Crosstalk

1
Crosstalk

• Calculation and SLEM

2
Topics

• Crosstalk and Impedance
• Superposition
• Examples
• SLEM

3
Cross Talk and Impedance

• Impedance is an electromagnetic parameter and is
  therefore effected by the electromagnetic
  environment as shown in the preceding slides.
• In the this second half, we will focus on looking
  at cross talk as a function of impedance and
  some of the benefits of viewing cross talk from
  this perspective.

4
Using Modal Impedances for Calculating Cross
Talk

• Any state can be described as a superposition of
  the system modes.
• Points to Remember
• Each mode has an impedance and velocity
  associated with it.
• In homogeneous medium, all the modal velocities
  will be equal.

5
Super Positioning of Modes
For a two line case, there are two modes

6
Two Coupled Line Example
Calculate the waveforms for two coupled lines
when one is driven from the low state to the high
and the other is held low.
Input
V
Line A
Time
1.0
V
Line B
Time
7
Two Coupled Line Example (Cont..)
First one needs the L and C matrices and then
I need the modal impedances and velocities. The
following L and C matrices were created in
HSPICE.

• Sanity Check
• The odd and even velocities are the same

Lo 3.02222e-007 3.34847e-008
3.02222e-007 Co 1.67493e-010
-1.85657e-011 1.67493e-010
Zodd 38.0 Ohms Vodd 1.41E08 m/s Zeven 47.5
Ohms Veven 1.41E08 m/s
8
Two Coupled Line Example (Cont..)
Now I deconvolve the the input voltage into the
even and odd modes
9
Two Coupled Line Example (Cont..)
Case i and Case ii are really the same A 0.5V
step into a Zeven47.5W line
Line B
Line A
Case ii
Case i
TdlenVeven8.98ns Vinit0.5VZeven/(Zeven30
Ohms) Vinit.306V Vrcvr2Vinit.612V
Zodd 38.0 Ohms Vodd 1.41E08 m/s Zeven 47.5
Ohms Veven 1.41E08 m/s
10
Two Coupled Line Example (Cont..)
Case iii is -0.5V step into a Zodd38W line
Line A
TdlenVodd8.98ns Vinit-0.5VZodd/(Zodd30O
hms) Vinit-.279V Vrcvr2Vinit-.558V
Case iii
Zodd 38.0 Ohms Vodd 1.41E08 m/s Zeven 47.5
Ohms Veven 1.41E08 m/s
11
Two Coupled Line Example (Cont..)
Case iv is 0.5V step into a Zodd38W line
Line B
Case iv
TdlenVodd8.98ns Vinit0.5VZodd/(Zodd30Oh
ms) Vinit.279V Vrcvr2Vinit.558V
Zodd 38.0 Ohms Vodd 1.41E08 m/s Zeven 47.5
Ohms Veven 1.41E08 m/s
Receiver (odd)
0.558V
0.279V
0.000V
0.0ns
9.0ns
12
Two Coupled Line Example (Cont..)
13
Two Coupled Line Example (Cont..)
Simulating in HSPICE results are identical to the
hand calculation
14
Assignment1

• Use PSPICE and perform previous simulations

15
Super Positioning of Modes
Continuing with the 2 line case, the following
L and C matrices were created in HSPICE for a
pair of microstrips

• Note
• The odd and even velocities are NOT the same

Zodd47.49243354 Ohms Vodd1.77E08m/s Zeven5
4.98942739 Ohms Veven1.64E08 m/s
Lo 3.02222e-007 3.34847e-008
3.02222e-007 Co 1.15083e-010
-4.0629e-012 1.15083e-010
16
Microstrip Example
The solution to this problem follows the same
approach as the previous example with one notable
difference. The modal velocities are different
and result in two different Tdelays Tdelay
(odd) 7.19ns Tdelay (even) 7.75ns This
means the odd mode voltages will arrive at the
end of the line 0.56ns before the even mode
voltages
17
Microstrip Cont..
HSPICE Results Single Bit switching, two coupled
microstrip example
18
HSPICE Results of Microstrip
The width of the pulse is calculated from the
mode velocities. Note that the widths increases
in 567ps increments with every transit
Calculation
567ps
1134ps
1701ps
2268ps
19
Assignment 2 and 3

• Use PSPICE and perform previous simulations

20
Modal Impedances for more than 2 lines

• So far we have looked at the two line crosstalk
  case, however, most practical busses use more
  than two lines.
• Points to Remember
• For N signal conductors, there are N modes.
• There are 3N digital states for N signal
  conductors
• Each mode has an impedance and velocity
  associated with it.
• In homogeneous medium, all the modal velocities
  will be equal.
• Any state can be described as a superposition of
  the modes

