Building Confidence in Simulation Tools

Eric Bogatin 2009

Slide -1

www.BeTheSignal.com

DL-210 Building Confidence on Simulation Tools

Building Confidence in Simulation Tools

Dr. Eric Bogatin, Signal Integrity Evangelist,

Bogatin Enterprises, www.beTheSignal.com

[email protected]

Eric Bogatin 2009

Slide -2

www.BeTheSignal.com


Outline

• Role of simulation tools

• A calibration process

� Compared to analytical exact expressions

� Compared to other simulators

� Compared to well characterized test vehicles

� Compared to test vehicles

• A simple Dk, Df measurement process

Eric Bogatin 2009

Slide -3

www.BeTheSignal.com


Why SI/PI Engineering is Hard

• Over riding product design goals

� Meet higher performance specs

� Meet shrinking schedules

� At lower cost

• Pick any two- it’s easy

• Doing all three- it’s hard

Eric Bogatin 2009

Slide -4

www.BeTheSignal.com


General Design Methodology

• Two categories of products

� Performance driven

� Cost-performance

• An efficient methodology;

� Identify the SI problems

� Find the root cause

� Establish design guidelines to minimize them

� “correct by design”: use analysis tools to develop prepre--layoutlayout design rules specific to your design

� Use post layoutpost layout verification tools to efficiently spin virtual prototypes

} Understand the essential principles

Eric Bogatin 2009

Slide -5

www.BeTheSignal.com


The Role of Analysis Tools

• Pre-layout phase

� Explore design tradeoffs

� Explore technology options

� Establish specific design rules/constraints for each custom design

• Post layout phase

� Final verification

� Robust to manufacturing variations

� Don’t pay extra for insurance (un-needed margin)

• Simulation/analysis as risk reduction

� Same risk level, lower added margin, lower added cost

Added margin �C

ost � simulation R

isk �

Eric Bogatin 2009

Slide -6

www.BeTheSignal.com


Design Cultures

“This is the one. We want you to pray for this one.”

Build it and test itEngineering discipline

model

measure simulate

create

“Test in performance” “Design in performance”

expertise

money

critical mass

“short term pain for

long term gain”leadership

Eric Bogatin 2009

Slide -7

www.BeTheSignal.com


Leverage Appropriate Analysis Tools

Rules of Thumb: Feeds your

intuition, useful for order of

magnitude estimating

1st order approximations:

Analytic approximations,

useful for quick estimates and

early design tradeoffs

Numerical simulation: field

solver, parasitic extraction,

SPICE, IBIS simulations,

can base a design on this

Ac

cu

rac

y

Effort required to get answerexpertise

moneytime

Eric Bogatin 2009

Slide -8

www.BeTheSignal.com


The Design Process

From Here To Here Or Here

?

Eric Bogatin 2009

Slide -9

www.BeTheSignal.com


Creativity is the key ingredient to the design process

IntuitionIntuition is the designer’s most important tool

Eric Bogatin 2009

Slide -10

www.BeTheSignal.com


One Role of Numerical Simulation

Tools: When Accuracy Counts

Characteristic impedance

Low

control

limit

High

control

limit

Accuracy error

Dis

trib

ution

Eric Bogatin 2009

Slide -11

www.BeTheSignal.com


How to Evaluate the Accuracy of a

Simulation Tool?

