Lecture2 Ee689 Channels
Transcript of Lecture2 Ee689 Channels
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Sam Palermo
Analog & Mixed-Signal Center
Texas A&M University
ECEN689: Special Topics in High-Speed
Links Circuits and SystemsSpring 2012
Lecture 2: Channel Components, Wires, & Transmission Lines
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Announcements
HW1 due NOW
Lab1 tomorrow in ZACH 203
Prelab 1 due tomorrow
Reference Material Posted on Website
TDR theory application note
S-parameter notes
2
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Agenda
Channel Components IC Packages, PCBs, connectors, vias, PCB Traces
Wire Models
Resistance, capacitance, inductance
Transmission Lines
Propagation constant
Characteristic impedance
Loss Reflections
Termination examples
Differential transmission lines
3
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Channel Components
4
Edge connector
Packaged SerDes
Line card trace
Backplane trace
Via stub
The ChannelTx IC
Pkg Line cardtrace
Edgeconnector
Line cardvia
Backplanevia
Backplane16 trace
Edgeconnector
Line cardtrace
Rx IC
Pkg Backplanevia
Line cardvia
[Meghelli (IBM) ISSCC 2006]
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IC Packages
Package style dependson application and pincount
Packaging technologyhasnt been able toincrease pin count atsame rate as on-chipaggregate bandwidth
Leads to I/O constraineddesigns and higher datarate per pin
5
Package Type Pin Count
Small Outline Package (SOP) 8 56
Quad Flat Package (QFP) 64 - 304
Plastic Ball Grid Array (PBGA) 256 - 420
Enhanced Ball Grid Array (EBGA) 352 - 896
Flip Chip Ball Grid Array (FC-BGA) 1089 - 2116
SOP
PBGA
QFP
FC-BGA
[Package Images - Fujitsu]
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IC Package Examples
Wirebonding is mostcommon die attach method
Flip-chip packaging allows
for more efficient heatremoval
2D solder ball array onchip allows for moresignals and lower signaland supply impedance
6
Standard Wirebond Package
Flip-Chip/Wirebond Package
Flip-Chip/Solder Ball Package
[Package Images - Fujitsu]
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IC Package Model
7
BondwiresL ~ 1nH/mmMutual L KCcouple
~20fF/mm
Package TraceL ~ 0.7-1nH/mmMutual L KClayer
~80-90fF/mmC
couple
~40fF/mm
[Dally]
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Printed Circuit Boards
Components soldered ontop (and bottom)
Typical boards have 4-8signal layers and anequal number of powerand ground planes
Backplanes can haveover 30 layers
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PCB Stackup
Signals typically on top andbottom layers
GND/Power plane pairs and
signal layer pairs alternate inboard interior
Typical copper trace thickness
0.5oz (17.5um) for signal layers 1oz (35um) for power planes
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Connectors
Connectors are usedto transfer signalsfrom board-to-board
Typical differentialpair density between
16-32 pairs/10mm
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[Tyco]
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Connectors
Important to maintain proper differentialimpedance through connector
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Crosstalk can be an issue in the connectors
[Tyco]
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Vias
Used to connect PCB layers
Made by drilling a hole throughthe board which is plated with
copper Pads connect to signal layers/traces
Clearance holes avoid power planes
Expensive in terms of signaldensity and integrity
Consume multiple trace tracks
Typically lower impedance and createstubs
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[Dally]
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Impact of Via Stubs at Connectors
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Legacy BPhas default straight vias
Creates severe nulls which kills signal integrity
Refined BP has expensive backdrilled vias
Edge connector
Packaged SerDes
Line card trace
Backplane trace
Via stub
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PCB Trace Configurations
Microstrips are signaltraces on PCB outersurfaces
Trace is not enclosed
and susceptible tocross-talk
Striplines aresandwiched between
two parallel groundplanes
Has increased isolation
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[Johnson]
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Wire Models
Resistance
Capacitance
Inductance
Transmission line theory
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Wire Resistance
Wire resistance is determined by materialresistivity, , and geometry
