high performance (copper) cable technology

Jay Diepenbrock

October, 2013

High Performance

(Copper) Cable

Technology

IEEESeptember, 2013 1

Outline

• What and where are “High Performance” cables?

• Cable types

• Differential links

• Cable assembly construction

• Cables and EMI

• Cable EMI mitigation

• Measuring EMC properties of Cables

• References

September, 2013 2

High Performance Cables

September, 2013 3

• Where? Everywhere• What?

• “Big Data” servers, networks• Ethernet, InfiniBand, SAS, PCI-Express

• PCs• SAS, USB 3.0

• Multimedia devices• USB 3.0, Thunderbolt

• TVs, entertainment• Coax (!), HDMI

High Performance Cables

September, 2013 4

40

80

128

1224

6 612

30

60

120

168

300

100.012 0.48 54.95 0

10.218

0.4 4

40

10

100

0

50

100

150

200

250

300

350

1990 1995 2000 2005 2010 2015

Ag

gre

ga

te t

hro

ug

hp

ut,

Gb

/s

Year

I/O Interface Data Rates PCI-Express Gen. 1

PCI-Express Gen. 2

PCI-Express Gen. 3

SAS 2.1

SAS 3

S-ATA 1.0

S-ATA 2.0

S-ATA 3.0

InfiniBand SDR

InfiniBand DDR

InfiniBand QDR

InfiniBand FDR

InfiniBand EDR

Thunderbolt

USB 1.1

USB 2

USB 3.0

HDMI 1.0

HDMI 1.3

HDMI 1.4

HDMI 2.0

Ethernet (100 Mb)

Ethernet (Gb)

Ethernet (802.3ba)

Ethernet (SFF-8431)

Ethernet (802.3bj)

Passive or ActiveCopper or Fiber

Bulk wire construction• Shielded or not• Single or multiconductor + Ground• Round or ribbonized• Flex• Laminated coax• Hybrid – misc. mixes (signals + power, etc.)

Connectors• Coax (F, SMA, N)• Direct attach multi-pin• Paddle card (soldered) multi-pin• Backplane style• “Pluggable” transceiver

Cable types

September, 2013 5

Cable Types

September, 2013 6

connectorconnector

Passive

Cu bulk wire

connectorconnector Cu bulk wire

connectorconnector

Half active(Tx or Rx end)

Cu bulk wire

Full active

connectorconnector Optical fiber

Active Optical

= electrical amp or eq. = O/E or E/O converter

Single-conductor Cable (coax)

September, 2013 7

Construction• Many sizes, materials• Majority are 50 or 75 Ohms• Single signal conductor• Dielectric – PE, PTFE, etc.• Shield (braid or foil+braid)• Jacket

Applications• TV, radio broadcasting• Cable TV• Commercial, amateur radio• Military• Cell phones• Anything RF (audio?)

D

𝑍0 =60

𝑒𝑟

𝐷

𝑑

shield

d

Center

Cond.

dielectric

Differential Pair Cables

September, 2013 8

Majority of high speed interfaces now differential• On chip, between functional islands• Memory• On-card• I/O

Why Differential signaling?• Higher system noise margin

• Power supply voltages decreasing -> lower voltage swing• Lower noise immunity (crosstalk)• Reduced EMI

Differential Pair Bulk wire

September, 2013 9

Construction• Two signal lines, many geometries• Typically 100 Ohms impedance• Twisted or parallel pair• Dielectric – air, PE, PTFE, etc.• Shielded (braid or foil+braid) or not • Jacketed or not

Applications• Networking (“Category”) – UTP, STP• HPC, Supercomputing , I/O (Fibre Channel, PCI-e, SAS, S-ATA,

InfiniBand, Ethernet, etc.) • Computer storage – SAS, S-ATA, USB• Consumer – HDMI, USB, Thunderbolt

Twisted Pair bulk wire

September, 2013 10

Application Lane

speed

# lanes

(pairs)

Cable

Type

Ethernet 1-1000

Mb/s

4 Cat. 3, 5,

5e UTP*

Ethernet 10 Gb/s 4 Cat. 6a

STP

PCI-e 2.5-16

Gb/s

2-32 SPP

FC, Enet,

IB,

2-25 Gb/s 2-24 SPP*

• Inexpensive• Various performance grades• (“Category” 5, 5e, 6, 6a, 7 cables)• Some shielded• Can be field terminated• Susceptible to crosstalk

Shielded Parallel Pair (“Twinax”) bulk wire

September, 2013 11

• Higher performance than TP• Individually Shielded Pairs• Various dielectrics -

PE, PTFE, etc.

• Foil and/or bulk braid shield• Outer jacket per application

• Flammability• Abrasion, chemical resistance

• Applications - I/O, networking

(FC, PCI-e, SAS, S-ATA, InfiniBand, Ethernet, etc.)

