ece 451 signal integrity - illinoisemlab.uiuc.edu/ece451/appnotes/si.pdf · far end response board...

1Copyright © by Jose E. Schutt-Aine , All Rights ReservedECE 451-Fall 2004

ECE 451Signal Integrity

Fall 2004

Jose E. Schutt-AineElectrical & Computer Engineering

University of [email protected]


TransmissionChannel

TransmissionChannel

TransmissionChannel

Ideal

Common

Noisy

Signal Integrity


• Attenuation & Loss (skin effect, on-chip loss)• Crosstalk (interconnect proximity, coupling)• Dispersion (frequency dependence of parameters)• Reflection (unmatched loads, reactive loads, ISI)• Distortion (nonlinear loads)• Interference & Radiation (EMI/EMC)• Rise time degradation• Clock skew (different electrical path lengths)

Signal Integrity


The Interconnect BottleneckThe Interconnect Bottleneck

TechnologyGeneration

MOSFET IntrinsicSwitching Delay

ResponseTime

1.0 um

0.01 um

~ 10 ps

~ 1 ps

~ 1 ps

~ 100 ps


ChipChip--Level Interconnect DelayLevel Interconnect DelayLine

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Vol

ts

0 0.4 0.8 1.2 1.6 2Time (ns)

Far End Response

BoardVLSISubmicronDeep Submicron

-0.1

0.175

0.45

0.725

1

0 0.4

Vol

ts

0.8 1.2 1.6 2Time (ns)

Near End Response

BoardVLSISubmicronDeep Submicron

Pulse Characteristics: rise time: 100 ps fall time: 100 ps pulse width: 4ns

Line Characteristics length : 3 mm near end termination: 50 Ω far end termination 65 Ω

LogicthresholdLogic

threshold


Signal Integrity

Crosstalk Dispersion Attenuation

Reflection Distortion Loss

Delta I Noise Ground Bounce Radiation

Sense Line

Drive Line

Drive Line

Interconnect BottleneckInterconnect Bottleneck


Reflection in Transmission Lines

1.

2.

3.


Metallic Conductors

Length

σArea

Re sist an ce : R

Package level:W=3 milsR=0.0045 Ω/mm

R = Le ng thσ Are a

Submicron level:W=0.25 micronsR=422 Ω/mm


Metal Conductivityσ (Ω-1 m−1 ×10-7)

Silver 6.1Copper 5.8Gold 3.5Aluminum 1.8Tungsten 1.8Brass 1.5Solder 0.7Lead 0.5Mercury 0.1

Metallic Conductors


RF SOURCE

Loss in Transmission Lines


δ

Low Frequency High Frequency Very High Frequency

Skin Effect in Transmission Lines


. .

Magnitude of current density

y

σ

w

t

e

D

V

J = Joe- y /d e

- jy / d

d

Skin Effect in Microstrip

εr


The electric field in a material medium propagates as

z

Eoe−γz = Eoe−αze− jβz

where γ = α + jβ. We also have

γ = ω µε(1+jσ

ωε) .

Skin EffectSkin Effect

Wint

δs

δs

Hint

CURRENT AREAS


Current density varies as

J = Joe−y / δe− jy / δ

Note that the phase of the current density varies as a function of y. The total

current is given by:

/ /

0 1y jy o

oJ wI J we e dy

jδ δ δ∞

− −= =+∫

oo o o

JE J Eσσ

= ⇒ =

The voltage measured over a section of the conductor of length L is:

oo

J DV E Dσ

= =

Skin effect and internal inductance


The “skin effect” impedance is therefore

(1 ) (1 )oskin

o

J DV j DZ j fI J w w

π µρσ δ

+= = = +

where ρ =1σ

is the bulk resistivity of the conductor

Zskin = Rskin + jXskin

with

skin skinDR X fw

π µρ= =

Skin effect and internal inductance


−∂V∂z

= (R+ jωL)I = ZI

−∂I∂z

= (G+ jωC)V = YV

Lossy Transmission LineL

∆z

C

I

V

+

-

G

R

Telegraphers Equation


z

R, L, G, C,

Lossy Transmission Line

forward wave

backward wave


Coupled Lines and Crosstalk

εr

w s

h

Cs

V1

V2

I1

I2

Cs

Cm Lm


50 Ωline 1

line 2

50 Ω

line 1

line 2

50 Ωline 1

line 2

line 1

line 2

Crosstalk noise depends on termination


50 Ωline 1

line 2

50 Ω

line 1

line 2

line 1

line 2

tr = 1 ns tr = 7 ns

Crosstalk depends on signal rise time


tr = 1 ns tr = 7 ns

Crosstalk depends on signal rise time

50 Ωline 1

line 2

line 1

line 2

line 1

line 2


-0.2

0

0.2

0.4

0.6

0.8

1

Vol

ts

0 5 10 15 20 25 30

Time (ns)

Drive Line at Near End

35 40

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Vol

ts

0 5 10 15 20 25 30

Time (ns)

Sense Line at Near End

35 40


ALS04 ALS240Drive Line 1

Drive Line 2

z=0 z=l

Drive Line 3

Sense Line 4

Drive Line 5

Drive Line 6

Drive Line 7

ALS04

ALS04

ALS04

ALS04

ALS04

ALS240

ALS240

ALS240

ALS240

ALS240

7-Line Coupled-Microstrip System

Ls = 312 nH/m; Cs = 100 pF/m;

Lm = 85 nH/m; Cm = 12 pF/m.


