microstrip line through slotline -...
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Differential Signaling
Ruey-Beei Wu
Rm. 340, Department of Electrical Engineering
E-mail: [email protected]
url: cc.ee.ntu.edu.tw/~rbwu
1
S. H. Hall et al., High-Speed Digital Designs, Chap.7
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R. B. Wu
What will you learn?
• What is differential signaling vs. single-ended
signaling
• Why differential signaling?
• What is the relation between differential and odd-
mode impedance?
• How common modes will be excited?
• How fiber weave effects affect differential
signaling
• How to suppress the presence of common mode?
2
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R. B. Wu 3
Outline
• Introduction
• Voltage Parameters
Definition
Impedance Design
• Mode Conversion
Mechanism
Suppression design
• Differential Line Loss
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R. B. Wu
Single Ended Signaling
• All electrical signal circuits require a loop or return path.
• Single ended signal subject to several distortions and noise.
– Ground or ref. may move due to switching currents (SSO noise).
– A “se” receiver only cares about a voltage ref. to its own ground.
– EMI can impose voltage on a single ended signal.
– Signal passing from one board to another are subject to local
ground disturbance.
• We can counteract these effects by adding more grounds.
• At freq. beyond 1GHz, 80% of the signal will be lost.
12/4/2002 Differential Signaling
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R. B. Wu
Review of Threshold Sensitivity
• Wave is ref. to either Vcc or Vss. Consequently, effective DC
value of the wave will be tied to one of these rails.
• Wave is att. around effective DC component of waveform, but
ref. does not change . Hence, clock trigger point btw various
clock load points is sensitive to distortion and attenuation.
12/4/2002 Differential Signaling
TxVssVref
Vss Rx2
VrefLong line
Vss Rx1Vref
Short line
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R. B. Wu
Noise Impact
• System noise can severely degrade SI on single-ended buses.
6
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R. B. Wu
Differential Signaling
• Any signal can be considered a loop is completed by two wires.
One of the “wires” in “se” signaling is the “ground”
• Differential signaling uses two conductors
– Tx translates signal into a pair of outputs, driven 180° out of phase.
– Rx, a diff. amp., recovers signal as voltage diff. on two lines.
12/4/2002
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R. B. Wu
Advantages of Differential Signaling
• Differential Signaling is not sensitive to SSO noise.
• A differential receiver is tolerant of its ground moving around, since
there is a virtual ground.
• If each “wire” of pair is on close proximity of another, EMI imposes
same voltage on both signals, and cancels out.
• Since AC currents in “wires” are equal but opposite and proximal,
radiated EMI is reduced.
• Signals passing from one board to another are not subject to local
ground disturbances.
• As frequencies increase beyond 1GHz, up to 80% of signal may be
lost, but difference still crosses 0 volts.
• Loss issues for differential signaling only come into play in high
loss system. Most single ended systems assume ~15% channel loss.
12/4/2002 Differential Signaling
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R. B. Wu
Removal of Noise
• Common-mode noise: noise on both legs of diff. pair.
( ) ( )
( )
diff D noise D noise
D D
v v v v v
v v
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R. B. Wu
Virtual Reference
• Extremely helpful to
preserving signal
integrity
10
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R. B. Wu
Differential Signaling - Cons
• The cost is doubling the signal
wires, not so bad as compared
to adding grounds to improve
single ended signaling.
• Routing constraint: Pair
signals need to be routed
together.
• Differential signal have
certain symmetry
requirements that may pose
routing challenges.
12/4/2002 Differential Signaling
1i 2i
1v 2v
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R. B. Wu
Differential Crosstalk
• Diff. crosstalk is in general
small, but can be larger
than “se” case occupying
same area.
12
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Voltage Parameters
13
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R. B. Wu
Propagation Terms to Consider
• Differential/Common mode propagation
• Single ended mode (uncoupled) propagation
– when the other line is quiet but terminated to absorb reflections.
– Transmission line matrices will reflect these modes.
