Upload
frankliu
View
217
Download
1
Embed Size (px)
Citation preview
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 1/44
Sam Palermo Analog & Mixed-Signal Center
Texas A&M University
ECEN720: High-Speed LinksCircuits and Systems
Spring 2013
Lecture 2: Channel Components, Wires, & Transmission Lines
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 2/44
Announcements
• HW1 due now• Lab1 will be on Jan. 28 in ZACH 203
• Prelab 1 is due on Jan 30 in class
• Reference Material Posted on Website• TDR theory application note• S-parameter notes
• Anyone graduating or want an intern this summer?• I know of several companies with openings in the high-
speed link area
2
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 3/44
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
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 4/44
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]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 5/44
IC Packages
• Package style dependson application and pincount
• Packaging technologyhasn’t 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 - 420Enhanced Ball Grid Array (EBGA) 352 - 896
Flip Chip Ball Grid Array (FC-BGA) 1089 - 2116SOP
PBGA
QFP
FC-BGA
[Package Images - Fujitsu]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 6/44
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]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 7/44
IC Package Model
7
Bondwires• L ~ 1nH/mm•Mutual L “K” • Ccouple
~ 20fF/mm
Package Trace• L ~ 0.7-1nH/mm•Mutual L “K” • Clayer
~ 80-90fF/mm•C
couple
~ 40fF/mm
[Dally]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 8/44
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
8
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 9/44
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
9
[Dally]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 10/44
Connectors
• Connectors are usedto transfer signalsfrom board-to-board
• Typical differentialpair density between
16-32 pairs/10mm
10
[Tyco]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 11/44
Connectors
• Important to maintain proper differentialimpedance through connector
11
• Crosstalk can be an issue in the connectors
[Tyco]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 12/44
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 create
“stubs”
12
[Dally]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 13/44
Impact of Via Stubs at Connectors
13
• Legacy BP has default straight vias• Creates severe nulls which kills signal integrity
• Refined BP has expensive backdrilled vias
Edge connector
Packaged SerDes
Line card traceBackplane trace
Via stub
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 14/44
PCB Trace Configurations
• Microstrips are signaltraces on PCB outersurfaces• Trace is not enclosed
and susceptible tocross-talk
• Striplines aresandwiched betweentwo parallel groundplanes• Has increased isolation
14
[Johnson]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 15/44
Wire Models
• Resistance
• Capacitance
• Inductance
• Transmission line theory
15
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 16/44
Wire Resistance
• Wire resistance is determined by materialresistivity, ρ , and geometry
• Causes signal loss and propagation delay
16
whl
Al
R ρ ρ ==2rl
Al
Rπ ρ ρ ==
[Dally]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 17/44
Wire Capacitance
• Wire capacitance is determinedby dielectric permittivity, ε ,and geometry
• Best to use lowest ε r
• Lower capacitance• Higher propagation velocity
17
sw
C ε =( )
12log
2rr
C πε =( )rs
Clog
πε =( )hss
wC
4log2πε ε +=
[Dally]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 18/44
Wire Inductance
• Wire inductance is determined by materialpermeability, µ , and closed-loop geometry
• For wire in homogeneous medium
• Generally
18
εµ =CL
H/m104 70 −×== π µ µ
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 19/44
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)
19
LR
f π 20
≥
Wire R L C >f (LC wire)
AWG24 Twisted Pair 0.08 Ω /m 400nH/m 40pF/m 32kHz
PCB Trace 5 Ω /m 300nH/m 100pF/m 2.7MHz
On-Chip Min. Width M6(0.18µm CMOS node) 40k Ω /m 4µH/m 300pF/m 1.6GHz
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 20/44
RLGC Transmission Line Model
20
( ) ( ) ( )t
txILtxRIx
txV∂
∂−−=∂
∂ ,,,
( ) ( ) ( )t
txVCtxGV
xtxI
∂∂−−=
∂∂ ,
,,
0dxAs →(1)
(2)
GeneralTransmissionLine Equations
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 21/44
Time-Harmonic Transmission Line Eqs.
