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A R C H I V E 2 0 0 6Tutorial“Differential Impedance And Insertion Loss Applied To Sockets”Eric BogatinChief Technical OfficerSynergetixCOPYRIGHT NOTICE• The papers in this publication comprise the proceedings of the 2006 BiTS Workshop. They reflect the authors’ opinions and are reproduced as presented , without change. Their inclusion in this publication does not constitute an endorsement by the BiTS Workshop, the sponsors, BiTS Workshop LLC, or the authors.• There is NO copyright protection claimed by this publication or the authors. However, each presentation is the work of the authors and their respective companies: as such, it is strongly suggested that any use reflect proper acknowledgement to the appropriate source. Any questions regarding the use of any materials presented should be directed to the author/s or their companies.• The BiTS logo and ‘Burn-in & Test Socket Workshop’ are trademarks of BiTS Workshop LLC.9„„9999999„99999„99999„9999999Tutorial 22006Differential Impedance and Insertion Loss Applied to Sockets Slide - 1 Differential Impedance and Insertion Loss Applied to Sockets Slide - 2OutlineWho cares?Differential Impedance and Insertion What design features influence insertion loss?Loss Applied to Sockets What is impedanceDr. Eric BogatinWhat design features influence impedance?CTO, SynergetixKansas City, KS What is differential impedanceericb@idinet.comWhat design features influence ...

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A R C H I V E 2 0 0 6
Tutorial
“Differential Impedance And Insertion Loss Applied
To Sockets”
Eric Bogatin
Chief Technical Officer
Synergetix
COPYRIGHT NOTICE
• The papers in this publication comprise the proceedings of the 2006 BiTS Workshop. They reflect the
authors’ opinions and are reproduced as presented , without change. Their inclusion in this publication
does not constitute an endorsement by the BiTS Workshop, the sponsors, BiTS Workshop LLC, or the
authors.
• There is NO copyright protection claimed by this publication or the authors. However, each presentation
is the work of the authors and their respective companies: as such, it is strongly suggested that any use
reflect proper acknowledgement to the appropriate source. Any questions regarding the use of any
materials presented should be directed to the author/s or their companies.
• The BiTS logo and ‘Burn-in & Test Socket Workshop’ are trademarks of BiTS Workshop LLC.9


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Tutorial 2
2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 1 Differential Impedance and Insertion Loss Applied to Sockets Slide - 2
Outline
Who cares?
Differential Impedance and Insertion What design features influence insertion loss?
Loss Applied to Sockets What is impedance
Dr. Eric Bogatin
What design features influence impedance?CTO, Synergetix
Kansas City, KS What is differential impedance
ericb@idinet.com
What design features influence differential impedance
2006 Burn-in and Test Socket Workshop What is differential insertion loss?
March 12-15, 2005
“It is better to uncover a little than to cover a lot”
- Francis Low
© Eric Bogatin 2006 www.BeTheSignal.com © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 3 Differential Impedance and Insertion Loss Applied to Sockets Slide - 4
Electrical Performance of Sockets For More Information
in Perspective
Constraints:• Performance • Vendors
• Corporate CultureCompliance
• Compatibility: Industry, Legacy
Pitchwww.BeTheSignal.com
Cycle lifetime
Online Lectures
Time between cleaning Cost: Feature Articles
$$$, TCOO, Electrical
PCD&M Monthly Signal Integrity Schedule, Risk
– DC resistance
Column: “No Myths Allowed” – Hi Frequency
– Signal IntegrityMaster Class Workshops Partitioning:» Bandwidth
» Insertion loss • Pin electronicsResources » Return loss • Wiring/cabling» SPICE models
• Loadboards– Power integrityPublished by Prentice Hall, 2004
» Loop inductance • Sockets
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 5 Differential Impedance and Insertion Loss Applied to Sockets Slide - 6
rdThe Socket as a Component 3 Best Alternative
• Specify values of model (circuit or behavioral) parameters• Purpose of an interconnect: “to transport a signal
Z
0from one point to another with an acceptable level of TD
Ldistortion”
C
Insertion loss
Return loss
• Specifications based on assumptions of the rest of the systemons
• Specifications are a pre-arranged compromise- sometimes based on:
System level simulation balancing cost-performance-constraints- (really hard!)
