
28. 28
Crosstalk Overview
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
+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
29. 29
Crosstalk Overview
Microstrip vs. Stripline Crosstalk
Crosstalk 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=4.2
Er=4.2
Stripline E field patterns
30. 30
Crosstalk Overview
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
11
12
11
12
11
12
12
11
12
11
11
12
12
11
12
11
12
11
12
11 )
)(
(
)
)(
(
C
C
L
L
C
L
C
L
C
L
C
L
C
C
L
L
C
C
L
L
TD
TD even
odd
0
2
)
_
(
11
12
11
12
C
C
L
L
T
LC
X
V
stripline
far
Crosstalk
r
input
Since far end crosstalk takes the following form:
Far end crosstalk is zero for a homogeneous Er
31. 31
Crosstalk Overview
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
R1
R2
R3
+1
Odd Mode
Equivalent
-1
R1
R2
Virtual Ground
in center
+1
Even Mode
Equivalent
+1
R1
R2
2R3
2R3
odd
Z
R
R
2
1
odd
even Z
Z
R
2
1
3
32. 32
Crosstalk Overview
Termination Techniques
Pi and T networks
The alternative is a PI termination
PI Termination
+1
Odd Mode
Equivalent
-1
R1
R2
R3
-1
½ R3
½ R3
+1
Even Mode
Equivalent +1
R1
R2
even
Z
R
R
2
1
odd
even
odd
even
Z
Z
Z
Z
R
2
3
R1
R2