• Assuming a traveling sinusoidal wave with angular frequency, ω
21
( ) ( ) ( )xIL jRdx
xdVω +−=
( ) ( ) ( )xVC jGdx
xdIω +−=
• Differentiating (3) and plugging in (4) (and vice versa)
( ) ( )xVdx
xVd 22
2
γ =
( ) ( )xIdx
xId 22
2γ =
• where γ is the propagation constant
( )( ) ( )-1m C jGL jR j ω ω β α γ ++=+=
(5)
(6)
Time-HarmonicTransmissionLine Equations
(3)
(4)
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 22/44
Transmission Line Propagation Constant
• Solutions to the Time-Harmonic Line Equations:
22
( ) ( ) ( ) xr
xf rf eVeVxVxVxV γ γ
00 +=+= −
• What does the propagation constant tell us?
• Real part ( α ) determines attenuation/distance (Np/m)• Imaginary part ( β) determines phase shift/distance (rad/m)• Signal phase velocity
( ) ( ) ( ) xr
xf rf eIeIxIxIxI γ γ
00 +=+= −
where ( )( ) ( )-1m C jGL jR j ω ω β α γ ++=+=
(m/s) β ω υ =
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 23/44
Transmission Line Impedance, Z 0
• For an infinitely long line, the voltage/current ratio is Z 0• From time-harmonic transmission line eqs. (3) and (4)
23
( )( ) ( )
Ω+
+== 0 C jG
L jR
xI
xVZ
ω
ω
• Driving a line terminated by Z 0 is the same as driving an
infinitely long line
[Dally ]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 24/44
Lossless LC Transmission Lines
• If Rdx=Gdx=0
24
LC
LC j j
ω β
α
ω β α γ
=
==+=
0
CL
Z
LC
=
==
0
1 β ω υ
No Loss!
• Waves propagate w/o distortion• Velocity and impedance
independent of frequency• Impedance is purely real
[Johnson ]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 25/44
Low-Loss LRC Transmission Lines
• If R/ ωL and G/ ωC << 1• Behave similar to ideal
LC transmission line,
but …• Experience resistive anddielectric loss
• Frequency dependent
propagation velocityresults in dispersion• Fast step, followed by slow
DC tail
25
( )( )
β α α
ω ω ω
ω ω
ω ω β α γ
j
CG
LR
LC jGZ
ZR
LCGLRC
jLC j
C jGL jR j
DR++=
+
+++≅
+−≅
++=+=
220
0
21
81
811
22
1
2
2
0
0
GZZR
D
R
≅
≅
α
α
+ +≅
22
81811 CGLRLC ω ω ω β
122
81
811
−
+
+≅
CG
LR
LCω ω
υ
Resistive Loss
Dielectric Loss
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 26/44
Skin Effect (Resistive Loss)
• High-frequency current density fallsoff exponentially from conductorsurface
• Skin depth, δ, is where current fallsby e -1 relative to full conductor
• Decreases proportional tosqrt(frequency)
• Relevant at critical frequency f swhere skin depth equals half conductor height (or radius)
• Above f s resistance/loss increasesproportional to sqrt(frequency)
26
δ d
e J−
= ( ) 21−= µσ π δ f
2
2
=h
f sπµ
ρ
( )21
=
sDC f f Rf R
21
02
=
s
DCR f
f Z
Rα
For rectangular conductor:
[Dally ]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 27/44
Skin Effect (Resistive Loss)
27
[Dally ]
MHzf mR sDC 43 ,7 =Ω=5-mil Stripguide
kHzf mR sDC 67 ,08.0 =Ω=30 AWG Pair
21
02
=
s
DC
R f
f
Z
Rα
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 28/44
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
28
CGD ω
δ =tan
LCf
CLfCGZ
D
DD
δ π
δ π α
tan2
tan22
0
=
==
[Dally ]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 29/44
Total Wire Loss
29
[Dally ]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 30/44
Reflections & Telegrapher’s Eq.
