ECE 451
Signal Integrity
Fall 2004
Jose E. Schutt-Aine
Electrical & Computer Engineering
University of Illinois
jose@emlab.uiuc.edu
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
1
Signal Integrity
Ideal
Transmission
Channel
Common
Transmission
Channel
Noisy
Transmission
Channel
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
2
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)
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
3
The Interconnect Bottleneck
Technology
Generation
MOSFET Intrinsic
Switching Delay
Response
Time
1.0 um
~ 10 ps
~ 1 ps
0.01 um
~ 1 ps
~ 100 ps
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
4
Chip-Level Interconnect Delay
Line
Pulse Characteristics:
Line Characteristics
rise time: 100 ps
fall time: 100 ps
pulse width: 4ns
length : 3 mm
near end termination: 50 Ω
far end termination 65 Ω
Near End Response
Far End Response
1
0.7
0.6
Volts
0.45
Logic
threshold
0.175
Board
VLSI
Submicron
Deep Submicron
0.5
Volts
Board
VLSI
Submicron
Deep Submicron
0.725
0.4
0.3
Logic
threshold
0.2
0.1
0
-0.1
-0.1
0
0.4
0.8
1.2
1.6
2
0
0.4
Time (ns)
ECE 451-Fall 2004
0.8
1.2
1.6
2
Time (ns)
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
5
Interconnect Bottleneck
Signal Integrity
Crosstalk
Dispersion
Attenuation
Reflection
Distortion
Loss
Delta I Noise
Ground Bounce
Radiation
Drive Line
Sense Line
Drive Line
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
6
Reflection in Transmission Lines
1.
2.
3.
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
7
Metallic Conductors
σ
Area
th
Leng
Re s ist an ce : R
Le ng th
R=
σ Are a
Package level:
W=3 mils
R=0.0045 Ω/mm
ECE 451-Fall 2004
Submicron level:
W=0.25 microns
R=422 Ω/mm
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
8
Metallic Conductors
Metal
Conductivity
σ (Ω-1 m−1 ×10-7)
Silver
Copper
Gold
Aluminum
Tungsten
Brass
Solder
Lead
Mercury
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
6.1
5.8
3.5
1.8
1.8
1.5
0.7
0.5
0.1
9
Loss in Transmission Lines
RF
SOURCE
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
10
Skin Effect in Transmission Lines
δ
Low Frequency
ECE 451-Fall 2004
High Frequency
Very High Frequency
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
11
Skin Effect in Microstrip
..
D
w
d
t
σ
y
-
J=
J oe
y /d
ude
gnit
a
M
/
- jy
d
e
ity
ens
d
t
n
urre
of c
V
εer
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
12
Skin Effect
The electric field in a material medium propagates as
E o e −γz = E oe −αz e− jβz
z
where γ = α + jβ. We also have
γ=ω
σ
µε(1+j ) .
ωε
Wint
Hint
δs
CURRENT AREAS
δs
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
13
Skin effect and internal inductance
Current density varies as
J = Jo e −y / δ e − jy / δ
Note that the phase of the current density varies as a function of y. The total
current is given by:
∞
I = ∫ J o we − y / δ e − jy / δ dy =
0
σ Eo = J o ⇒ Eo =
J o wδ
1+ j
Jo
σ
The voltage measured over a section of the conductor of length L is:
V = Eo D =
ECE 451-Fall 2004
Jo D
σ
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
14
Skin effect and internal inductance
The “skin effect” impedance is therefore
Z skin =
V J o D (1 + j ) D
=
= (1 + j ) π f µρ
σ J o wδ w
I
1
is the bulk resistivity of the conductor
where ρ =
σ
Z skin = R skin + jX skin
with
Rskin = X skin =
ECE 451-Fall 2004
D
π f µρ
w
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
15
Lossy Transmission Line
I
L
R
+
V
C
G
∆z
Telegraphers Equation
∂V
−
= (R+ jωL)I = ZI
∂z
∂I
− = (G + jωC)V = YV
∂z
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
16
Lossy Transmission Line
z
R, L, G, C,
forward wave
backward wave
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
17
Coupled Lines and Crosstalk
w
εr
h
V1
V2
s
Cs
I1
Cm
I2
Lm
Cs
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
18
Crosstalk noise depends on termination
50 Ω
50 Ω
line 1
line 1
line 2
50 Ω
line 1
line 2
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
line 2
line 1
line 2
19
Crosstalk depends on signal rise time
50 Ω
line 1
line 2
50 Ω
tr = 1 ns
tr = 7 ns
line 1
line 2
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
line 1
line 2
20
Crosstalk depends on signal rise time
50 Ω
line 1
line 2
tr = 1 ns
tr = 7 ns
line 1
line 2
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
line 1
line 2
21
Sense Line at Near End