20010000

1

2

3

4

5Drive line 3 at Near End

Time (ns)2001000

-1

0

1

2

3

4

5Drive Line 3 at Far End

Time (ns)

Drive Line 3


2001000-1

0

1

2Sense Line at Near End

Time (ns)2001000

-1

0

1

2Sense Line at Far End

Time (ns)

Sense Line


Multiconductor Simulation


• Signal launched on a transmission line can be affected by previous signals as result of reflections

• ISI can be a major concern especially if the signal delay is smaller than twice the time of flight

• ISI can have devastating effects

• Noise must be allowed to settled before next signal is sent

Intersymbol Interference (ISI)


Volts

Time

Waveform beginning transition from low to highwith unsettled noise on the bus

Different starting point due to ISI

Receiver switching threshold

Timing differencedue to ISI

Ideal waveform beginning transistionfrom low to high with no noise on the bus

Intersymbol Interference


-2

-1

0

1

2

3

4200 MHz switching on above bus

400 MHz switching on above bus

Ideal 400 MHz waveformTime

Probe pointZo = 65 ohms30 ohms

V

Intersymbol Interference and Signal Integrity


• Minimize reflections on the bus by avoiding impedance discontinuities

• Minimize stub lengths and large parasitics from package sockets or connectors

• Keep interconnects as short as possible (minimize delay)

• Minimize crosstalk effects

Minimizing ISI


Vih

Vil

Threshold

(waveform into reference load)

(waveform at receiver)

Maximum flight time measuredat last crossing of Vih or Vil

Time

Ringback and Rise Time Control

• Violation into threshold region• Detrimental even if threshold is not crossed• Can exacerbate ISI• Can be aggravated by nonlinear (time varying) terminations • Can increase skew between signals


• Power supply effects (SSN, ground bounce, rail collapse)• Noise from IC• Receiver transistor mismatches• Return path discontinuities • Coupling to reference voltage circuitry

Voltage Reference Uncertainty

Threshold region

Vref + uncertainty

Vref - uncertainty

Time

Threshold

Vih

Vil

Major Contributors


Jitter is difference in time of when somethingwas ideally to occur and when it actually did occur.

Some devices specify the amount of marginal jitter and totaljitter that it can take to operate correctly. If the cable addsmore jitter than the receiver’s allowed marginal jitter and total jitter the signal will not be received correctly. In this case the jitter is measured as in the below diagram

Jitter Definition


Eye Diagrams

• Eye diagrams are a time domain display of digital data triggered on a particular cycle of the clock. Each period is repeated and superimposed. Each possible bit sequence should be generated so that a complete eye diagram can be made


Eye Diagram


Pseudorandomsequencegenerator

Transmitter Receiver

Scope

Trig Vert

Clk

Data

FiberEye Pattern Analysis


• Jitter is a signal timing deviation referenced to a recovered clock from the recovered bit stream

• Measured in Unit Intervals and captured visually with eye diagrams

• Two types of jitter– Deterministic (non Gaussian)– Random

• The total jitter (TJ) is the sum of the random (RJ) and deterministic jitter(DJ)

Jitter


• Crosstalk– Noisy neighboring signals

• Interference

• Reflections– Imperfect terminations– Discontinuities (e.g. multidrop buses, stubs)

• Simultaneous switching noise (SSN)– Noisy reference plane or power rail– Shift in threshold voltages

Causes of Deterministic Jitter


Eye Diagram - ADS Simulation


Eye Diagram - ADS SimulationIdeal Matched Line


Eye Diagram - ADS Simulation5 GHz Data Transmission


Eye Diagram - ADS Simulation


• The Bit-error rate (BER) quantifies the likelihood of a bit being interpreted at the receiver incorrectly due to jitter- or amplitude-induce degradation on the received signal

• No higer than a 10-16 BER is tolerable => no more than 1 error out of 1016 bits.

• BER can be measured directly or quantified with statistical calculations

• Deterministic jitter(DJ) can be easily measured via S-parameters obtained in the frequency domain

Bit-Error Rate

ece 451 signal integrity - illinoisemlab.uiuc.edu/ece451/appnotes/si.pdf · far end response board...

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