A guess
Because it worked in the last design
Enough margin for designer to sleep at night
Assuming performance is freeSimulated with HyperLynxWhat’s important to know?
Incorrect assumptions
1. Will the system work? Information that was passed from engineer to engineer to engineer to engineer…(only
one of whom might have an idea of what they want)2. Is the socket “good enough?”
3. How do you know before you build it and test it?
© Eric Bogatin 2006 © Eric Bogatin 2006
1March 12 - 15, 20069
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Differential Impedance and Insertion Loss Applied to Sockets Slide - 7 Differential Impedance and Insertion Loss Applied to Sockets Slide - 8
Is this acceptable?
Universally used metric to define
Sometimes the frequency domain “goodness” of a socket:
offers an easier path to the answer
-1 dB insertion loss bandwidth No new information in the frequency domain
The only reason we’d ever leave the time
domain to go to the frequency domain:
To get to the answer faster.
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 9 Differential Impedance and Insertion Loss Applied to Sockets Slide - 10
Transmitted Signals in the
Two World Views
Frequency Domain
Time domain view
What are signals in the frequency domain?
only sine waves
incident
transmitted
Frequency domain view reflected
amplitude amplitude
amplitudeincident phase phase
phase
transmitted
Everything you ever wanted to know about the performance of a socket is
contained in the reflected and transmitted signalsUp to the highest sine wave frequency that is significant
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 11 Differential Impedance and Insertion Loss Applied to Sockets Slide - 12
Terminology Most Important Caveat
Source Termination incident impedance impedance
= 50 Ohms = 50 Ohms
transmitted
incident
transmitted
Vtransmitted
What’s important: at each frequency • The source impedance and the load impedance when defining V S21 is always 50 Ohms.incident
• Also called: • Insertion loss has significance if the end use environment is 50
Insertion loss Ohms
S21
Transfer function S21 isS21 is dommi inated by how the i immpedance of the socket
matches the impedance of the test environment!
There is a magnitude and a phase at each frequency
© Eric Bogatin 2006 © Eric Bogatin 2006
2March 12 - 15, 20069
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Differential Impedance and Insertion Loss Applied to Sockets Slide - 13 Differential Impedance and Insertion Loss Applied to Sockets Slide - 14
The Value of “-1 dB Insertion Loss
Good and Bad Insertion Loss
Bandwidth” as a Metric
1.0 • Relative comparison
0.9 good
0.8 • First pass screening
0.7
0.6 • Rough, rule of thumb for usable operating frequency0.5
0.4 Is this good? Is it good enough?
0.3 • Should not be used to sign off on a design
1.0
0.2 bad 0.9 too approximate
0.1 0.8
0.0
0.7 too much margin? Too little?
022 4 6 8 10 12 14 16 180
0.6
Too many assumptions0.5freq, GHz
0.4
0.3 • Multiple approximations:
Simulated with Agilent ADS 0.2
Bandwidth of the signal
0.1
0.0 Is the system a 50 Ohm system?
022 4 6 8 10 12 14 16 180
Total system budget
freq, GHz• Is there a difference between Allocation to the socket
good
-1 dB = 90% transmitted signal amplitude • A better approach (and much more expensive):good enough
-2 dB = 80% tritted signal amplitude Model and simulatebetter ?
-3 dB = 70% transmitted signal amplitude
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 15 Differential Impedance and Insertion Loss Applied to Sockets Slide - 16
The Simplest Model of a
What Affects Insertion Loss of a Socket?
Transmission Line
Microstrip1. Matched Impedance
2. Controlled impedance A "-1" order model:
Any two conductors with length3. Discontinuities of load board
4. Length
5. Dielectric loss
6. Conductor loss Length
7. DC contact resistance
Lead frame of an IC Package
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 17 Differential Impedance and Insertion Loss Applied to Sockets Slide - 18
Labeling the Conductors The Signal
Vsignal
V
Signal path
Signal path
V Vin
Return path
GROUND GROUND
Return path
© Eric Bogatin 2006 © Eric Bogatin 2006
3March 12 - 15, 2006
Insertion Loss (magnitude)
Insertion Loss (magnitude)Tutorial 2
2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 19 Differential Impedance and Insertion Loss Applied to Sockets Slide - 20
How fast does a signal move down
Instantaneous Impedance
a line?