12/4/2002 Differential Signaling
1i 2i
1v 2v
1i 2i
1v 2v
odd-mode propagation even-mode propagation
E-field H-field
E-field
H-field
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R. B. Wu
Even Mode Pattern
E field H field
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R. B. Wu
Odd Mode Pattern
E field H field
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R. B. Wu
Differential Signal Parameters
12/4/2002 Differential Signaling
Line 1 Line 2
Reference
( )1 1'
( )1 1
oddD
evenD
v tv
v tv
V mv M v
• Differential voltage: vdiff = vD+- vD-
• Common-mode voltage: vcom= (vD+-+vD-)/22 ; / 2diff odd com evenZ Z Z Z
12
( )1 1'
( )1 1
oddD
evenD
i ti
i ti
I mi M i
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R. B. Wu 18
Differential vs. Odd Mode Impedance
sV
oZ0sV
oZ0
aldifferentiZ
sV2
aldifferentiZ
aldifferentiZ
oaldifferenti ZZ 02
P. E. Fornberg, M. Kanda, C. Lasek, M. Piket-May, and S. H. Hall, “The impact of a nonideal return path
on differential signal integrity,” IEEE Trans. EMC, vol. 44, Feb. 2002.
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R. B. Wu
Differential Microstrip Example
12/4/2002 Differential Signaling
SE: single ended = uncoupled
11 22 12 21
11 22 12 21
6.60; 1.29 nH in
4.18; 0.67pF in
L L L L
C C C C
11 12
11 12
11 12
11 12
11
11
1 12 2
2 2 66.2
23.7
39.7
L L
diff odd C C
L L
com even C C
L
se C
Z Z
Z Z
Z
11 12 11 12
11 12 11 12
11 11
( )( ) 1.924 ns ft
( )( ) 1.996ns ft
1.992 ns ft
diff odd
com even
se
L L C C
L L C C
L C
Mode impedance
Mode delay
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R. B. Wu 20
Impedance Design
Variations in Impedance of Coupled Microstrip Line
2.4r
w ws
d
tw/d = 1,1.5,2
w/d = 1,1.5,2
t/d = 0,0.04,0.07,0.1
0 1 2 3
s/d
20
40
60
80
100
Z
Imp
ed
an
ce
(
)
Zeven
Zodd
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R. B. Wu 21
Design Examples
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R. B. Wu
Propagation Velocities
• For TEM structures, (striplines)
– Differential mode, common mode, and single ended velocities are the same
• For non TEM and quasi-TEM structures (microstrip)
– Differential mode, common mode, and single ended velocities and impedances are not the same.
– Common mode can be converted to differential mode at a receiver and results in a differential signal disturbance.
12/4/2002 Differential Signaling
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R. B. Wu
Balanced vs. Unbalanced
• Unbalanced signaling in reference to ground
• Balanced signaling is referenced only to the other
port terminal.
– If each channel is identical, then this suggests a virtual AC
ground between the two terminals.
– It is often useful to allow this AC ground to be a DC voltage
to biasing devices.
• Good Agilent technologies article on balanced and
unbalanced signaling
– http://we.home.agilent.com/upload/cmc_upload/tmo/dow
nloads/EPSG084733.pdf
12/4/2002 Differential Signaling
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R. B. Wu
Ethernet 10/100BASE-T example
TN1
TP1
12/4/2002 Differential Signaling
50
50
50
50
TransformerFilter
Common-mode choke
Unbalanced Balanced
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Common Mode Conversion
25
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R. B. Wu
Asymmetry-caused Mode Conversion
26
Length difference Impedance difference
Crosstalk difference Length difference
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R. B. Wu
Example of Common Mode
• Line 1 and line 2 have the same DC offset.
– This is DC common mode.
– It can be defined as an average DC for time duration of many UI cycles value as well.
• Line1 and line 2 have the same AC offset
– This is AC common mode
• AC common mode also results from time differences (skew) between signals on line 1 and line 2. This can result in AC common mode and differential signal loss.
