Microstrip Line Through Slotline -...

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

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

R. B. Wu 3

Outline

• Introduction

• Voltage Parameters

Definition

Impedance Design

• Mode Conversion

Mechanism

Suppression design

• Differential Line Loss

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

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.


TxVssVref

Vss Rx2

VrefLong line

Vss Rx1Vref

Short line

R. B. Wu

Noise Impact

• System noise can severely degrade SI on single-ended buses.

6

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

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.


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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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Virtual Reference

• Extremely helpful to

preserving signal

integrity

10

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.


1i 2i

1v 2v

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Differential Crosstalk

• Diff. crosstalk is in general

small, but can be larger

than “se” case occupying

same area.

12

Voltage Parameters

13

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.


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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Even Mode Pattern

E field H field

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Odd Mode Pattern

E field H field

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Differential Signal Parameters


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

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.

R. B. Wu

Differential Microstrip Example


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

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

R. B. Wu 21

Design Examples

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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.


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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


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Ethernet 10/100BASE-T example

TN1

TP1


50

50

50

50

TransformerFilter

Common-mode choke

Unbalanced Balanced

Common Mode Conversion

25

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Asymmetry-caused Mode Conversion

26

Length difference Impedance difference

Crosstalk difference Length difference

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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.


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

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.


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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”


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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.


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.


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


R. B. Wu

Diff. to Common Mode Conversion (ACCM)

34

( 0) ( 0)

D Dv vACCM

v z v z

Fiber Weave Effects & Design

35

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Fiber-Weave Effect

36

3.73 3.5 0.23eff

2 (40.4 39.2)eff GHzf c f

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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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Fiber-Weave Effect

38

3.73 3.5 0.23eff

2 (40.4 39.2)eff GHzf c f

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.

Common-Mode Suppression

Filters

40

R. B. Wu

PWB structures that introduce Skew


An escape from a BGA or connector pinsintroduces skew

This is an example of skew compensation

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Bends introduce skew


Back to back bendscompensate for skew from frequencies below 2 GHz.

Back to back bendscompensate for skew from frequencies below 2 GHz.

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

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

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

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

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

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

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

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

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,”

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

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.

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.

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.

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

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