Differential Pair Transmission Lines - West Michigan

1 Differential Pair Transmission Lines2014 IEEE International Symposium on Electromagnetic CompatibilityDifferential Pair Transmission Lines are Differential Pair Transmission Lines ? Makes a Good Differential Pair Transmission line ? : A Good Differential Makes a Bad Differential Pair Transmission line ? : Effect of Different Trace : Effect of Symmetrical and Asymmetrical : Tight or Loose? : Symmetrical Loose and Tight Mode : Common Mode Choke on : Common Mode Choke Measurements with What are Differential Pair Transmission Lines ? Differential Pair Transmission Lines4A Differential Pair Transmission line is any two conductive paths used to transfer Pair Transmission Lines5 The signals are equal in amplitude but opposite in Pair Transmission Lines6 The majority of the return currents for each line are in the Ground/Power Planes.

2 Differential Pair Transmission Lines : Any signal on a Differential pair can be described by a Differential signal component and a common signal component. Each component will see a different impedance as it propagates down the Pair Transmission Lines : Differential signaling has many signal integrity advantages over single ended signals, such as: Contributing to less rail collapse, Less EMI, Better noise immunity, and Less sensitivity to Pair Transmission Lines :910II. What Makes a GoodDifferential Pair Transmission line ?What Properties make a Good Differential Pair Transmission line : UNIFORM cross section is the most important property. MATCHED TIME DELAY (electrical trace length) between each line is the second most important property. SYMMETRY the same line width and dielectric spacing IMPEDANCE MATCHING of Source, Transmission line , and , IMPEDANCE MATCHINGMATCHEDTIME DELAY12 III: Demonstration: A Good Differential PairA12A Good Differential Pair:13 Tight Coupled Symmetrical Differential PairNote the well defined Differential signals and minimum skew trademarks of a well designed Differential is Skew?

3 Skew the time delay between two or more nets. It can be controlled .. by matching the length of the nets.*14* Dr. Eric Bogatin: Signal and Power Integrity Simplified, Pages 9 and 533 Common Mode Signal generated by skew even with the common signal What Makes a BadDifferential Pair?What Properties make a Bad Differential Pair Transmission line : Asymmetry: Between Differential Traces (Test Pads/Stubs/Plane Jumps/vias) Between Differential Trace Electrical Lengths (Time/Phase Delay) Changes in: Distances between Traces (Impedance Changes) Changes in Trace Width (Impedance Changes) Inconsistent Return Paths: Breaks in Planes Layer JumpingAny asymmetries will convert Differential signals into common mode signals1617V. Demonstration of the Effect of Different Trace Lengths: B1218 Skew: Close Traces with Different Trace Lengths vs.

4 Close Traces with Equal Trace LengthsClose Traces with Different Trace Lengths Close Traces with Equal Trace LengthNote the increase of Skew19 Skew: Far Traces with Different Trace Lengths vs. Close Traces with Different Trace LengthsFar Traces with Different Trace Lengths Note the equal increase in amplitude of Skew with change with Close or Far CouplingClose Traces with Different Trace Lengths 20VI. Demonstration of the Effect of Symmetrical and Asymmetrical StubsD1221 Skew: Tight Coupling Asymmetrical Stubs vs. Tight Coupled Symmetrical StubsTight Coupled Asymmetrical StubsTight Coupled Symmetrical StubsNote that in this instance of lower frequency and rise/fall times, the placement of the stubs has little or no effect. However at higher frequencies, expect to see an affect on the signal and increased Coupling: Tight or Loose?

5 Loose Coupling: Loose Coupling provides the opportunity to use wider trace widths to maintain the target impedance. Loose Coupling Differential impedance depends only on the single ended impedance of either trace, not on the spacing (Cross Coupling) of the traces. Loose Coupling: only equal trace lengths are of Coupling: More Return Currents Coupled from Trace to Trace Reduces unwanted coupling from other traces Thinner trace width to maintain the target impedance and increase circuit density Greater Effect on Transmission line Impedance with change of Trace Spacing Greater Losses at High Frequency due to Skin Effect24 Differential Pair Transmission line Impedance, Edge Coupled StriplineThese Fields Determine the Transmission line Impedance ZOThese Fields Determine the Transmission line Coupling and Coupling Impedance ZCOUPLINGZDIFF= 2 x ZODDZODD= ZO ZCOUPLING25Mr.