12/4/2002 Differential Signaling
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R. B. Wu
Individual signals Plot individual line voltages and offset voltage
0 0.83 1.67 2.5 3.33 4.17 51
0.33
0.33
1
1.67
2.33
3
ai
bi
offseti
ti
ns• Devices need to have enough common mode dynamic range to receive
or transmit waveforms. In this case, signals swing between -0.1 and 2.1.
• The sine wave amplitude is 1 and peak to peak is 2.
• Signal a and b is what would be observed with 2 oscilloscope probes
12/4/2002Differential Signaling
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R. B. Wu
Differential Mode Signal
0 0.83 1.67 2.5 3.33 4.17 52
1
0
1
2
a b
t
ns
Plot Differential voltage
• The differential amplitude is 2 and peak to peak is 4 which is 2 times the individual signal peak to peak amplitude.
• Notice the distortions are gone.
12/4/2002Differential Signaling
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R. B. Wu
Common Mode Signal
Plot common mode voltage
0 1 2 3 4 50.8
0.9
1
1.1
ai bi
2
ti
ns
• The DC common mode signal is 1
• The AC common mode signal is .2 v peak to peak
– Some specifications may call this 0.1 v peak from the DC average
• We will add this common mode to the signals “a” and “b”
12/4/2002Differential Signaling
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R. B. Wu
Add 150 ps skew to signal b
Plot individual line voltages and offset voltage
0 0.83 1.67 2.5 3.33 4.17 51
0.33
0.33
1
1.67
2.33
3
ai
bi
offseti
ti
ns
• Waveforms do not look so good.
• We even have what appears to be non-monotonic behavior.
12/4/2002Differential Signaling
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R. B. Wu
Differential signal looks OK
0 0.83 1.67 2.5 3.33 4.17 52
1
0
1
2
a b
t
ns
Plot Differential voltage
max a b( ) min a b( ) 3.562
• However we lost differential signal amplitude.
• It used to be 4 peak to peak and now is 3.562.
12/4/2002Differential Signaling
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R. B. Wu
Common mode measurements are different
0 1 2 3 4 50.5
1
1.5
2
ai bi
2
ti
ns
Plot common mode voltage
meana b
2
1 maxa b
2
mina b
2
0.944
max meana b
2
maxa b
2
meana b
2
mina b
2
0.504
• Average is still 1. Peak to peak is 0.944 but peak is 0.504
• AC common mode signals can be converted to differential
12/4/2002Differential Signaling
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R. B. Wu
Diff. to Common Mode Conversion (ACCM)
34
( 0) ( 0)
D Dv vACCM
v z v z
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Fiber Weave Effects & Design
35
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R. B. Wu
Fiber-Weave Effect
36
3.73 3.5 0.23eff
2 (40.4 39.2)eff GHzf c f
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R. B. Wu
Fiber-Weave Effect
37
• Common glass cloths used in PCB manufacture.
• FR4 dielectric is a composite material.
• Homogeneous dielectric assumption inaccurate when signals with multi-GHz frequency content.
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R. B. Wu
Fiber-Weave Effect
38
3.73 3.5 0.23eff
2 (40.4 39.2)eff GHzf c f
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R. B. Wu
Minimizing Fiber Weave Effect
39
1. Offset routing.
Routing the trace in a straight line with offset in the middle equal to a glass bundle pitch.
2. Zig-zag routing or slanted routing.
Flowing a zig-zag or slanted routing of differential traces.
3. Image Rotation.
Rotating layout database relative to the edge of PCB board.
4. Using alternate PCB material.
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Common-Mode Suppression
Filters
40
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R. B. Wu
PWB structures that introduce Skew
12/4/2002 Differential Signaling
An escape from a BGA or connector pinsintroduces skew
This is an example of skew compensation
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R. B. Wu
Bends introduce skew
12/4/2002Differential Signaling
Back to back bendscompensate for skew from frequencies below 2 GHz.