1. Compare the results to analytically exact equations

2. Compare the results to other, well established industry standard simulation tools

3. Compare the results to well characterized test vehicles

4. Compare the results to typical interconnect structures

Eric Bogatin 2009

Slide -12

www.BeTheSignal.com


Characteristic Impedance of Twin

Rods and Ansoft Field Solver

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Rod Radius (mm)

Ch

ara

cte

ris

tic

Im

pe

da

nc

e

(OH

ms

)

analytic

field solver

−

+

ε= 1

22ln

1202

0r

s

r

sZ

r

r r

s

s = 30 mm

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Radius (mm)

Ab

so

slu

te E

rro

r

Eric Bogatin 2009

Slide -13

www.BeTheSignal.com


Comparing Polar Si9000 to Ansoft SI2D

0

1020

3040

50

60

70

8090

100110

120

0 2 4 6 8 10 12 14 16 18 20

Line Width, mils

Ch

ara

cte

ris

itc

Im

pe

da

nc

e, o

hm

s

Ansoft

Polar

0%

1%

2%

3%

4%

5%

0 2 4 6 8 10 12 14 16 18 20

Line Width, milsA

bs

olu

te E

rro

r

H1 = 5 milsEr1 = 4T1 = 1.4 milsW1 = W2 = variable.

Eric Bogatin 2009

Slide -14

www.BeTheSignal.com


Comparing the Polar Si9000 to

Agilent ADS

-1%

0%

1%

2%

3%

4%

0 2 4 6 8 10 12 14 16 18 20

Line Width, mils

Ab

so

lute

Err

or

Absolute error for field solvers ~ <1%

Eric Bogatin 2009

Slide -15

www.BeTheSignal.com


Calculated Single-ended S-Parameters: Comparing Agilent ADS and Polar Si9000

H1 = 5 milsEr1 = 4S1 = 5 milsW1 = W2 = 5 milsT1 = 0.7 milsDf = 0.02Length = 6 inches

2 4 6 80 10

-30

-20

-10

-40

0

freq, GHz

S-Parameter in dB

Return loss

Insertion loss

2 4 6 80 10

-50

-40

-30

-20

-10

-60

0

freq, GHz

S-P

arameter in dB S41

S31

Red is ADSBlue is Polar

Eric Bogatin 2009

Slide -16

www.BeTheSignal.com


Calculated Differential S-Parameters: Comparing Agilent ADS and Polar Si9000

2 4 6 80 10

0.85

0.90

0.95

1.00

1.05

0.80

1.10

freq, GHz

Delay in nsec

Common signal delay

Differential signal delay

2 4 6 80 10

-30

-20

-10

-40

0

freq, GHz

Differential S-Parameter in dB

SDD21

SDD11

Comparative accuracy is within pen width

Red is ADSBlue is Polar

Eric Bogatin 2009

Slide -17

www.BeTheSignal.com


“Typical” test vehicle

• Stripline

• Closely spaced

• Nominal values: � T= 1.4 mils

� H1 = H2 = 10 mils

� w = 8.6 mils

� s = 8.4 mils

� Dk = 4.5

� Df = 0.02

Eric Bogatin 2009

Slide -18

www.BeTheSignal.com


Converting Test Line Parameters

into S-Parameters

Eric Bogatin 2009

Slide -19

www.BeTheSignal.com


0.5 1.0 1.5 2.0 2.50.0 3.0

42

44

46

48

50

52

54

40

55

time, nsec

Single-ended Impedance, ohms

0.5 1.0 1.5 2.0 2.50.0 3.0

0

20

40

-20

60

time, nsec

Near End Cross Talk, x1000

0.5 1.0 1.5 2.0 2.50.0 3.0

-0.0

0.2

0.4

0.6

0.8

-0.2

1.0

time, nsec

Transmitted Single-ended Signal

0.5 1.0 1.5 2.0 2.50.0 3.0

0

2

4

6

8

-2

10

time, nsec

Far End C

ross Talk, x 1000

Initial Comparison:

