crosstalk
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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
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