Drive Line at Near End
0.2
1
0.15
0.8
0.1
Volts
Volts
0.6
0.05
0.4
0
-0.05
0.2
-0.1
0
-0.15
-0.2
0
0
5
10
15
20
25
30
35
5
10
15
20
25
30
35
40
40
Time (ns)
Time (ns)
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
22
7-Line Coupled-Microstrip System
ALS04
ALS240
Drive Line 1
ALS04
Drive Line 2
ALS04
Drive Line 3
ALS240
ALS240
Sense Line 4
ALS04
Drive Line 5
ALS04
Drive Line 6
ALS04
ALS240
ALS240
ALS240
Drive Line 7
z=0
z=l
Ls = 312 nH/m; Cs = 100 pF/m;
Lm = 85 nH/m;
ECE 451-Fall 2004
Cm = 12 pF/m.
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
23
Drive Line 3
Drive line 3 at Near End
Drive Line 3 at Far End
5
5
4
4
3
3
2
2
1
1
0
0
0
ECE 451-Fall 2004
100
Time (ns)
200
-1
0
100
200
Time (ns)
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
24
Sense Line
Sense Line at Near End
2
2
Sense Line at Far End
1
1
0
0
-1
-1
0
100
200
0
Time (ns)
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
100
200
Time (ns)
25
Multiconductor Simulation
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
26
Intersymbol Interference (ISI)
• 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
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
27
Intersymbol Interference
Volts
Ideal waveform beginning transistion
from low to high with no noise on the bus
Timing difference
due to ISI
Receiver switching threshold
Time
Different starting point due to ISI
Waveform beginning transition from low to high
with unsettled noise on the bus
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
28
Intersymbol Interference and Signal Integrity
30 ohms
V
4
Probe point
Zo = 65 ohms
200 MHz switching on above bus
400 MHz switching on above bus
3
2
1
0
-1
Ideal 400 MHz waveform
-2
ECE 451-Fall 2004
Time
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
29
Minimizing ISI
• 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
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
30
Ringback and Rise Time Control
(waveform into reference load)
(waveform at receiver)
Vih
Threshold
Vil
Maximum flight time measured
at last crossing of Vih or Vil
Time
•
•
•
•
•
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
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
31
Voltage Reference Uncertainty
Vref + uncertainty
Vih
Threshold region
Threshold
Vil
Vref - uncertainty
Time
Major Contributors
•
•
•
•
•
Power supply effects (SSN, ground bounce, rail collapse)
Noise from IC
Receiver transistor mismatches
Return path discontinuities
Coupling to reference voltage circuitry
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
32
Jitter Definition
Jitter is difference in time of when something
was ideally to occur and when it actually did occur.
Some devices specify the amount of marginal jitter and total
jitter that it can take to operate correctly. If the cable adds
more 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
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
33
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
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
34
Eye Diagram
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
35
Eye Pattern Analysis
Fiber
Pseudorandom
sequence
generator
Data
Clk
Transmitter
Receiver
Scope
Trig Vert
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
36
Jitter
• 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)
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
37
Causes of Deterministic 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
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
38
Eye Diagram - ADS Simulation
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
39
Eye Diagram - ADS Simulation
Ideal Matched Line
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
40
Eye Diagram - ADS Simulation
5 GHz Data Transmission
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
41
Eye Diagram - ADS Simulation
5 GHz Data Transmission
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
42
Eye Diagram - ADS Simulation
10 GHz Data Transmission
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
43
Eye Diagram - ADS Simulation
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
44
Bit-Error Rate
• 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 Sparameters obtained in the frequency domain
ECE 451-Fall 2004
Copyright © by Jose E. Schutt-Aine , All Rights Reserved
45