Causes signal loss and propagation delay
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wh
l
A
lR
==
2r
l
A
lR
==
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Wire Capacitance
Wire capacitance is determinedby dielectric permittivity, ,
and geometry
Best to use lowest r
Lower capacitance
Higher propagation velocity
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s
wC
=
( )12
log
2
rrC
=
( )rsC
log
=
( )hssw
C4log
2+=
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Wire Inductance
Wire inductance is determined by materialpermeability, , and closed-loop geometry
For wire in homogeneous medium
Generally
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=CL
H/m104 70 ==
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Wire Models
Model Types Ideal
Lumped C, R, L
RC transmission line
LC transmission line
RLGC transmission line
Condition for LC or RLGC model (vs RC)
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L
Rf
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Wire R L C >f (LC wire)
AWG24 Twisted Pair 0.08/m 400nH/m 40pF/m 32kHz
PCB Trace 5/m 300nH/m 100pF/m 2.7MHz
On-Chip Min. Width M6(0.18m CMOS node)
40k/m 4H/m 300pF/m 1.6GHz
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RLGC Transmission Line Model
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( ) ( ) ( )ttxILtxRI
xtxV
=
,,,
( )( )
( )t
txVCtxGV
x
txI
=
,,
,
0dxAs (1)
(2)
GeneralTransmissionLine Equations
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Time-Harmonic Transmission Line Eqs.
Assuming a traveling sinusoidal wave with angular frequency,
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( )( ) ( )xILjR
dx
xdV+=
( )( ) ( )xVCjG
dx
xdI+=
Differentiating (3) and plugging in (4) (and vice versa)
( )( )xV
dx
xVd 22
2
=
( ) ( )xIdx
xId 22
2=
where is the propagation constant
( )( ) ( )-1mCjGLjRj ++=+=
(5)
(6)
Time-HarmonicTransmission
Line Equations
(3)
(4)
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Transmission Line Propagation Constant
Solutions to the Time-Harmonic Line Equations:
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( ) ( ) ( ) xrx
frf eVeVxVxVxV
00 +=+=
What does the propagation constant tell us?
Real part () determines attenuation/distance (Np/m) Imaginary part () determines phase shift/distance (rad/m)
Signal phase velocity
( ) ( ) ( ) xrx
frf eIeIxIxIxI
00 +=+=
where ( )( ) ( )-1mCjGLjRj ++=+=
(m/s)=
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Transmission Line Impedance, Z0
For an infinitely long line, the voltage/current ratio is Z0 From time-harmonic transmission line eqs. (3) and (4)
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( )
( ) ( )
+
+==
0 CjG
LjR
xI
xVZ
Driving a line terminated by Z0 is the same as driving aninfinitely long line
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Lossless LC Transmission Lines
If Rdx=Gdx=0
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LC
LCjj
=
=
=+=
0
C
LZ
LC
=
==
0
1
No Loss!
Waves propagate w/o distortion Velocity and impedance
independent of frequency
Impedance is purely real
[Johnson]
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Low-Loss LRC Transmission Lines
If R/L and G/C
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Skin Effect (Resistive Loss)
High-frequency current density fallsoff exponentially from conductorsurface
Skin depth, , is where current fallsby e-1relative to full conductor
Decreases proportional tosqrt(frequency)
Relevant at critical frequency fswhere skin depth equals half
conductor height (or radius) Above fsresistance/loss increases
proportional to sqrt(frequency)
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d
eJ
= ( ) 21
= f
2
2
=
hfs
( )2
1
=
s
DCffRfR
2
1
02
=
s
DCR
f
f
Z
R
For rectangular conductor:
[Dally]
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Skin Effect (Resistive Loss)
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[Dally]
MHzfmR sDC 43,7 ==
5-mil Stripguide
kHzfmR sDC 67,08.0 ==
30 AWG Pair
2
1
02
=
s
DC
R f
f
Z
R
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Dielectric Absorption (Loss)
An alternating electric fieldcauses dielectric atoms torotate and absorb signalenergy in the form of heat
Dielectric loss is expressedin terms of the losstangent
Loss increases directlyproportional to frequency
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CG
D
=tan
LCf
CLfCGZ
D
D
D
tan
2tan2
2
0
=
==
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Total Wire Loss
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Reflections & Telegraphers Eq.
