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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 Crosstalk Overview Crosstalk Induced Noise 3 Key Topics: Mutual Inductance and capacitance Coupled noise Circuit Model Transmission line matrices Crosstalk Overview Mutual Inductance and Capacitance 4 Crosstalk is the coupling of energy from one line to another via: Mutual capacitance (electric field) Mutual inductance (magnetic field) Mutual Capacitance, Cm Mutual Inductance, Lm Zo Zo Zo Zo far far Cm Lm Zs near near Zs Zo Zo Crosstalk Overview 5 Mutual Inductance and Capacitance “Mechanism of coupling” The circuit element that represents this transfer of energy are the following familiar equations dI dV VLm Lm I Cm Cm dt dt 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 Crosstalk Induced Noise “Coupled 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 Lm ILm Zs near near Zs Zo Zo I near I Cm I Lm I far I Cm I Lm Crosstalk Overview 7 Crosstalk Induced Noise “Voltage Profile of Coupled Noise” 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 Zo Zo Far End Driven Line Un-driven Line “victim” Zs Near End Driver Zo Crosstalk Overview 8 Graphical Explanation Time = 0 Near end crosstalk pulse at T=0 (Inear) ~Tr Near end V crosstalk Zo TD Far end crosstalk pulse at T=0 (Ifar) Time= 1/2 TD ~Tr 2TD V Zo far end Zo crosstalk Time= TD V Zo Zo Far end of current terminated at T=TD Time = 2TD V Near end current Zo terminated at T=2TD Zo Crosstalk Overview 9 Crosstalk Equations TD Zo Vinput LM CM A Terminated Victim Zo 4 L C Far End TD X LC Driven Line Un-driven Line Vinput X LC LM CM B L C “victim” 2Tr A B Zs Near End Driver Zo Tr ~Tr Tr TD 2TD Far End Zo Open Victim Vinput LM C M A 4 L C Far End Driven Line Un-driven Line “victim” A 1 B B C C 2 Zs Near End Driver Zo Tr ~Tr ~Tr Vinput X LC LM C M C L C Tr 2TD Crosstalk Overview 10 Crosstalk Equations TD Near End Open Victim Vinput LM C M Zo A Zo 2 L C Far End A C Driven Line Vinput LM C M C B Un-driven Line 4 L C “victim” Tr Tr Tr Vinput X LC LM C M Zs Near End B L C 2TD Driver 2Tr 3TD The Crosstalk noise characteristics are dependent on the termination of the victim line Crosstalk Overview Creating a Crosstalk Model 11 “Equivalent Circuit” The circuit must be distributed into N segments as shown in chapter 2 C12 Line 1 Line 2 L12 K C1G C2G L11L22 L11(1) L11(2) L11(N) Line 1 C1G(1) C1G(2) C1G(N) K1 K1 K1 C12(n) C12(1) C12(2) Line 2 L22(1) C2G(1) L22(2) C2G(2) L22(N) C2G(N) Crosstalk Overview Creating a Crosstalk Model 12 “Transmission 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 L11 L12 ... L1N L L22 Inductance Matrix = 21 LN 1 LNN Crosstalk Overview Creating a Crosstalk Model 13 “Transmission Line Matrices” The Capacitance matrix is shown, where: CNN = the self capacitance of line N per unit length where: C NN C NG Cm utuals CNG = The capacitance between line N and ground CMN = Mutual capacitance between lines M and N C11 C12 ... C1 N C C22 Capacitance Matrix = 21 C N 1 C NN For example, for the 2 line circuit shown earlier: C11 C1G C12 Crosstalk Overview 14 Example Calculate near and far end crosstalk-induced noise magnitudes and sketch the waveforms of circuit shown below: v R1 R2 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: 9.869nH 2.103nH L / inch = 2.103nH 9.869nH 2.051 pF 0.239 pF C / inch = 0.239 pF 2.051 pF L11 9.869nH The characteristic impedance is: ZO 69.4 C11 2.051 pF Therefore the system has matched termination. The crosstalk noise magnitudes can be calculated as follows: Crosstalk Overview 15 Example (cont.) Near end crosstalk voltage amplitude (from slide 12): Vinput L12 C12 1V 2.103nH 0.239 pF Vnear 9.869nH 2.051 pF 0.082V 4 L11 C11 4 Far end crosstalk voltage amplitude (slide 12): Vinput ( X LC ) L12 C12 1V * 2inch * 9.869nH * 2.051 pF 2.103nH 0.239 pF V far L C 9.869nH 2.051 pF 0.137V 2Trise 11 11 2 *100 ps The propagation delay of the 2 inch line is: TD X LC 2inch * (9.869nH * 2.051nH 0.28ns 200mV/div Thus, Crosstalk Overview 100ps/div 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 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) Even Mode Odd Mode The interaction of the fields will cause the system electrical characteristics to be directly dependent on patterns Crosstalk Overview Odd Mode Transmission 18 Potential difference between the conductors lead to an increase of the effective Capacitance equal to the mutual capacitance +1 -1 +1 -1 Electric Field: Magnetic Field: Odd mode Odd mode Because currents are flowing in opposite directions, the total inductance is reduced by the mutual inductance (Lm) dI d ( I ) Drive (I) V V L Lm dt dt Induced (-ILm) I dI Induced (ILm) Lm ( L Lm) dt Drive (-I) -I Crosstalk Overview Odd Mode Transmission 19 “Derivation of Odd Mode Inductance” I1 L11 Mutual Inductance: Consider the circuit: + V1 - Lm k