s-parameters, signal integrity and you or s-parameters...

Slide -1VL-156 S-parameters, Signal Integrity and You

Bogatin Enterprises 2010 www.BeTheSignal.com

S-Parameters, Signal Integrity and You

or

S-Parameters without tears

Dr. Eric Bogatin,

Signal Integrity Evangelist,

Bogatin Enterprises

www.beTheSignal.com

April 2010Presented at the Huntsville EMC Symposium, April 2010



Outline

• Download a copy of the presentation from beTheSignal.com: under SI content, select “slides presentation”, PPT-VL-156

Why do I care about S-parameters

What are they?

What sorts of questions can they answer

Examples of “patterns”



Copyright © 2010 by

Bogatin Enterprises, LLC

All rights reserved. No material contained in

this presentation may be distributed or

reproduced in whole or in part without the

written permission of Bogatin Enterprises.

Please respect the great deal of effort that

has gone into the preparation of these

lectures and use these materials for your

personal use only.

Bogatin Enterprises, LLC26235 W. 110th Terr.Olathe, KS 66061v: 913-393-1305f: 913-393-0929e: [email protected]



For More Information

www.BeTheSignal.com

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My Blog: What I learned this monthPublished by Prentice Hall, 2009



The Role of Interconnects

• Purpose of an interconnect: “to transport a signal from one point to another with an acceptable level of distortion”

• How do you characterize how an interconnect affects a signal?



Characterize an Interconnect with “Precision” Reference Signals

Incident Wave

Transmitted Wave

S11 S21

Incident Wave

Transmitted wave

TDRTDT

t

t

TDR

VNA

Time Domain

Frequency Domain



S-Parameters Describe the

“Behavior” of an Interconnect

Intrinsic

system

response,

20 GHz BW

Impulse

response

through 1 m

backplane

200 psec/div

Transmitted sine wave

transfer function for

backdrilled via stub

In the frequency domain

Transmitted sine wave transfer

function for a via stubIn the time domain

For single ended signals, for

differential and common signals



An Incredibly Powerful Formalism to

Store Information

Backplane viaDaughter Card via

Daughter CardSMA launch

Connector

ML2CTL_V

CLin9

ReuseRLGC=no

RLGC_File=

Layer[2]=2

Layer[1]=2

W[2]=W2_CON_1 milS [1]=S_CON_1 mil

W[1]=W1_CON_1 mil

Length=Len_CON_1 in

Subst="Subst2"

CLINP

TL45

Ao=0

Ae=0

Ko=Dk1_DC_1Ke=Dk1_DC_1

L=Len_D Cvia mil

Zo=(Z11_DCvia-Z12_D Cvia) Ohm

Ze=(Z11_DCvia+Z12_D Cvia) Ohm

ML2CTL_V

CLin10

ReuseR LGC=no

RLGC_File=

Layer[2]=2

Layer[1]=2

W[2]=W 2_DC_1 milS [1]=S_DC_1 mil

W[1]=W 1_DC_1 mil

Length=Len_DC_1 in

Subst="Subst1"

TLINTL47

F=(1/TD_launch) GHz

E=360

Z=Z_launch Ohm

TLIN

TL48

F=(1/TD_launch) GHz

E=360

Z=Z_launch Ohm

TLIN

TL49

F=(1/TD_SMA) GHz

E=360

Z=Z_SMA Ohm

TLINTL50

F=(1/TD_SMA) GHz

E=360

Z=Z_SMA Ohm

R

R40

R=50 Ohm

R

R16

R=50 Ohm

C

C 4

C =C_DCvia pF

C

C5

C=C_D Cvia pF

C

C7

C=C_BPvia pF

VtPulseSRC3

Period=100 nsec

Width=100 nsec

Fall=(2.25*RT) nsec

Rise=(2.25*RT) nsec

Edge=erf

Delay=0 nsec

Vhigh=1 V

V low=0 Vt

TDR

VNA

EM Sim

Single ended time domain

Single ended frequency domain

Differential time domain

Differential frequency domain

Circuit Simulation

Characterization

Hacking



A Behavioral Model Can Be Used to

Emulate System Performance

PRBS, 6.25 Gbps,

211 – 1 bits

Convolution

integral

=

Overlay each bit,

synchronous

with the clock+

Impulse response



Bit Rate and Collapse of the Eye

3.125 Gbps 6.25 Gbps 10 Gbps

25 inch length backplane channel



A Behavioral Model Can Be Integrated

Directly in a Circuit Simulation

HyperLynx simulation



What are S-Parameters?