Single-Ended Performance

measured

simulated

One conclusion: simulator is accurate to ~ 10%

Eric Bogatin 2009

Slide -20

www.BeTheSignal.com


Alternative Conclusions

• What is simulated is not what is measured

� SMA launch not in simulation

� ¼ inch uncoupled trace feed not in simulation

• The as designed parameters are not the same as the as manufactured parameters

� H, w

� Dk, Df

Eric Bogatin 2009

Slide -21

www.BeTheSignal.com


Including SMA Launch and Feed

Effects

0.5 1.0 1.5 2.0 2.50.0 3.0

42

44

46

48

50

52

54

40

55

time, nsec


0.5 1.0 1.5 2.0 2.50.0 3.0

-0

10

20

30

40

50

-10

60

time, nsec

Near End C

ross Talk, x1000

0.5 1.0 1.5 2.0 2.50.0 3.0

-0.0

0.2

0.4

0.6

0.8

-0.2

1.0

time, nsec

Transmitted S

ingle-ended Signal

0.5 1.0 1.5 2.0 2.50.0 3.0

0

2

4

6

8

10

-2

12

time, nsec

Far End Cross Talk, x 1000

measured

simulated

Eric Bogatin 2009

Slide -22

www.BeTheSignal.com


What about Cross Section and

Materials?

• Cross section shows� H: 10 mils nominal � 10.2 mils actual

� W: 8.6 mils nominal � 8 mils actual

� S: 8.4 mils nominal � 9 mils actual

• Materials :� Dk from fab: 4.5 � 4.0 (measured)

� Df assumed 0.02, stays the same

Eric Bogatin 2009

Slide -23

www.BeTheSignal.com


Single Ended Response

0.5 1.0 1.5 2.0 2.50.0 3.0

42

44

46

48

50

52

54

40

55

time, nsec


0.5 1.0 1.5 2.0 2.50.0 3.0

-0

10

20

30

40

50

-10

60

time, nsec

Near End C

ross Talk, x1000

0.5 1.0 1.5 2.0 2.50.0 3.0

-0.0

0.2

0.4

0.6

0.8

-0.2

1.0

time, nsec

Transmitted Single-ended Signal

0.5 1.0 1.5 2.0 2.50.0 3.0

2

4

6

8

0

10

time, nsecFar End Cross Talk, x 1000

measured

simulated

What accounts for the slightly different slopes in NEXT?

Eric Bogatin 2009

Slide -24

www.BeTheSignal.com


Asymmetry:

Welcome to the Real World

0.5 1.0 1.5 2.0 2.50.0 3.0

42

44

46

48

50

52

54

56

58

40

60

time, nsec

Z11_meas

Z22_meas

TDR response measured from opposite ends: T11, T22

0.5 1.0 1.5 2.0 2.50.0 3.0

0

10

20

30

40

50

-10

60

time, nsec

Near End C

ross Talk, x1000

NEXT response measured from opposite ends: T31, T42

Eric Bogatin 2009

Slide -25

www.BeTheSignal.com


2 4 6 8 10 12 14 16 180 20

-50

-40

-30

-20

-10

-60

0

freq, GHz

Return Loss, db

2 4 6 8 10 12 14 16 180 20

-20

-10

-30

0

freq, GHz

Insertion loss, dB

2 4 6 8 10 12 14 16 180 20

-50

-40

-30

-20

-10

-60

0

freq, GHz

S31, near end cross talk, db

2 4 6 8 10 12 14 16 180 20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0

1.0

freq, GHzDelay, nsec

Frequency Domain, Single-ended

BW of the simple transmission line model ~ 12 GHz

measured

simulated

Eric Bogatin 2009

Slide -26

www.BeTheSignal.com


Measure Dk, Df of Materials

• Practical: easy, routine, robust

• Many standards

• IPC is generating new methods

• Fundamental issue: removing connector effects

• A solution:� Build uniform line with a few precision discontinuities

� Model the launches

� Model the discontinuities

� Fit to entire topology and Dk, Df values

� Instant evaluation of the quality of the connector/launch model

Eric Bogatin 2009

Slide -27

www.BeTheSignal.com


Simple Test Structures

SMA launches on back sideNon-uniform launches

Precision discontinuities spaced 3 inches in the line

Eric Bogatin 2009

Slide -28

www.BeTheSignal.com


0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.60.0 2.8

35

40

45

50

30

55

time, nsec

Z11_meas

Z22_meas

Measured TDR Response

From port 1 side

From port 2 side

Note: - some asymmetry in the line- very symmetric launches

Eric Bogatin 2009

Slide -29

www.BeTheSignal.com


Modeling an SMA-

or other Discontinuity

TD

1

10

1BW <

Len

For a general, non uniformnon uniform section:

Up to what frequency will a T model match its behavior?