I2 + V2 - L11L22 dI 1 dI V1 LO Lm 2 dt dt L22 dI dI V2 LO 2 Lm 1 dt dt Since the signals for odd-mode switching are always opposite, I1 = -I2 and V1 = -V2, so that: V L dI 1 L d ( I 1 ) ( L L ) dI 1 1 O m O m dt dt dt dI d ( I 2 ) dI V2 LO 2 Lm ( LO Lm ) 2 dt dt dt Thus, since LO = L11 = L22, Lodd L11 Lm L11 L12 Meaning that the equivalent inductance seen in an odd-mode environment is reduced by the mutual inductance. Crosstalk Overview Odd Mode Transmission 20 “Derivation of Odd Mode Capacitance” V2 Mutual Capacitance: Consider the circuit: C1g Cm C1g = C2g = CO = C11 – C12 C2g V2 So, dV1 d (V1 V2 ) dV dV I1 CO Cm (C O C m ) 1 C m 2 dt dt dt dt dV d (V2 V1 ) dV dV I 2 CO 2 C m (C O C m ) 2 C m 1 dt dt dt dt And again, I1 = -I2 and V1 = -V2, so that: dV1 d (V1 (V1 )) dV I 1 CO Cm (C1g 2C m ) 1 dt dt dt dV2 d (V2 (V2 )) dV I 2 CO Cm (C O 2C m ) 2 dt dt dt Thus, Codd C1g 2Cm C11 Cm Meaning that the equivalent capacitance for odd mode switching increases. Crosstalk Overview Odd Mode Transmission 21 “Odd Mode Transmission Characteristics” Impedance: Thus the impedance for odd mode behavior is: Lodd L11 L12 Z odd Codd C11 C12 ( Note : Z differential 2 Z odd ) Explain why. Propagation Delay: and the propagation delay for odd mode behavior is: TDodd LoddCodd ( L11 L12 )(C11 C12 ) Crosstalk Overview Even Mode Transmission 22 Since the conductors are always at a equal potential, the effective capacitance is reduced by the mutual capacitance +1 +1 +1 +1 Electric Field: Magnetic Field: Even mode Even mode Because currents are flowing in the same direction, the total inductance is increased by the mutual inductance (Lm) dI d (I ) Drive (I) V V L Lm dt dt Induced (ILm) I dI Induced (ILm) Lm ( L Lm) dt Drive (I) I Crosstalk Overview Even Mode Transmission 23 Derivation of even Mode Effective Inductance L11 Mutual Inductance: I1 Again, consider the circuit: + V1 - Lm dI dI k V1 LO 1 Lm 2 I2 + V2 - L11L22 dt dt dI dI L22 V2 LO 2 Lm 1 dt dt Since the signals for even-mode switching are always equal and in the same direction so that I1 = I2 and V1 = V2, so that: dI1 d ( I1 ) dI V1 LO Lm ( LO Lm ) 1 dt dt dt dI d (I 2 ) dI V2 LO 2 Lm ( LO Lm ) 2 dt dt dt Thus, Leven L11 Lm L11 L12 Meaning that the equivalent inductance of even mode behavior increases by the mutual inductance. Crosstalk Overview Even Mode Transmission 24 Derivation of even Mode Effective Capacitance V2 Mutual Capacitance: Again, consider the circuit: C1g Cm C2g V2 dV1 d (V1 V1 ) dV I 1 CO Cm CO 1 dt dt dt dV d (V2 V2 ) dV I 2 CO 2 C m CO 2 dt dt dt Thus, Ceven C0 C11 Cm Meaning that the equivalent capacitance during even mode behavior decreases. Crosstalk Overview Even Mode Transmission 25 “Even Mode Transmission Characteristics” Impedance: Thus the impedance for even mode behavior is: Leven L11 L12 Z even Ceven C11 C12 Propagation Delay: and the propagation delay for even mode behavior is: TDeven LevenCeven ( L11 L12 )(C11 C12 ) Crosstalk Overview 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) Line 1 Probe point v1 v2 Line2 V2 Delay difference due to modal velocity differences Crosstalk Overview Microstrip vs. Stripline Crosstalk 27 Crosstalk Induced Velocity Changes Chapter 2 defined propagation delay as T r pd c 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 Crosstalk Overview Microstrip vs. Stripline Crosstalk 28 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 Er=1.0 Er=1.0 Er=4.2 Er=4.2 The effective dielectric constant, and subsequently the propagation velocity depends on the electric field patterns Crosstalk Overview Microstrip vs. Stripline Crosstalk 29 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 +1 +1 +1 -1 Er=4.2 Er=4.2 Subsequently, if the transmission line is implemented in a homogeneous dielectric, the velocity must stay constant between even and odd mode patterns Crosstalk Overview Microstrip vs. Stripline Crosstalk 30 Crosstalk Induced Noise The constant velocity in a homogeneous media (such as a stripline) forces far end crosstalk noise to be zero TDodd TDeven ( L11 L12 )(C11 C12 ) ( L11 L12 )(C11 C12 ) L12C11 L11C12 L11C12 L12C11 L12 C12 L11 C11 Since far end crosstalk takes the following form: Vinput X LC L12 C12 Crosstalk ( far _ stripline) 0 2Tr L11 C11 Far end crosstalk is zero for a homogeneous Er Crosstalk Overview Termination Techniques 31 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 Odd Mode +1 R1 Equivalent R1 R3 -1 R2 R2 Virtual Ground in center -1 2R3 R1 R2 Z odd Even Mode +1 R1 Equivalent R3 Z even Z odd 1 +1 2R3 R2 2 Crosstalk Overview Termination Techniques 32 Pi and T networks The alternative is a PI termination PI Termination R1 R1 Odd Mode +1 ½ R3 R3 Equivalent -1 ½ R3 R2 R2 -1 +1 R1 R1 R2 Z even Even Mode Equivalent +1 R2 Z evenZ odd R3 2 Z even Z oddCrosstalk Overview

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