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Via and Return Path Discontinuity Impact on High Speed Digital Signal. Qinglun Chen, Intel WPD Jin Zhao, Sigrity Inc. Contributor: Weimin Shi, Intel DPG, Raymond Chen (Sigrity) Chi-te Chen Intel WPD. Agenda. The motivation of this study How via impact on high speed digital signal - PowerPoint PPT Presentation
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IEEE EPEP2000
Via and Return Path Discontinuity
Impact on High Speed Digital Signal
Qinglun Chen, Intel WPD
Jin Zhao, Sigrity Inc.
Contributor: Weimin Shi, Intel DPG, Raymond Chen (Sigrity) Chi-te Chen Intel WPD
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IEEE EPEP2000
1) The motivation of this study
2) How via impact on high speed digital signal3) How Signal return path discontinuity impact on
high speed digital signal 4) Correlation and measurements5) Summary
Agenda
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IEEE EPEP2000
Motivations of This StudyRambus needs less than 5 ps skew accuracy and +/- 125 ps skew budget. What are the major skew contributors for Rambus channel interconnection?
• Via discontinuity and coupling
• Return path discontinuity
• Manufacturing variation
• Impedance discontinuity
• Buffer Strength difference
• Cross talking ---even and odd modes
• Load variation
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IEEE EPEP2000
Via DiscontinuityHow via length impact the signal wave form and delay?
How via coupling impact the signal wave form and delay?
How termination via impact the signal wave form?
Test Board for Rambus Study
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IEEE EPEP2000
How via length impact on high Speed digital signal quality
Signal trace
GND
GND
Via
Via Inductance depending on its length -> impedance either
1
2
3
4
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IEEE EPEP2000
Waveform and Delay Comparison
8.00E-0109.00E-0101.00E-0091.10E-0091.20E-0091.30E-0091.40E-0091.50E-0091.60E-009
1.0
1.2
1.4
1.6
1.8
Case 1 Case 2 Case 3 Case 4
Voltag
e (
V)
Time (second)
delay(ps) Skew pure skew Via impactCase1 1095.8 -120.3 -102.1 17.9Case2 1216.1 0 0 0Case3 1216.7 0.6 0 0.6Case4 1239.5 18.5 12 6
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IEEE EPEP2000
How via coupling impact on high Speed digital signal quality
Signal trace
GND
GND
Via even coupling increase via inductance increase impedance
Via odd coupling decrease via inductance decrease impedance
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IEEE EPEP2000
Via Coupling impact on waveform and Delay
0.00E+000 5.00E-010 1.00E-009 1.50E-009 2.00E-009 2.50E-009 3.00E-009
0.8
1.0
1.2
1.4
1.6
1.8 Single Line (Driver) Single Line (Receiver) Odd Mode (Driver) Odd Mode (Receiver) Even Mode (Drivers) Even Mode (Receivers)
Vo
lta
ge
(V
)
Time (second)
with ground via delay(ps) Maximum SkewSingle line 1300.9Odd 1262 39even 1344.8 43.9
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IEEE EPEP2000
Gnd
Pwr
~35 Ohm 28 Ohm Vcc
Gnd
Pwr
~35 Ohm 28 Ohm Vcc
Figure 6. Configuration of Rambus signal net
How termination via impact the signal wave form?
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IEEE EPEP2000
1.00E-009 1.20E-009 1.40E-009 1.60E-009 1.80E-009 2.00E-009
0.8
1.0
1.2
1.4
1.6
1.8 without power vias with power vias
Vo
lta
ge
(V
)
Time (second)
delay (ps) SkewWith power via 1241.1 0Without power via 1277.5 36.4
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IEEE EPEP2000
PCB Signal Return Path Discontinuity
How does a power reference plane impact the signal wave form and delay?
How does via return path impact the signal wave form and delay?
What is the difference between single line vs multiple line switching for RPD?
How do different layer traces impact the signal wave form and delay?
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IEEE EPEP2000
How a power reference plane impact the signal wave form and delay?
Signal trace
GND
VCC
Via
• AC path through inductance and capacitance
• Local capacitance is frequency dependent
• Current have to flow to the ground plane through local capacitance and induce imaging current
• Referring a power plane will create return path discontinuity
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IEEE EPEP2000
Return Path Discontinuity & Via Coupling impact on waveform and Delay
0.00E+000 5.00E-010 1.00E-009 1.50E-009 2.00E-009 2.50E-009 3.00E-009
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Single Line (Driver) Single Line (Receiver) Odd Mode (Driver) Odd Mode (Receiver) Even Mode (Drivers) Even Mode (Receivers)
Vo
lta
ge
(V
)
Time (second)
delay(ps) Maximum SkewSingle line 1312Odd 1252 60even 1400 88
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IEEE EPEP2000
What is the difference between single line vs multiple line switching for RPD?
