Document 244689
Slide -1 Why We Need EM Why You Need Electromagnetics for Signal Integrity Analysis – Sometimes (presented at DesignCon 2011 with Stephen Hall, Olufemi(Femi) Oluwafemi, Jeff Loyer Intel Corporation) Dr. Eric Bogatin, Signal Integrity Evangelist, Bogatin Enterprises Copies of this presentation are available on James Clerk Maxwell (1831-1879) www.beTheSignal.com George Simon Ohm (1789-1854) 5/1/2012 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -2 Why We Need EM Overview • Two world views Circuit elements Electromagnetics • Success stories Transmission lines and circuits Real capacitors for the PDN • Examples where circuit model view is not accurate enough FEXT PDN design: does location matter? Return current through planes Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -3 Why We Need EM Two World Views Circuit elements and sources: Electromagnetic Fields and Boundary Conditions Courtesy of Ansys Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -4 Why We Need EM The Obvious Need for Electromagnetics Converting the Physical World into the Circuit World Capacitance Matrix [pF/m]: 1 2 1 118.052 -4.451 2 -4.451 118.052 Inductance Matrix [nH/m]: 1 2 1 280.236 30.059 2 30.059 280.236 DC Resistance Matrix [ohms/m]: 1 2 1 4.949 0.000 2 0.000 4.949 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -5 Why We Need EM Two Worlds: Similar Diff Eqs, Similar Behavior (in most cases) • • A signal is a voltage or current A signal is a propagating electromagnetic wave i v vs Rs Rt L C Electric field ( E) (H) Magnetic field Field quantity Lumped Circuit Elements i( t ) = C dv(t) dt v(t) = −L di(t) dt ∂H y ∂E a x − = ε x ∂t ∂z ∂H y ∂E a y x = − µ ∂t ∂z Similar differential equations, ….but with some differences 2 important “cheats” cheats” expand the use of lumped circuit elements Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -6 Why We Need EM Cheat #1: Add Spatial Distribution to Electrical Model of a Lossless Transmission Line x Telegraphers’ equation ∂ ∂ V (x, t ) = −L I(x, t ) ∂x ∂t ∂ ∂ I(x, t ) = −C V (x, t ) ∂x ∂t Wave equation ∂2 1 ∂2 V (x, t ) = V (x, t ) 2 ∂t LC ∂x 2 ∂2 1 ∂2 I(x, t ) = I(x, t ) 2 ∂t LC ∂x 2 A new ideal circuit element: the lossless transmission line Z0 = L C TD = Len x LC Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -7 Why We Need EM Circuit Model View is Incredibly Successful! V4 V3 V2 V1 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -8 Why We Need EM Cheat #2: How Current Flows in Transmission lines Our Elementary School View of Current Flow How does current flow? Leads to miss conception of how current flows in a transmission line Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -9 Why We Need EM Current propagates as a signal-return path loop with a direction of propagation and a direction of circulation signal +++ +++ I = displacement current +++ --- The current loop has two directions associated with it: 1. A direction of propagation 2. A direction of circulation They are independent! Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -10 Why We Need EM When Do We Need Maxwell’s Equations? • When there is spatial variation in E, H field • When there are propagating EM fields • When propagation modes change • Typically when Len > ~ 1/10th λ Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -11 Why We Need EM Application #1: Where Far End Cross Talk Really Comes From 1 v incident signal NEXT = Vnear Vsignal RT Near end Measured Near and Far End XTK in Two Uniform Microstrips: 5 mil wide line and space, 4 inches long Far end FEXT = Vfar Vsignal Incident signal Transmitted signal RT = 100 psec NEXT: Measured near end cross talk 10% noise/div FEXT: Measured far end cross talk Very different signatures Very different magnitudes 200 psec/div Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -12 Why We Need EM Thinking About Cross Talk in Terms of Mutual L and C ∆x = RT x v v = 6 inches/nsec V 1 2 1 CW signal current 2 CCW induced current @ Far end: IC - IL @ Near end: IC + IL What is far end noise if IC = IL? What geometry has IC = IL? Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -13 Why We Need EM Eliminate FEXT with Stripline Measured at Far end, Near end terminated, TDT end Open TDR 50 Ohms open Far end noise Differences: TDR response Far end cross talk in microstrip: IL > IC Stripline Microstrip Noise response No far end cross talk in stripline: IL = IC Why? Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -14 Why We Need EM An Alternative Way of Thinking About Far End Cross Talk Odd mode Even mode TDD21 TCC21 Which signal travels faster? 500 psec/div Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -15 Why We Need EM An Alternative Way of Thinking About Far End Cross Talk = ½ + ½ What comes out? Far end cross talk is really due to the difference in speed between a differential and common signal What is the far end noise expected in stripline? Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -16 Why We Need EM Microstrip vs. Stripline Odd mode Even mode SCC21 SDD21 SCC21 SDD21 1. No difference in speed between diff and comm signal 2. No far end cross talk Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -17 Why We Need EM Application #2: Capacitors and Planes Sample courtesy of X2Y 1E1 Impedance, Ohms Measured Impedance of Real, 220 nF, 0603 MLCC Capacitor 1 1E-1 1E-2 1E6 1E7 1E8 2E8 freq, Hz Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -18 Why We Need EM Behavior of Real, 220 nF, 0603 MLCC Capacitor: fitting RLC Model Sample courtesy of X2Y Impedance, Ohms 1E1 1 1E-1 C Measured impedance ESR = 0.017 Ω ESL Simulated impedance ESL = 1.3 nH ESR C = 180 nF 1E-2 1E6 1E7 1E8 2E8 SRF1 = 1 = 2π ESL x C freq, Hz Note: ESL is not intrinsic to the capacitorcapacitor- related to mounting 1 Bogatin Enterprises, a LeCroy Company 2012 1 160 MHz ESL1 x C1 www.beTheSignal.com Slide -19 Why We Need EM X2Y Capacitor Example Sample courtesy of X2Y Impedance, Ohms 1E1 Measured Simulated RLC model 0603 MLCC 1 1E-1 X2Y 1E-2 1E6 C Like 4 capacitors in parallel L R 220nF X2Y C = 180 nF L = 0.42 nH R = 0.012 Ohms 1E7 1E8 freq, Hz X2Y capacitors are an important low inductance alternative Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -20 Why We Need EM Unlike Real Estate, Does Position Matter with Capacitors? 4 capacitors, 0.1uF, ESL = 3 nH, 2 in x 2 inch cavity Does position matter? When cavity spreading inductance is small, and caps have high ESL, cavity is transparent, position does not matter much Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com 2E8 Slide -21 Why We Need EM The Other Extreme: Thick Cavity, Low ESL Capacitors 4 capacitors, 0.1uF, ESL = 0.5 nH, 2 in x 2 inch cavity Does position matter? When cavity spreading inductance is large, and caps have small ESL, cavity is NOT transparent, position does matter Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -22 Why We Need EM Application #3: Return Current Through Two Planes • When the return plane changes, return current is injected into the plane cavity Current travels like a radial wave through the impedance of the plane-plane cavity This is the most important way high frequency noise is injected into the board planes • Set up: 50 ohm microstrip top and bottom 30 mil thick dielectric between the planes 10 inch x 10 inch board 1 v signal, 20 mA at 0.2 nsec rise time Simulated voltage between the planes with HyperLynx/PI (20 mV full scale ( 2%) Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -23 Why We Need EM How to Minimize the Switching Noise in Signal Vias Changing Return Planes? Add an adjacent return via “a lot is good, more is better and too many is just right” - Frank Schonig Add 4 adjacent return vias HyperLynx 8.0 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -24 Why We Need EM Use Thinner Dielectric Between Return Planes h = 30 mils h = 3 mils HyperLynx 8.0 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -25 Why We Need EM The Use of Potential Return Vias Drives Return Plane Selection • Voltage on return plane has no impact on the impedance of the signal line • Return plane selection is all about the via design • Add return via adjacent to each signal via, as close to signal via as practical Best return plane selection 1 Second best return plane selection 1 2 2 3 signal 3 signal 4 4 As fall back: use thin dielectric between all adjacent planes, add low L DCblocking caps between changing return planes: limited value with shorter RT Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -26 Why We Need EM “Sometimes a lie tells more of the truth than the truth” – Francis Low • “Engineering is the art of approximation” • Use the approach that gets you “an acceptable answer fastest” • For most signal integrity problems, voltage, current and circuits is just fine- provided you think about wave propagation of signals! • But, some effects are inherently electromagnetic field related and can’t be approximated by voltage, current and circuits: How geometry and material properties affect circuit element values Electromagnetic interactions with materials- conductors and dielectrics Distributed electrical properties of interconnects EMI and radiation effects Coupling between waveguides Signal transmission when the propagation mode changes • Electromagnetics is in your future. • Embrace Maxwell’s Equations Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -27 Why We Need EM For More Information www.BeTheSignal.com SI Library Webinars, feature articles, presentations, hands on labs Class schedules Blog: www.beTheSignal.com/blog @beTheSignal Published by Prentice Hall, 2009 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com
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