• A collection of scattered responses (at every frequency):

Reflected sine waves

Transmitted sine waves

• Stored in a special format: Touchstone file

.s1p: scattered data from 1 port

.s2p: scattered data from 2 ports

R

1 port into wavesine

1 port from out wavesineS11 =

1 port into wavesine

2 port from out wavesineS21 =

Freq units are MHz

S-parameters

dB and degrees

Reference Z 50 ohm

Magnitude,

phase



General Labeling Scheme for

S-Parameters

1 2

3

4

S11

S31

S21

S41 S22

S43

S12

k port into wavesine

j port from out wavesine=jkS

Sout in



Everything You Ever Wanted to Know

about an Interconnect: Time, frequency, single-ended, differential

Single ended S-parameters Differential S-parameters

Differential T-parametersSingle ended T-parameters

TDR

Term

Term3

Z=50 Ohm

Num=3

Term

Term1

Z=50 Ohm

Num=1

Term

Term2

Z=50 Ohm

Num=2

Term

Term4

Z=50 Ohm

Num=4ML2CTL_V

CLin1

S[1]=S_1 mil

W [1]=W _1 m il

Length=Len in

Subst="Subst1"

ADS

VNA

Frequency Domain

Time Domain



An Important Caveat

• S-parameters are just a format to store information

• The quality of the information depends on:

The quality of the measurement: calibration, fixturing, de-embedding,R.

The quality of the model used in the simulation: materials properties, geometry information, de-featuring, input information, intrinsic simulator accuracy

• Just because you have the S-parameters doesn’t mean you have a high quality description

Courtesy of Brad Brim, Sigrity



The “Ends” as “Ports”

incident

reflected

~50Ω Z0 = 50Ω

Vsource

50 Ω source impedance

Magnitudeand phaseDetector

Port 1Port 2

In principle, source impedance can be set to anything

In practice: unless there is STRONG compelling reasons, ALWAYS use

source impedance of 50 Ohms

measure

simulate

Agilent VNA



Reflected Response has All Reflections

from all Interfaces

+

+

=

• A “good” interconnect

Close to 50 Ohms

Little reflected amplitude

Large, negative dB

10% reflected, -20 dB





Convert S11 into Impedance Profile:

T11

Reflection coefficient (mRho)

11T1

11T150~Z

−

+Ω

Imp

ed

an

ce

Pro

file

First order estimates only!)

1.5 GHz/div

Time domain



Is the Slope an Impedance Change

or From Series Resistance?

Is the slope real or due to series R?

Compare T11 with T22

Very symmetrical interconnect!

Slope up is real- very lossy traces



2 Port S-Parameters

• S11: Return loss

It is the reflected signal

Impedance mismatch from 50 ohms throughout the interconnect

A little about losses

• S21: Insertion loss

It is the transmitted signal

Impedance mismatches throughout the interconnect

Losses

~50Ω Z0 = 50Ω

Vsource

Magnitudeand phaseDetector

DUT

magnitude/phase

detector50Ω

Z0= 50Ω

SPICE simulation circuit



36 inch Trace, 50 Ohms, FR5,

Rogers Duroid 5880

2 4 6 80 10

-35

-30

-25

-20

-15

-10

-5

-40

0

freq, GHz

Re

turn

Lo

ss,

dB

2 4 6 80 10

-35

-30

-25

-20

-15

-10

-5

-40

0

freq, GHz

Inse

rtio

n L

oss,

dB

Duroid

FR5

Both interconnects have similar impedance

Gradual decline in insertion loss usually an indication of losses

Small “ripples” usually due to impedance mismatches

Sharp dips usually due to coupling to resonances

Measured with Agilent VNA, displayed with Agilent ADS



S11 from PBGA Package Leads

(open at far end)

1

2

Signal path

via

1 2 3 40 5

-8

-6

-4

-2

-10

0

freq, GHz

S11, dB

Measured return loss of package leads open at

far end,

Spectrum of cavity resonances



Via to Cavity Coupling

3.25 inches

1.187 inches

0.8 inches

Plane resonances expected:

Len = 3.25 in fres = 0.92 GHz



Len

GHz3

Lenx2Dk

GHz12fres ==

0.8 inches

S21

wo return vias

HyperLynx 8.0



Being Bilingual: Frequency

and Time Domain Views

Each domain has a slightly difference

sensitivity

Tight coupling

Weak coupling

Tight coupling

Weak coupling

Weak coupling1 2

Tight coupling

1 2

(other line terminated)

Measured with

Agilent VNA,

displayed with

Agilent PLTS



4-Port Single Ended S-Parameters

1

3

2

4(and their return paths!)