v
Vsignal
V
signal
Signal pathε
return
Return path
in air: v = 186,000 miles per sec v = 12 inches/nsec
• Signal sees an “instantaneous impedance” each step along the pathinches inches12 12n sec n sec inches • Instantaneous impedance depends on the geometry of signal and return pathv = = = 6 n sec • A controlled impedance when instantaneous impedance is constant24
• One impedance that characterizes the interconnect:
• Characteristic impedance
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 21 Differential Impedance and Insertion Loss Applied to Sockets Slide - 22
Characteristic Impedance and Most Important Features of
Characteristic Impedance Capacitance per Length
capacitance per length • Characteristic impedance is not about the signal path
decreases, the characteristic
increase h impedance increases • Characteristic impedance is not about the return path
• Characteristic impedance will depend both signal and return
w = 10 mils
path, inseparably
h = 5 mils
• There is no such thing as the characteristic impedance of a
50 Ohm PCB cross section single pin
the capacitance per length
increases, characteristic increase w • Change the return path configuration, you change the impedance decreases
characteristic impedance
• (Obvious(Obviously,ly, the sa the same goes for goes for insertioninsertion loss!)ss!)
1Z ~0 CL
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 23 Differential Impedance and Insertion Loss Applied to Sockets Slide - 24
Return Path Selection Strongly Influences Ideal, Lossless transmission lines
Single Ended Impedance have just Two Parameters:
Return Path Patterns
• Characteristic Impedance: Z0
•T im e d elay: TD
© Eric Bogatin 2006 © Eric Bogatin 2006
4March 12 - 15, 2006Tutorial 2
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Differential Impedance and Insertion Loss Applied to Sockets Slide - 25 Differential Impedance and Insertion Loss Applied to Sockets Slide - 26
How well do these pins look like an ideal
Pattern 2a: current flow at 20 GHz transmission line?
ret sig ret
?
=
Agilent ADS Momentum
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 27 Differential Impedance and Insertion Loss Applied to Sockets Slide - 28
All Transmission Lines with the Same Characteristic Minimizing Insertion Loss Principle #1:
Impedance and Time Delay Behave Exactly the Same Match Impedance to 50 Ohms
1. Uniform impedance interconnect3D EM sim of the pin fieldAgilent ADS
Simulated ideal transmission line (Z0 = 42 ohms) 2. Match socket to 50 Ohms
3. Keep: 30 Ohms < Z < 80 Ohms and insertion loss will never be 0Return Loss Insertion Loss greater than -1 dB0.0
0
-0.5
-20 0
-1.0-40
Z = 80 Ohms0-1
-60 -1.5 Z = 30 Ohms
0BBWW o off t thhe model ~ 12 GHe model ~ 12 GHzz
Z = 20 Ohms
-2.0 0-80 -2
022 4 6 8 10 12 14 16 180022 4 6 8 10 12 14 16 180
freq, GHz freq, GHz
0 -3200
100 -50 -4
0 Simulated with Agilent ADS
-100 -5-100
022 4 6 8 10 12 14 16 180
-200 -1502 4 6 8 10 12 14 16 1802 4 6 8 10 12 14 16 180
freq, GHzfreq, GHz freq, GHz
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 29 Differential Impedance and Insertion Loss Applied to Sockets Slide - 30
What if the Impedance is not Controlled? Three Impedance Discontinuities
0
0
-1
-1 -2
30 Ω 80 Ω-3
-2
-4
Total length = 0.2 inches Total length = 0.2 inches
-5-3 30 Ω 80 Ω
-6
-4
-7 30 Ω 80 Ω 30 Ω
-8
-5
022 4 6 8 10 12 14 16 180
022 4 6 8 10 12 14 16 180
freq, GHzfreq, GHz
• Low frequency behavior is related to ~ average impedance- can be better than either one • Low frequency behavior is related to ~ average impedance- can be better than either one
• Highest insertion loss can be much worse than either discontinuity (> 3x) • Highest insertion loss can be much worse than either discontinuity (> 7x)
© Eric Bogatin 2006 © Eric Bogatin 2006
5March 12 - 15, 2006
Insertion Loss, dB
phase(S(3,3)) dB(S(3,3))
phase(S(1,1)) dB(S(1,1))
phase(S(4,3))
dB(S(4,3))
phase(S(2,1))
dB(S(2,1))
Insertion Loss, dB
Insertion Loss, dBÆ
Æ
Æ
Æ
Æ
Æ
Æ
Æ
Æ
Æ
Tutorial 2
2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 31 Differential Impedance and Insertion Loss Applied to Sockets Slide - 32
Minimizing Insertion Loss Principle #2: 7 Principles of Socket Design for
Use a controlled impedance interconnect Optimized Insertion Loss
1. match characteristic impedance of socket to 50 Ohms
2. Keep the impedance constant through socket
• Match average impedance to 50 Ohms 3. Optimize (minimize) pad stack up capacitance
4. Keep socket short (shorter is better, but long may be
good enough)• Design for controlled impedance- uniform
cross section 5. Dielectric loss of socket not critical
6. Conductor loss of socket not critical
7. Contact resistance of socket not critical
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 33 Differential Impedance and Insertion Loss Applied to Sockets Slide - 34
The Highest Speed Signals
What is a differential signal?