6 Rick Hartley: The Truth about Differential Pairs in High Speed PCBsTight Coupling and Transmission line Impedance(Microstrip Example):Tight Coupling:Requires Constant Trace to Trace Spacing to maintain Impedance to a greater change in Trace Impedance with changes in Trace to Trace 100 4 mil wide line mil separation6 mils above the planeZDIFF= 131 milseparationZDIFF= 100 7 mil wide line 14 mil separation6 mils above the planeZDIFF= 108 milseparation26Mr. Rick Hartley: The Truth about Differential Pairs in High Speed PCBsTight Coupling and Skin Effect4 mil wide traces mil separation6 mils above plane 100 7 mil wide traces 14 mil separation6 mils above plane 100 ZCOUPLINGI llustration (24 long traces)4 mil wide traces mil wide traces Rick Hartley: The Truth about Differential Pairs in High Speed PCBsGeneral Observations on Coupling: When loss is important, loosely coupled Differential pairs should be used.

7 When interconnect density and noise immunity are important, tightly coupled Differential pairs should be used. With no overriding constraint, loose coupling with a spacing equal to twice the trace width offers a reasonable compromise in providing the lowest loss at the highest interconnect Demonstration: Symmetrical Loose vs. Tight CouplingA12 Demonstration: Loose vs. Tight Coupling30 Loose Coupled SymmetricalTight Coupled SymmetricalNote no difference in skew or signal31IX. EmissionsEmissions Avoid crossing splits in the return path PCB artwork Cable interconnections Keep the pairs tightly coupled Field containment impacts emissions level Use of Stripline Use of tightly coupled Microstrip32 Near Field Emissions H Field Scan of Tight vs. Loose Coupling:Loosely coupled vs.

8 Tightly coupled Differential pairLoosely coupled vs. Tightly coupled Differential pair with gap in return path3334X. Signal SkewSignal Skew35 Signal Skew conclusions PCB geometries can fix skew issues or make it worse. Common Mode Chokes provide improvement36 Signal Integrity Guidelines Symmetry matters Skew Physical length and mismatch impact rise/fall time37XI. Common Mode Chokes38So What is a Common Mode Choke? Two Equal Value Inductors wound on the same high Core Phased Identically With a Mutual Inductance Coupling approaching: : Even when the frequencies of signals and noise overlap, their different conduction modes enable suppression of only noise. Remember: Common Mode is Noise Differential Mode is Signal Performance does not decrease even with a large Differential Mode current, as long as the core does not become Mode Chokes (CMC) come in all different sizes for different current and circuit applications:41 CMC Differential Response: First, Some AssumptionsAt frequencies greater than = 5RC2/L2, virtually all the Differential currents IGround Planereturn to the source through L2and not through the ground plane.

9 *RC1and RC2(trace and inductor parasitic resistance) can be combined into a single resistor RCwhich is much less than the Load Resistance *Mr. Henry Ott: Electromagnetic Compatibility Engineering, Page 14942 Differential Mode Simplification:VSL1L22 MRLRCL1L2 RCRLVSISIS+_(Remember: L1+ L2 2LM= LTotal)And: M = LM43 Final Simplification:RLRLRCVSVSBut: RC<< RLThe Inductors DisappearLoss Resistors can be S PROVE THIS MATHEMATICALLY:So What? What does this mean? 45 Let s do a little Mathematics describing the Common Mode Choke s Differential Mode response:** Dr. Clayton Paul s and Mr. Henry Ott s equations and illustrations First, we can simplify the circuit diagram:L2 RLL1+ L2 2 MRLVSVSRCRCK (coupling) 1L1= L2= ML146 Kirchhoff Voltage Loop:VS= (L1+L2)IS 2 MIS+ (RL+ RC)ISIf:k 1 then:if L1= L2= M= LVS= 2 LIS 2 LIS+ (RL + RC)ISVS= (RL+ RC2)ISif RC<< RLVS= RLISAll your signal is developed across RLL1+ L2 2M 47(Signal)VS= RLISSo what does this mean?

10 The Inductors Disappear from the circuit in the Differential RC2are << RL; so those losses can be Plot Simulation LTSpice Model49 Overlay of Differential Mode Input and Differential Mode OutputNote no change in phase or CMC Disappears in the Differential Mode51 Mathematics of the Common Mode Currents:Let s find out how the Common Mode currents are majority of the equations are from Mr. Henry Ott s: Electromagnetic Compatibility, pages 144 to Common Mode Currents for Conducted Emissions:I1I2L1L2 RLRCVGVN= I1 RLVL153 Current Loop 1 Current Loop 2(Noise)Kirchhoff Law for Loop I1and Loop I2: Loop 1: VG= L1I1+ MI2+ I1RL Loop 2: VG= L2I2+ MI1+ RCI2 Solving Loop 2 for I2: I2= (VG MI1)/( L2+ R2) If: L1= L2= M=L Substituting I2into Loop 1: I1= (VGRC)/( L(RC+ RL) + RCRL)54 Solving For VN: I1= (VGRC)/( L(RC+ RL) + RCRL) If: RC <<RL I1= (VGRC/(( LRL) + RCRL) VN= I1RL VN= RL(VGRC)/( LRL+ RCRL) VN= (VGRC)/( L+ RC) By Multiplying Eq.)