Back to back bendscompensate for skew from frequencies below 2 GHz.
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R. B. Wu 43
Bent Differential Lines
Discontinuities
Coupled TL
Differential
Common mode
21 VVVd
2
21 VVVc
When differential signaling travels
through the bends, common mode
noise is generated.
MLIN
L=
W=
ID=
0 mm
2.1 mm
TL1
MLIN
L=
W=
ID=
0 mm
2.1 mm
TL2
MLIN
L=
W=
ID=
0 mm
2.1 mm
TL3
MLIN
L=
W=
ID=
0 mm
2.1 mm
TL4
IND
L=
ID=
0.12 nH
L1
IND
L=
ID=
2.69 nH
L2
RES
R=
ID=
0.002 Ohm
R1
CAP
C=
ID=
0.020136 pF
C1
RES
R=
ID=
1.0424 Ohm
R2
CAP
C=
ID=
0.020136 pF
C2
CAP
C=
ID=
0.074 pF
C3 CAP
C=
ID=
0.074 pF
C4
CAP
C=
ID=
0.423 pF
C5 CAP
C=
ID=
0.423 pF
C6
PORT
Z=
P=
50 Ohm
1
PORT
Z=
P=
50 Ohm
2
PORT
Z=
P=
50 Ohm
3
PORT
Z=
P=
50 Ohm
4
1
2
4
3
Freq = 0.6GHz ( L:nH / C:pF / R:Ohm )
L13 =1.20e-01 R13 =2.00e-03
L24 =2.6940e+00 R24 =1.0424e+00
C12 =2.0136e-02C34 =2.0136e-02
C11 = 7.3978e-02 C33 = 7.3978e-02
C22 = 4.2320e-01C22 = 4.2320e-01
C12KL
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R. B. Wu 44
De-embedding for Bent Structures
Coupled Tx-line
IA
VA
+
_
IB
VB
+
_
DUT( Differential Bend ) _
IC
VC
+
Coupled Tx-Line_
ID
VD
+
DUT (Differential bend)
Coupled Tx-Line
Coupled Tx-Line Coupled Tx-Line
Coupled Tx-line
IA
VA
+
_
IB
VB
+
_Coupled Tx-Line
_
IC
VC
+
ID
VD
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R. B. Wu 45
Common Mode Conversion Noise
0 200 400 600 800 1000 1200
-0.2
-0.1
0
0.1
0.2
Tr=50ps
Tr=100ps
Voltage(V)
Time (ps)
0 200 400 600 800 1000 1200
-0.2
-0.1
0
0.1
0.2
Tr=50ps
Tr=100ps
Voltage(V)
Time (ps)
common mode noise at receiver reflected differential mode noise at sending end
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R. B. Wu 46
0 200 400 600 800 1000 1200
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
Round corner bends
Right-angle bends
Voltage(V)
Time (ps)
0 200 400 600 800 1000 1200
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
Round corner bends
Right-angle bends
Voltage(V)
Time (ps)
common mode noise at receiver reflected differential mode noise at sending end
For outer R ~ 3w
inner R ~ 1wL13 = 4.237e-2L24 = 3.025e-1
Lm = 4.805e-2C12 = 3.729e-3C11 = 6.071e-3
C22 = 2.132e-002
R=3W
3 4
1
2
424.01413
LL
LK m
( 17 mil )
( 6 mil )
L13 = 0.198L24 = 1.93Lm = 0.115
C12 = 0.02 = C24C11 = 0.05 = C33C22 = 0.23 = C44
KL=0.1855
Round Corner Bend can’t solve it
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R. B. Wu 47
Dual Bend can’t solve either
0 200 400 600 800 1000 1200 1400 1600Time (ps)
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
Vol
tage
(V
)
L = 5cm
L = 1cm
1
2
3
5
6
L
common mode noise at receiver
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R. B. Wu 48
4
3
2
1
44434241
34333231
24232221
14131211
4
3
2
1
44434241
34333231
24232221
14131211
4
3
2
1
1
1
1
1
SSSS
SSSS
SSSS
SSSS
a
a
a
a
a
a
a
a
b
b
b
b
Due to :
and
112
132422
2C
Z
LLCC
o
C
02,1,21
212.