30
T
i T ZZ
VI +
=0
2
+
−=
+−=
−==
0
0
0
00
0
2
,
ZZ
ZZ
Z
VI
ZZV
ZVI
IIIZ
VI
T
Tir
T
iir
Tf ri
f
0
0
ZZZZ
VV
IIk
T
T
i
r
f
rr +
−===
Termination Current:• With a Thevenin-equivalent model of the line:
• KCL at Termination:Telegrapher’s Equation or
Reflection Coefficient
[Dally ]
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 31/44
Termination Examples - Ideal
31
R S = 50Ω
Z0 = 50 Ω , t d = 1ns
R T = 50 Ω
05050
5050
050505050
5.05050
501
=+
−=
=+−=
=
+=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
source
termination
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 32/44
Termination Examples - Open
32
R S = 50Ω
Z0 = 50 Ω , t d = 1ns
R T ~ ∞ (1M Ω )
05050
5050
15050
5.05050
501
=+
−=
+=+∞−∞=
=
+=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
source
termination
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 33/44
Termination Examples - Short
33
R S = 50Ω
Z0 = 50 Ω , t d = 1ns
R T = 0 Ω
05050
5050
1500500
5.05050
501
=+
−=
−=+−=
=
+=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
source
termination
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 34/44
Arbitrary Termination Example
34
R S = 400Ω
Z0 = 50 Ω , t d = 1ns
R T = 600 Ω
778.050400
50400
846.05060050600
111.050400
501
=+
−=
=+−=
=
+=
rS
rT
i
k
k
VVV
in (step begins at 1ns)
sourcetermination
0.111V 0.205V
0.278V 0.340
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 35/44
Lattice Diagram
35
R S = 400Ω
R T = 600 Ω
Z0 = 50 Ω , t d = 1ns
in (step begins at 1ns)
Rings up to 0.6V (DC voltage division)
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 36/44
Termination Reflection Patterns
36
R S = 25 Ω , RT = 25 Ω
kr S & kr T < 0
Voltages Converge
R S = 25 Ω , RT = 100 Ω
kr S < 0 & kr T > 0
Voltages Oscillate
R S = 100 Ω , RT = 25 Ω
kr S > 0 & kr T < 0
Voltages Oscillate
R S = 100 Ω , RT = 100 Ω
kr S > 0 & kr T > 0
Voltages Ring Up
source
termination
sourcetermination
source
termination
source
termination
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 37/44
Termination Schemes
37
• No Termination• Little to absorb line energy• Can generate oscillating
waveform• 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=> −
LClt porch 2≅
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 38/44
Termination Schemes
38
• Receiver Termination• No reflection from receiver• Watch out for intermediate
impedance discontinuities• Little to absorb reflections at driver
• Double Termination• Best configuration for min
reflections• Reflections absorbed at both driver
and receiver• Get half the swing relative to
single termination• Most common termination scheme
for high performance serial links
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 39/44
Differential Signaling
39
• Differential signaling advantages• Self-referenced• Common-mode noise rejection
• Increased signal swing• Reduced self-induced power-supply noise
• Requires 2x the number of signaling pins
relative to single-ended signaling• But, smaller ratio of supply/signal (return) pins• Total pin overhead is typically 1.3-1.8x (vs 2x)
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 40/44
Odd & Even Modes
40
[Hall ]
• Even mode• When equal voltages drive both lines, only one mode propagates called even more
• Odd mode• When equal in magnitude, but out of phase, voltages drive both lines, only one
mode propagates called odd mode• For a differential pair (odd mode), a virtual reference plane exists between
the conductors that provides a continuous return current path• Electric field is perpendicular to the virtual plane• Magnetic field is tangent to the virtual plane
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 41/44
Balanced Transmission Lines
• Even (common) modeexcitation• Effective C = C C
• Effective L = L + M• Odd (differential) mode
excitation
• Effective C = C C + 2C d• Effective L = L – M
41
21
21
2
+−=
+=
dcodd
c
even
CCML
Z
C
MLZ
[Dally ]
2 ,2 even
CModdDIFFZ
ZZZ ==
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 42/44
PI-Termination
42
1RZeven=
2||2|| 221 RZRRZ evenodd==
−
=oddeven
evenodd
ZZZZ
R 22
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 43/44
T-Termination
43
12 2RRZeven +=
( )oddeven
odd
ZZR
RZ
−=
=
21
1
2
7/29/2019 Lecture2 Ee720 Channels
http://slidepdf.com/reader/full/lecture2-ee720-channels 44/44
Next Time
• Channel modeling• Time domain reflectometer (TDR)• Network analysis