21
Three Conductor Considerations
There are 3N digital states for N signal
conductors
22
Three Coupled Microstrip Example
From HSPICE Lo 3.02174e-007
3.32768e-008 3.01224e-007 9.01613e-009
3.32768e-008 3.02174e-007 Co 1.15088e-010
-4.03272e-012 1.15326e-010
-5.20092e-013 -4.03272e-012 1.15088e-010
23
Three Coupled Microstrip Example
Actual modal info
Using the approximations gives
Z1,1,159.0Ohms
Z1,-1,144.25Ohms
Modal velocities
The three mode vectors
The Approx. impedances and velocities are pretty
close to the actual, but much simpler to
calculate.
24
Three Coupled Microstrip ExampleSingle Bit
Example HSPICE Result
25
Points to Remember

• The modal impedances can be used to hand
  calculate crosstalk waveforms
• Any state can be described as a superposition of
  the modes
• For N signal conductors, there are N modes.
• There are 3N digital states for N signal
  conductors
• Each mode has an impedance and velocity
  associated with it.
• In homogeneous medium, all the modal velocities
  will be equal.

26
Crosstalk Trends

• Key Topics
• Impedance vs. Spacing
• SLEM
• Trading Off Tolerance vs. Spacing

27
Impedance vs Line Spacing

• As we have seen in the preceding sections,
• 1) Cross talk changes the impedance of the line
• 2) The further the lines are spaced apart the the
  less the impedance changes

28
Single Line Equivalent Model (SLEM)

• SLEM is an approximation that allows some cross
  talk effects to be modeled without running fully
  coupled simulations
• Why would we want to avoid fully coupled
  simulations?
• Fully coupled simulations tend to be time
  consuming and dependent on many assumptions

29
Single Line Equivalent Model (SLEM)

• Using the knowledge of the cross talk impedances,
  one can change a single transmission lines
  impedance to approximate
• Even, Odd, or other state coupling

Equiv to Even State Coupling
Equiv to Odd State Coupling
30
Single Line Equivalent Model (SLEM)

• Limitations of SLEM
• SLEM assumes the transmission line is in a
  particular state (odd or even) for its entire
  segment length
• This means that the edges are in perfect phase
• It also means one can not simulate random bit
  patterns properly with SLEM (e.g. Odd - Single
  Bit - Even state)

The edges maybe in phase here, but not here
1 2 3
1 2 3
Three coupled lines, two with serpentining
31
Single Line Equivalent Model (SLEM)

• How does one create a SLEM model?
• There are a few ways
• Use the L and C matrices along with the
  approximations
• Use the L and C matrices along with Weimins
  MathCAD program
• Excite the coupled simulation in the desired
  state and back calculate the equivalent impedance
  (essentially TDR the simulation)

32
Trading Off Tolerance vs. Spacing

• Ultimately in a design you have to create
  guidelines specifying the trace spacing and
  specifying the tolerance of the motherboard
  impedance
• i.e. 10mil edge to edge spacing with 10
  impedance variation
• Thinking about the spacing in terms of impedance
  makes this much simpler

33
Trading Off Tolerance vs. Spacing

• Assume you perform simulations with no coupling
  and you find a solution space with an impedance
  range of
• Between 35W to 100W
• Two possible 65W solutions are
• 15mil spacing with 15 impedance tolerance
• 10mil spacing with 5 impedance tolerance

34
Reducing Cross Talk

• Separate traces farther apart
• Make the traces short compared to the rise time
• Make the signals out of phase
• Mixing signals which propagate in opposite
  directions may help or hurt (recall reverse cross
  talk!)
• Add Guard traces
• One needs to be careful to ground the guard
  traces sufficiently, otherwise you could actually
  increase the cross talk
• At GHz frequency this becomes very difficult and
  should be avoided
• Route on different layers and route orthogonally

35
In Summary

• Cross talk is unwanted signals due to coupling or
  leakage
• Mutual capacitance and inductance between lines
  creates forward and backwards traveling waves on
  neighboring lines
• Cross talk can also be analyzed as a change in
  the transmission lines impedance
• Reverse cross talk is often the dominate cross
  talk in a design
• (just because the forward cross talk is small or
  zero, does not mean you can ignore cross talk!)
• A SLEM approach can be used to budget impedance
  tolerance and trace spacing