ccdad
cdc a
bSMin
DUT(Differential bend)
Coupled TX line
PORT
Z=P=
50 Ohm1
PORT
Z=P=
50 Ohm2
PORT
Z=P=
50 Ohm3
PORT
Z=P=
50 Ohm4
C1g
C12
C2g
C3g
C34
C4g
L34
L13
Lm
Derived optimal value of
compensating capacitance :CC
CC
VcVd
0Vd
Vc
Compensating Capacitance
G. H. Shiue, W. D. Guo, C. M. Lin, and R.-B. Wu, “Noise reduction using compensation capacitance for bend discontinuities of differential transmission lines,” IEEE T-AdvP, pp. 560-569, Aug. 2006.
p1p2
p3 p4
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R. B. Wu 49
W S
TEr=4.3H
W=1.75mm;S=0.75mm;H=1.5mm; T=0.1mm;
formula
0 200 400 600 800 1000
Time (ps)
-0.04
0
0.04
0.08
0.12
Voltage (
Volt)
Common Mode NoiseWithout Cc
With 1 X Cc'
X = 3.8mm
X = 4.8mm
X = 5.8mm
X = 6.8mm
90 1Cc' X=3.8 X=4.8 X=5.8 X=6.8
Vc_pp 0.108 0.053 0.084 0.066 0.046 0.045
Vr_pp 0.085 0.132 0.312 0.347 0.368 0.377
times3.3
Simulation Results
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R. B. Wu 50
Common-Mode Suppression Filter by DGS
• Wideband filter for common-
mode suppression by using two
U-shaped and one H-shaped
coupled patterned ground.
• 15dB reduction over 3.6-9.1GHz,
with FBW of 87% with size 0.44
x 0.44 g2
• No significant deterioration in
insertion loss and group delay in
freq. domain and eye diagram in
time domain.
• 75% reduction in peak noise in
time domain, and 10dB
improvement in EMI is also
noticed.
S.-J. Wu, C.-H. Tsai, T.-L. Wu, and T. Itoh, “A novel wideband common-mode suppression filter for gigahertz
differential signals using coupled patterned ground structure,” IEEE T-MTT, pp. 848-855, April 2009
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R. B. Wu
Suppresion by Tightly Coupled Bend
51C.Gazda1, D. Vande Ginste1, H. Rogier1, R.-B. Wu, and D. De Zutter, “A wideband common-mode
suppression filter for bend discontinuities in differential signaling using tightly coupled microstrips,”
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R. B. Wu
Have you learned?
• How differential signaling better in noise
removal?
• How to design differential lines?
• What is ACCM?
• How to design for suppressing fiber weave effects
on differential signaling
• How to design common mode suppression filters,
by compensation capacitance, DGS, and tightly
coupled bends ?
52
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R. B. Wu 61
Conclusions
Differential signaling has the property of low noise
generation and the ability to reject common mode noise.
An equivalent circuit of differential bends is extracted by
field solver and verified.
The bend demonstrate signal integrity effects.
Using EM software to simulate local structures and
extract model parameters, Spice can be evoked to
analyze the whole system with better knowledge.
Structural compensation by capacitances was proposed.
Other approaches for common mode suppression are
attracting studies currently.
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R. B. Wu 62
Further Reading
H. Wu, W. T. Beyene, N. Chen, C.-C. Huang, and C. Yuan, “ Design and
verification of differential transmission lines,” IEEE Proc. EPEP, pp.
85-88, 2001.
Y. Massoud, J. Kawa, D. MacMillen, and J. White, “Modeling and analysis
of differential signaling for minimizing inductive crosstalk,” IEEE Proc.
DAC. pp. 804-809, 2001.