Are All Differential
Example:
National Semi DS92LV010A • Serial Data Interface (SDI): 0.27 1.488 Gbps/pin
Output swing: V , V1 2
• HyperTransport: 0.4 1.2 Gbps/pin 1.125v to 1.375 v into 27 Ohm load
• Fibre Channel: 1.062 2.125 4.25 Gbps/pin
V = V -V1 2™• Serial RapidIO : 1.25 2.5 3.125 Gbps/pin
• PCI Express: 2.5 5 Gbps/pin
• XAUI 3.125 6.25 Gbps/pin
• Proprietary (Basic) x 2x 3x Gbps/pin
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 35 Differential Impedance and Insertion Loss Applied to Sockets Slide - 36
Differential and Common
Differential Signals
® ®Signals Differential I/O Standards Supported By Altera Stratix Devices
4.01V = V1 − V2 ()V = V1 + V2diff comm 2 3.3 V
PCLM
Diff Signal Comm Signal3.0 V3.0
0.3 V 3.15 VVcomm
2.1 V 0.4 V 1.9 V
2.0 LVPECL
1.7 V
0.4 V 1.2 V1.4 V
LVDS
0.6 V 0.6 VVdiff 1.0 V1.0
0.9 V
Hyper-
Transport Note: There Is a Very Large
0.3 V Common Component
0.0
Technology
Courtesy of ALTERA Corp.There is a large common voltage component!
© Eric Bogatin 2006 © Eric Bogatin 2006
6March 12 - 15, 2006
Voltage (V)9
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Differential Impedance and Insertion Loss Applied to Sockets Slide - 37 Differential Impedance and Insertion Loss Applied to Sockets Slide - 38
What’s a Differential Pair
Very Important Principle!Transmission Line?
Answer: …..any two, coupled transmission lines (with their return paths).
1
2 Differential impedance is the
instantaneous impedance the
difference signal sees
Optimized high speed performance for the special case:
a symmetric pair, with matched time delay of both paths
What’s differential impedance?
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 39 Differential Impedance and Insertion Loss Applied to Sockets Slide - 40
What is the Impedance the “…It Depends”
Differential Signal Sees?
The Differential Impedance
• No coupling: Z0 = single-ended characteristic
impedance
• With coupling: depends on how the other line is
driven
Other line is tied low
Other line is driven opposite
(differential signal)Z = Z + ZZ Z diff 0 00 0 Other line is driven the same
(common signal)
What is the impedance of each line?
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 41 Differential Impedance and Insertion Loss Applied to Sockets Slide - 42
Other Line Is Tied Low Other Line Driven Opposite
58
56
Second Trace Pegged Low 54
Z , Second Trace Pegged Low052+1 v -1 v
58 50
56 48
54 return
4652 Z , Both Traces Driven Opposite0
50 44Differential Signal
48 Z , Second Trace Pegged Low 42Odd Mode State0
46
40
44 0 5 10 15 20 25 30 35 40 45 50
42
Edge to Edge Spacing Between the Traces (mils)40
0 5 10 15 20 25 30 35 40 45 50
Edge to Edge Spacing Between the Traces (mils)
Z = 2 x Zdiff odd
Polar Instruments SI8000
© Eric Bogatin 2006 © Eric Bogatin 2006
7March 12 - 15, 2006
Single-Ended Impedance (Ohms)
Single-Ended Impedance (Ohms)9
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Differential Impedance and Insertion Loss Applied to Sockets Slide - 43 Differential Impedance and Insertion Loss Applied to Sockets Slide - 44
Pop Quiz
+1 v -1 v
return Differential mode• If there is no coupling between the lines
How would we implement this?