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T
iT
ZZ
VI
+=
0
2
+
=
+=
=
0
0
0
00
2
ZZ
ZZ
Z
VI
ZZ
V
Z
VI
III
T
Tir
T
iir
Tfr
0
0
ZZ
ZZ
V
V
I
Ik
T
T
i
r
i
rr
+
===
Termination Current: With a Thevenin-equivalent model of the line:
KCL at Termination:
Telegraphers Equation or
Reflection Coefficient
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Termination Examples - Ideal
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RS = 50
Z0= 50, td = 1ns
RT = 50
0
5050
5050
05050
5050
5.05050
501
=
+
=
=+
=
=
+
=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
source
termination
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Termination Examples - Open
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RS = 50
Z0= 50, td = 1ns
RT ~ (1M)
0
5050
5050
150
50
5.05050
501
=
+
=
+=+
=
=
+
=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
source
termination
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Termination Examples - Short
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RS = 50
Z0= 50, td = 1ns
RT = 0
0
5050
5050
1500
500
5.05050
501
=
+
=
=+
=
=
+
=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
source
termination
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Arbitrary Termination Example
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RS = 400
Z0= 50, td = 1ns
RT = 600
778.0
50400
50400
846.050600
50600
111.050400
501
=
+
=
=+
=
=
+
=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
sourcetermination
0.111V
0.205V
0.278V0.340
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Lattice Diagram
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RS = 400
RT = 600
Z0= 50, td = 1ns
in (step begins at 1ns)
Rings up to 0.6V(DC voltage division)
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Termination Reflection Patterns
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RS = 25, RT = 25
krS& krT< 0
Voltages Converge
RS = 25, RT = 100
krS < 0 & krT> 0
Voltages Oscillate
RS = 100, RT = 25
krS > 0 & krT< 0
Voltages Oscillate
RS = 100, RT = 100
krS > 0 & krT> 0
Voltages Ring Up
source
termination
source
termination
source
termination
source
termination
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Termination Schemes
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No Termination Little to absorb line energy
Can generate oscillatingwaveform
Line must be very short
relative to signal transition time n = 4 - 6
Limited off-chip use
Source Termination Source output takes 2 steps up Used in moderate speed point-
to-point connections
LCnlnTt triproundr 2=>
LCltporch 2
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Termination Schemes
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Receiver Termination No reflection from receiver
Watch out for intermediateimpedance discontinuities
Little to absorb reflections at driver
Double Termination
Best configuration for minreflections
Reflections absorbed at both driver
and receiver
Get half the swing relative tosingle termination
Most common termination schemefor high performance serial links
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Differential Transmission Lines
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Differential signaling advantages
Self-referenced
Common-mode noise rejection
Increased signal swing
Reduced self-induced power-supply noise
Requires 2x the number ofsignaling pins relative to single-ended signaling
But, smaller ratio ofsupply/signal (return) pins
Total pin overhead is typically1.3-1.8x (vs 2x)
[Hall]
Even mode
When equal voltages drive bothlines, only one mode propagatescalled even more
Odd mode
When equal in magnitude, but outof phase, voltages drive both lines,only one mode propagates calledodd mode
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Balanced Transmission Lines
Even (common) modeexcitation
Effective C = CC
Effective L = L + M Odd (differential) mode
excitation
Effective C = CC + 2Cd Effective L = L M
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21
21
2
+
=
+=
dc
odd
c
even
CC
MLZ
C
MLZ
[Dally]
2,2 evenCModdDIFF
ZZZZ ==
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PI-Termination
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1RZeven =
2||2|| 221 RZRRZ evenodd ==
=
oddeven
evenodd
ZZ
ZZR 22
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T-Termination
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12 2RRZeven +=
( )oddeven
odd
ZZR
RZ
=
=
21
1
2
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Next Time
Channel modeling Time domain reflectometer (TDR)
Network analysis