M. Sung, W. Ryu, H. Kim, J. Kim, and J. Kim, “Reduction of crosstalk
noise in modular jack for high-speed differential signal interconnection,”
IEEE T-AdvP, pp. 260-267, Aug. 2001.
P. E. Fornberg, M. Kanda, C. Lasek, M. Piket-May, and S. H. Hall, “The
impact of a nonideal return path on differential signal integrity,” IEEE T-
EMC, vol. 44, Feb. 2002.
H. Johnson and M. Graham, High-Speed Signal Propagation. New Jersey:
Prentice-Hall, 2003, ch. 6.
H. Chen, Q. Li, L. Tsang, C.-C. Huang, and V. Jandhyala, “Analysis of a
large number of vias and differential signaling in multilayered
structures,” T-MTT, vol. 51 pp. 818 -829, Mar. 2003.
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R. B. Wu 63
P. E. Fornberg, M. Kanda, C. Lasek, M. Piket-May, and S. H. Hall, “The
impact of a nonideal return path on differential signal integrity,” T-EMC,
vol. 44, Feb. 2002.
N. Orhanovic, R. Raghuram, N. Matsui, “Signal propagation and radiation
of single and differential microstrip traces over split Image Planes,”
IEEE EMC Symp., pp. 339-343, 2000.
F. Gisin, Z. Pantic-Tanner, “Routing differential I/O signals across spilt
ground planes at the connector for EMI control,” IEEE EMC Symp., pp.
325-327, 2000.
T. E. Moran, K. L. Virga, G. Aguirre, and J. L. Prince, “Methods to reduce
radiation from split ground plane in RF and Mixed signal packaging
structure,” IEEE T-AdvP , vol. 25, no. 3, Aug. 2002.
G.-H. Shiue, and R.-B. Wu, “ Reduction in reflections and ground bounce
for signal line through a split power plane by using differential coupled
microstrip lines,” IEEE T-AdvP, 2003.
G. H. Shiue, W. D. Guo, C. M. Lin, and R.-B. Wu, “Noise reduction using
compensation capacitance for bend discontinuities of differential
transmission lines,” IEEE T-AdvP, pp. 560-569, Aug. 2006.
S.-J. Wu, C.-H. Tsai, T.-L. Wu, and T. Itoh, “A novel wideband common-
mode suppression filter for gigahertz differential signals using coupled
patterned ground structure,” IEEE T-MTT, pp. 848-855, April 2009.
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R. B. Wu
• G.-H. Shiue, et al., "A comprehensive investigation of a common-mode filter
for gigahertz differential signals using quarter-wavelength resonators," IEEE
T-CPMT, Jan. 2014.
• J. Kim, et al., "High-frequency scalable modeling and analysis of a differential
signal through-silicon via," IEEE T-CPMT, Apr. 2014.
• T.-W. Weng, et al., "Synthesis model and design of a common-mode bandstop
filter (CM-BSF) with an all-pass characteristic for high-speed differential
signals," IEEE T-MTT, Aug. 2014.
• G. H. Shiue, et al., "Significant reduction of common-mode noise in weakly
coupled differential serpentine delay microstrip lines using different-layer-
routing-turned traces," IEEE T-CPMT, 2014.
• F. Grassi, et al., "On mode conversion in geometrically unbalanced differential
lines and its analogy with crosstalk,” IEEE T-EMC, April 2015.
• Q. Lu, et al., "Electrical modeling and characterization of shield differential
through-silicon vias," IEEE T-ED, May 2015.
• C.-Y. Hsiao, et al., "A new broadband CM noise absorption circuit for high-
speed differential digital systems," IEEE T-MTT, Jun. 2015.
• C.-C. Yeh, et al., "Common-mode noise suppression of differential serpentine
delay line using timing-offset differential signal," IEEE T-EMC, Dec. 2015.
• C.-C. Yeh, et al., "Reduction of common-mode and differential-mode noises
using timing-offset differential signal," IEEE T-CPMT, Dec. 2015.64