• If each line had a single ended impedance of 50 ohms
What would be the differential impedance of the pair?
There is:
Ans: 100 ohms • Odd mode impedance
• Differential signalsIf 50 ohms is the universally used single ended impedance,
100 ohms is the universally used differential impedance
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 45 Differential Impedance and Insertion Loss Applied to Sockets Slide - 46
As coupling dominates-What geometry terms influence
different intuition is needed differential impedance?
Re-train your intuition 150
140 s = 2 w
130
Z11 ~ single ended impedance to the 120
return path 110
s = w100100Z21 ~ the relative coupling between Z21 ~ the relative coupling between
90Z21Z21 the two signal lines 90the two signal lines
T = 0.7 mils 80
Dk = 3.8V 702~ induced noise on w = 5 milsZ ~ Z21 11 Z11 >> Z21 Z11 ~ Z21s = w, 2w 60Z11 second signal line V1
compared to the first 50
signal line 0 5 10 15 20 25 30 35 40
Height (H1))Z = 2 x (Z11 – Z21)diff
Easy: when Z11 >> Z21: no coupling, single ended case
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 47 Differential Impedance and Insertion Loss Applied to Sockets Slide - 48
Another way of thinking about coupling: Return Current in Closely Coupled
the return current distribution Differential pair: plane close and far
Current distribution in 2, 50 Ohm microstrips, @100 MHz
s = 3 x w
Z11 >> Z21
100 MHz
Z11 ~ Z21
s = w
Return currents overlap in the h = 20 mils
return plane
Return plane plays no role
Diff impedance = single ended
How much return current overlaps in the return plane? impedance between the lines
What is Z11 ?? Z21
Ansoft 2D field solver
© Eric Bogatin 2006 © Eric Bogatin 2006
8March 12 - 15, 2006
Zdiff
ZdiffTutorial 2
2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 49 Differential Impedance and Insertion Loss Applied to Sockets Slide - 50
When signal to signal coupling dominates- all return If Signal To Signal Coupling Is Much Tighter
currents in planes overlap and cancel out. Planes play
Than Signal To Return no role in diff impedance
Return current is carried by adjacent trace when the signal lines in
the differential pair look like two isolated traces as part of a single
170ended transmission line. Dielectric
160thickness very
140 150largeReturn current carried by adjacent trace
140
100 MHz 120 130
100 120
110s
80 100h = 20 mils
90
60 w = 5 mils 80
40 70
60
20
Current density scale expanded by 10x 50
0 0 5 10 15 20 25
0 5 10 15 20 25 30 35 40 45 50 Separation, s
Plane to Trace Separation, h (mils)
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 51 Differential Impedance and Insertion Loss Applied to Sockets Slide - 52
Return Currents in
Single ended current in (-) pin
Differential Pairs
@ 10 MHz
+ signal
- signal return
(floating)Most return current is carried by the plane when
signal to return coupling >> signal to signal
coupling
Ex: most board level interconnects @ 10 MHz
Mentor Graphics Hyperlynx
oX X
Most return current is carried by the other signal
when signal to plane coupling << signal to signal
coupling
Magnitude of current
Ex: most connectors, shielded twisted pair, twisted
What would 1 GHz look like?pair, sockets
© Eric Bogatin 2006 © Eric Bogatin 2006
Differential Impedance and Insertion Loss Applied to Sockets Slide - 53 Differential Impedance and Insertion Loss Applied to Sockets Slide - 54
Return Current for Differential Signal:
Single ended signal in (+) pin Pattern 1a
@ 10 MHz @ 10 MHz
- signal
+ signal return + signal - signal return
(floating)
oo ooX X oo XX
Small amount of
residual return
current- mostly
cancelled out
© Eric Bogatin 2006 © Eric Bogatin 2006
9March 12 - 15, 2006
Differential Impedance (Ohms)
Zdiff