ppt-305 why we need em for si

Slide -1Why We Need EM

Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com

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

www.beTheSignal.com

5/1/2012

James Clerk Maxwell (1831-1879) George Simon Ohm (1789-1854)



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



Two World Views

Circuit elements and sources:Electromagnetic Fields

and Boundary Conditions

Courtesy of Ansys



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



Two Worlds: Similar Diff Eqs,

Similar Behavior (in most cases)

• A signal is a voltage or current

∂

∂−=

∂∂

t

H

z

Ea

yx

y µ

∂∂

=∂

∂−

t

E

z

Ha xy

x ε

di(t)v(t) L

dt= −

Field quantityLumped Circuit Elements

LC

RtRs

vs

i

v

Electric field ( E )

Magnetic field ( H )

• A signal is a propagating electromagnetic wave

Similar differential equations, Similar differential equations, …….but with some differences.but with some differences

2 important 2 important ““cheatscheats”” expand the use of lumped circuit elementsexpand the use of lumped circuit elements

d v ( t )i( t ) C

d t=



Cheat #1: Add Spatial Distribution to

Electrical Model of a Lossless

Transmission Line

( ) ( )t,xIt

Lt,xVx ∂

∂−=

∂∂ ( ) ( )t,xV

tCt,xI

x ∂∂

−=∂∂

( ) ( )t,xVxLC

1t,xV

t 2

2

2

2

∂∂

=∂∂ ( ) ( )t,xI

xLC

1t,xI

t 2

2

2

2

∂∂

=∂∂

Telegraphers’ equation

Wave equation

C

LZ0 =

x

A new ideal circuit element: the lossless transmission line

TD Len x LC=



Circuit Model View is Incredibly

Successful!

V1

V2

V3V4



Cheat #2: How Current Flows in

Transmission lines

How does current flow?

Leads to miss conception of how current flows in a transmission line

Our Elementary School View of

Current Flow



Current propagates as a signal-return path loop

with a direction of propagation and a direction of

circulation

signal

The current loop has two directions associated with it:

1. A direction of propagation

2. A direction of circulation

They are independent!

+++

=

+++

- - - +++

I

displacement

current



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



Application #1: Where Far End

Cross Talk Really Comes From

Very different signatures

Very different magnitudes

RT

1 v incident

signal

Near endsignal

near

V

VNEXT =

Far endsignal

far

V

VFEXT =

Incident signal

Transmitted signal

NEXT: Measured near end cross talk

FEXT: Measured far end cross talk

200 psec/div

10% noise/div

RT = 100 psec

Measured Near and Far

End XTK in Two Uniform

Microstrips: 5 mil wide

line and space, 4 inches

long



Thinking About Cross Talk in

Terms of Mutual L and C

1

2

V

v = 6 inches/nsec

∆x = RT x v

1

2CW signal current

CCW induced current

@ Near end: IC + IL

@ Far end: IC - ILWhat is far end noise if IC = IL?

What geometry has IC = IL?



Eliminate FEXT with StriplineMeasured at Far end, Near end terminated, TDT end Open

Differences:

Far end cross talk

in microstrip:

IL > IC

No far end cross

talk in stripline:

IL = IC

MicrostripStripline

TDR open50 Ohms Far end noise

TDR response

Noise response

Why?



An Alternative Way of Thinking

About Far End Cross Talk

Odd mode Even mode

500 psec/div

TCC21

TDD21

Which signal travels faster?



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?



Microstrip vs. Stripline

1. No difference in speed between diff and comm signal

2. No far end cross talk

SCC21

SDD21

SCC21

SDD21

Odd mode Even mode



1E7 1E81E6 2E8

1E-1

1

1E-2

1E1

freq, Hz

Impedance,

Ohm

s

Application #2:

Capacitors and Planes

Sample courtesy of X2Y

Measured Impedance of Real, 220 nF,

0603 MLCC Capacitor



1E7 1E81E6 2E8

1E-1

1

1E-2

1E1

freq, Hz

Impedance,

Ohm

s

Behavior of Real, 220 nF,

0603 MLCC Capacitor: fitting RLC Model


Measured

impedance

Simulated

impedance

C = 180 nF

ESR = 0.017 Ω

ESL = 1.3 nH

Note: ESL is not intrinsic to the capacitorNote: ESL is not intrinsic to the capacitor-- related to mountingrelated to mounting1111

1CxESL

MHz160

CxESL2

1SRF =

π=

CESL

ESR



1E7 1E81E6 2E8

1E-1

1

1E-2

1E1

freq, Hz

Impedance,

Ohm

s

X2Y Capacitor Example

220nF X2Y

C = 180 nF

L = 0.42 nH

R = 0.012 Ohms

X2Y capacitors are an important low inductance alternativeX2Y capacitors are an important low inductance alternative

Measured

Simulated RLC model

C

L

R

Like 4 capacitors in parallel

0603 MLCC

X2Y




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



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



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%)



How to Minimize the Switching Noise in

Signal Vias Changing Return Planes?

“a lot is good, more is better and too many is just right”

- Frank SchonigAdd an adjacent return via

Add 4 adjacent return vias

HyperLynx 8.0



Use Thinner Dielectric

Between Return Planes

h = 30 mils h = 3 mils

HyperLynx 8.0



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

1

2

3

4

Second best return plane selection

signal

1

2

3

4

Best return plane selection

signal

As fall back: use thin dielectric between all adjacent planes, add low L DC-

blocking caps between changing return planes: limited value with shorter RT



“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



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Published by Prentice Hall, 2009

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