At Receivers: 1 line switching on S1 1 line switching on s6 8 line switching on s6
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IEEE EPEP2000
How via return path impact on high Speed digital signal quality
Signal trace
GND
VCC
GND
Via
Delay order: near ground via, far ground via, no ground via
GND
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IEEE EPEP2000
How Ground Via impact on Skew and Waveform
1.10E-009 1.20E-009 1.30E-009 1.40E-009 1.50E-009 1.60E-009
1.0
1.2
1.4
1.6
1.8
Ref. Bottom Ref. Bottom with 3 near vias Ref. Bottom with 3 far vias
Vo
lta
ge
(V
)
Time (second)
delay (ps) skewNo ground via 1312.5 9.7Near Via 1302.8 0Far via 1308.6 5.8
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IEEE EPEP2000
Measurement Results for Four layers with 3 Line coupling
L1 Voltage for Even, Odd and Single
-0.2
0
0.2
0.4
0.6
0.8
1
0
23
4
46
8
70
2
93
6
11
70
14
04
16
38
18
72
21
06
23
40
25
74
28
08
30
42
32
76
35
10
37
44
39
78
42
12
44
46
46
80
49
14
Time in ps
Vo
lta
ge
V Even
V Odd
V Single
L2 Voltage for Even, Odd and Single
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Time in ps
Vo
lta
ge
V EvenV OddV Single
L3 Voltage for Even, Odd and Single
-0.2
0
0.2
0.4
0.6
0.8
1
Time in ps
Volta
ge
V Even
V Odd
V Single
L4 voltage for Even,Odd and Single
-0.2
0
0.2
0.4
0.6
0.8
1
Tim
e
36
9
73
9
11
09
14
79
18
49
22
19
25
89
29
59
33
29
36
99
40
69
44
39
48
09
Time in ps
Vo
lta
ge V Even
V Odd
V Single
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IEEE EPEP2000
OR840 WS Board Benefits From Fundamental Studies With Speed97
More than 4000 outrigger system boards are shipped per week
Fundamental studies:
1. Return path; 2. Via coupling; 3. Die-to-Termination channel analysis
This is a very competitive product: Rambus IS on 4 Layer stack-up
That is why IBM has ordered a big volume
S1
GND
S3
VCC
Rambus signalsOutrigger Board Stack-up:
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IEEE EPEP2000
Correlation
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IEEE EPEP2000
Power plane as ground return test pattern
Via coupling test patternPropagation delay test pattern for manufacturing variation
Via characterization Pattern
Test Board For Correlation(1998)
Via characterization pattern
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IEEE EPEP2000
S1
GND
S3
S4
VCC
S6
400 mils
62 mils
100 mils
50 ohm TraceVia
Via Coupling Pattern
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IEEE EPEP2000
Red Line --- no coupling, Blue Line -----Odd coupling
Test Board Measurement of Via Even Mode Coupling
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IEEE EPEP2000
Red Line --- no coupling, Blue Line -----Odd coupling
Test Board Measurement of Via Odd Mode Coupling
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IEEE EPEP2000
Via Coupling Skew Measurement and Simulation (Even/odd Modes)
Input Signal skew
0 ps 25 ps 100 ps 200 ps
Via coupling skew Speed97 Sim
75 ps 80ps
59 ps 48 ps 32 ps
Via coupling skew depends on input signal skew
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IEEE EPEP2000
S1
GND
S3
S4
VCC
S6
400 mils
62 mils
50 ohm Trace
Via
Two Series Via Patterns for Correlation
S1
GND
S3
S4
VCC
S6
62 mils
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IEEE EPEP2000
Speed97 Simulation Vs TDR MeasurementPattern 1
Red Line --- Measurement, Blue Line -----Speed97
( done in 12/1998 at New York University at Binghamton by Weimin Shi )
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IEEE EPEP2000
Speed97 Simulation Vs TDR MeasurementPattern 2
Red Line --- Measurement, Blue Line -----Speed97
( done in 12/1998 at New York University at Binghamton by Weimin Shi )
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IEEE EPEP2000
Summary
• Via discontinuity and coupling become a major signal skew contributor for high speed digital signals
• Power reference plane is another major signal skew contributor for high speed digital signals
• Synchronized signals should reference a same ground plane to avoid additional signal skew
• Return path discontinuity is due to non-uniform inductance and capacitance in signal paths
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