Two, single ended transmission lines with coupling

44434241

34333231

24232221

14131211

SSSS

SSSS

SSSS

SSSS

Stimulus

Re

sp

on

se Interpreting single ended measurements:

S11 : return loss, single ended

S21= S12 : insertion loss, single ended

S31= S13 : near end cross talk

S41= S14 : far end cross talk

in

outin,out

V

VS =

1

3

2

4



Major Source of Industry Confusion

1

3

2

4

Single

ended

ports

Single

ended

ports(and their return paths!)

There is no Convention for Labeling the

Ports for Transmission Line Structures

1

2

3

4

(and their return paths!)

S31 is NEXT

S31 is insertion loss

Recommended:

- Odd on the left, even on the right

- Scalable to n ports



Being Ambidextrous

Diff pair

port 1

Diff pair

port 2


1

3

2

4(and their return paths!)

Two, single ended transmission lines with coupling

One, differential pair

1

3

2

4



Signals in the Two World Views

Diff signal in/out Common signal in/out


1

3

2

4

TWO Single Ended

Transmission Lines

Port 1 Port 2(and their return paths!)

ONE Differential Pair

Transmission Line

Single ended signal in

Single ended signal out:



Four Cases

• SDD Differential signal in differential signal out

• SCC Common signal in common signal out

• SCD Differential signal in common signal out

• SDC Common signal in differential signal out

Diff pair

port 1

Diff pair

port 2


One, differential pair



Every Combination

Differential

Pair Port 1

Differential

Pair Port 2(and their return paths!)

Diff signal in Common signal out

Sxx21

Common signal in

Sxx11

Diff signal out



16 Differential S-Parameter Matrix

Elements

SDD

SCD

SDC

SCC

SDD11: diff impedance profile

SDD21: diff impedance losses

SCC11: comm impedance profile

SCC21: comm impedance losses

SCD11: mode conversion:

reflected comm signal

SCD21: mode conversion:

transmitted comm signal

SDC11: mode conversion:

reflected diff signal

SDC21: mode conversion:

transmitted diff signal

Displayed with Agilent PLTS



Mode Conversion in 3 Different

Channels of a Backplane

Mode conversion is very small, <

-30 dB, peaks at 2 GHz

Suggests skew is very small, <

~3% of a cycle or ~ 15 psec

1 GHz/div

Step response of mode conversion

Peak signal is ~ 4%

If it were on twisted pair,

might generate ~ 50 µA of common current

Might fail FCC

Relative skew

between lines varies



Impedance, in Ohms

XAUI Backplane Measured with

Agilent VNA: Displayed with PLTS

SMA Launch

Daughter card

traces

Daughter

card via

Backplane via

connector Backplane traces

SMA Launch

Daughter card traces

Daughter

card via

Backplane viaconnector

TDD11



S-Parameter Behavioral Models

• A black box

Can come from measurement or EM simulation

They represent “point” solutions

• Data mining

They contain “everything you ever wanted to know” about the interconnect

Can be transformed: time, frequency domains, single-ended, differential forms

• Emulate

Can be integrated in system simulations to evaluate performance

Answer “will this component work in this system”

• Limitations

You cannot modify them

Sometimes difficult to identify root cause of performance limitation

U1

CMOS,3.3V,FAST

2

1

V1

TOP

BO...

TL1

83.5 ohms

447.547 ps

3.000 in

StackupTL2

83.5 ohms

447.547 ps

3.000 in

Stackup

J2

m1_1234.s4p

Port1 Port2

Port3 Port4

J3

m1_1234.s4p

Port1 Port2

Port3 Port4

U2

CMOS,3.3V,FAST

2

1

TL3

83.5 ohms

447.547 ps

3.000 in

Coupled StackupTL4

83.5 ohms

447.547 ps

3.000 in

Stackup

TL5

83.5 ohms

447.547 ps

3.000 in

StackupTL6

83.5 ohms

447.547 ps

3.000 in

Stackup

J4

m1_1234.s4p

Port1 Port2

Port3 Port4



The End!

www.BeTheSignal.com

Signal Integrity Certification Programs

Continuing Education Curriculums

Signal integrity public classes

No Myths Allowed webinar series

Streaming recorded lectures

Hands on labs

Feature articles and columns

SI-Insights quarterly publication

Monthly Pop Quiz

My Blog: What I learned this monthPublished by Prentice Hall, 2009

s-parameters, signal integrity and you or s-parameters...

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