Crosstalk - How can we avoid it - Herv Grabas Mutual Inductance and - - PowerPoint PPT Presentation

Crosstalk

• How can we avoid it -

Hervé Grabas

• 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

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
• pposite 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

• 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    

• 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

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

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

Crosstalk Overview

Creating a Crosstalk Model

“Equivalent Circuit”

• The circuit must be distributed into N segments

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

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

• Potential difference between the conductors lead to an increase
• f 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

• 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

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

• dd

Z R R  

2 1

 

• dd

even

Z Z R   2 1

3

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

• dd

even

• dd

even

Z Z Z Z R   2

3 R1 R2

Formula : near end cross talk

Near end cross talk is usually dominant

Formula far end cross talk

Odd and even impedance

Impedance Variation for a Three Conductor Stripline (Width=5[mils]) 20 40 60 80 100 120 5 10 15 20 Edge to Edge Spacing [mils] Impedance[Ohms] Z single bit states Z odd states Z even states

Some results

Some results

Jason & Ossy ceramic plate

4GHz of bandwidth

• 10dB of coupling up to

2.3GHz

Some results

Some results

LAPPD glass plate

800MHz of bandwidth

• 10dB of coupling up to

300MHz

Conclusion

More work needed on the coupling. Plan :

• Look back at the odd and even mode

impedance values

• Make a Pi termination
• Create a model of the coupling and test it

Different topic: MCP simulation

Can we create a model of this?

The answer is yes

This is for a perfect Dirac distributed current with constant velocity

Plot of previous function

Parameters : High voltage across gap V = 100V Gap = 1e-3 m 3dB = 1.14GHz

This is for a perfect Dirac distributed current with constant acceleration

Plot of previous function

3dB = 2.3GHz

Case of space distributed charge

This is for a Rect distributed over a distance s current with constant speed

3dB = 830MHz

Space charge : the small the electron cloud spreading the better