As long as Len < 1/10th λ

secnin6

LenTD =

Len

6.0BW < inches

BW

6.0Len <

For a 10 GHz BW, longest non-uniform element should be < 60 mils

With SMA length ~ 150 mils, and a 10 GHz BW, need no more than 3 T elements

Eric Bogatin 2009

Slide -30

www.BeTheSignal.com


Circuit Topology to Fit:

10 different Parameters

ML1CTL_CTL62

ReuseRLGC=noRLGC_File=Layer=2

W=w_1 milLength=Len_c inSubst="Subst1"

ML1CTL_CTL80


W=w_1 milLength=Len_b inSubst="Subst1"

ML1CTL_CTL68


W=w_1 milLength=Len_a inSubst="Subst1"

CC7C=C_a pF

CC6C=C_a pF

Term

Term3

Z=50 Ohm

Num=3

TLINTL65

F=(1/TD_3b) GHzE=360Z=Z0_3b Ohm

TLINTL64

F=(1/TD_3a) GHzE=360Z=Z0_3a Ohm

Term

Term4

Z=50 Ohm

Num=4

TLINTL71

F=(1/TD_4b) GHzE=360Z=Z0_4b Ohm

TLINTL72

F=(1/TD_4a) GHzE=360Z=Z0_4a Ohm

TLINTL77

F=(1/TD_4c) GHzE=360Z=Z0_4c Ohm

TLINTL78

F=(1/TD_3c) GHzE=360Z=Z0_3c Ohm

SMA launch Len = 3 inches

6 parameters;Zo, TD x 3 Parameter nominal extracted

W 13.1 mils 12.5H 10 mils 10.5Dk 4.5 3.94

Df 0.02 0.017

Eric Bogatin 2009

Slide -31

www.BeTheSignal.com


TDR Response

0.5 1.0 1.5 2.0 2.5 3.0 3.50.0 4.0

10

20

30

40

50

0

60

time, nsec

Z11_meas

Z11_sim

Eric Bogatin 2009

Slide -32

www.BeTheSignal.com


S-Parameter Match

2 4 6 80 10

-30

-20

-10

-40

0

freq, GHz

dB(S11_meas)

dB(S

11_sim

)

2 4 6 80 10

-100

0

100

-200

200

freq, GHz

phase(S11_meas)

phase(S11_sim

)

2 4 6 80 10

-5

-4

-3

-2

-1

-6

0

freq, GHz

dB(S21_meas)

dB(S

21_sim

)

2 4 6 80 10

-100

0

100

-200

200

freq, GHz

phase(S21_meas)

phase(S21_sim

)

measuredsimulated

Eric Bogatin 2009

Slide -33

www.BeTheSignal.com


Conclusions

• Accuracy of field solver can be ~ 2% at BW > 10 GHz between field solver simulations and well characterized test vehicles

• Requires

� Accurate materials properties

� As manufactured cross section

• Even using constant Dk, Df values

• The Real World is more complex than we would like it to be

� Uniform transmission lines aren’t

� As manufactured features not equal to as specified features

� Dielectric thicknesses as manufactured not equal to as specified

� Etch back of signal lines variable

� Launches may have asymmetry

Eric Bogatin 2009

Slide -34

www.BeTheSignal.com


Concerns for the Future

• Moving the industry toward “engineering discipline”

• Overcoming the barriers

expertisemoney

critical mass

“short term pain for long term gain”leadership

Documents

Building Confidence in Simulation Tools