Bulk shield

dielectric

s

Drain wired

D

Shielded Parallel Pair (“Twinax”)

September, 2013 12

Advantages• Good performance• Low crosstalk

Pitfalls• Symmetry important

• Non-uniform materials• Geometric structure

• Common Mode generation• Skew• System asymmetries

• Manufacturing

good

bad

Shielded Parallel Pair shield topology

September, 2013 13

EXD versus Standard Spiral Shield 24 AWG 100 Ohm

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Frequency MHz

SD

D21 d

B / 1

0 m

ete

r

thru fixture

EXD 1

EXD 2

EXD 3

EXD 4

Optimized for High Frequency 1




EXD

Spiral 10 meter data, fixture not removed

Longitudinal

shield

Spiral shield

Quad

September, 2013 14

Construction• Four signal lines (two pairs), but smaller• Dielectric – PE, PTFE• Unshielded quad• Bulk shield• Jacket

Applications• HPC, Supercomputing • Limited usage

• Expensive, hard to make – orthogo-nality critical to CM, xtalk perf.

• Hard to terminate

1+

1-

2+ 2-

shield

Connectors

September, 2013 15

7-pin Serial ATA right-angle 7-pin Serial ATA straightSFF-8482 SAS 29 pin w /power

SFF-8470 SAS SFF-8088 external mini SAS SFF-8487 internal mini SAS

HDMI

Connectors

September, 2013 16

PCI-Expressx16

SFF-8038 (SFP+)

(QSFP)

September, 2013 17

Raw Cable

Spring

Cover Shell

Screw

PCBA

Latch

Base

Insert Molding

Spacer

Cover

Tear-down – QSFP (SFF-8088)

Wire termination

September, 2013 18

QSFPSFF-8088 (12X InfiniBand)

Differential Links

Each signal transmitted by a pair of conductors, driven

180 degrees out of phase

Considerations:

–greater common mode noise immunity than single-ended

–less EMI radiation than single-ended

–must consider and measure differential quantities

analysis, simulation methods

test equipment, fixtures

–additional propagation modes are possible

+ -

Signal conductors

Drain wire Foil shield

Dielectric

+ -

card wirecable

September, 2013 19

Differential Impedance

• “Modes" are now possible

• Case 1

L/CC11 C12

C21 C22

L11 L12

L21 L22

L, C, Z

L, C, Zcommon mode

September, 2013 20

Differential Impedance

• “Modes" are now possible

• Case 1

L/CC11 C12

C21 C22

L11 L12

L21 L22

L, C, Z

L, C, Z

common mode

• Case 2

L, C, Z

L, C, Z

differential mode

the modes have different impedances,

and different propagation delays!

It's still , but now C= and L=

September, 2013 21

Differential Measurements

• Options–Make multiple single-ended measurements and do the math yourself

–Buy differential test equipment, build differential fixtures

Differential TDR - measure M1=C1-C2

Four port VNA or two port with external test set - measure sdd21, not s21,

and sdd11, not s11

Provides additional information over use of baluns (no common mode data)

Z11 Z12 Z22

see Carey, Scott, and Weeks: "Characterization of Multiple Parallel Transmission Lines,"

IEEE Trans. Instr. and Meas., Sept. 1969

September, 2013 22

Differential Pair Skew

• Two types:–in-pair (between legs of pair)Due to difference in propagation delay between legs of pair

Manifested as "excess attenuation"

Spec. limits pretty tight - causes differential imbalance, and can

cause EMI problems due to common mode energy

not uniform with length!

–pair to pair (between pairs)difference in propagation delay between pairs

modern interfaces relatively insensitive to it (500 ps limit) - it's

corrected in the design

September, 2013 23

Skew

September, 2013 24

Skew

September, 2013 25

• Small amounts of skew create significant common mode noise

• As little as 1% of bit width for skew can have significant EMI effects

• As little as 10% of bit width skew creates CM signal of equivalent amplitude to initial signals

Skew

September, 2013 26

Individual Channels of Differential Signal with Skew

2 Gb/s with 50 ps Rise and Fall Time (+/- 1.0 volts)

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09

Time (seconds)

Vo

lta

ge

Channel 1

No Skew

10 ps

20 ps

50 ps

100 ps

150 ps

200 ps

Skew

September, 2013 27

Common Mode Voltage on Differential Pair Due to In-Pair Skew

2 Gb/s with 50 ps Rise and Fall Time (+/- 1.0 volts)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09 3.5E-09 4.0E-09 4.5E-09 5.0E-09

Time (seconds)

Am

pli

tud

e (

vo

lts)

10 ps

20 ps

50 ps

100 ps

150 ps

200 ps

Rise/fall time mismatch

September, 2013 28

• Small amounts of mismatch create significant CM noise

• Not as significant as skew, but harder to control!• Telltale is significant 2nd harmonic content


September, 2013 29

Example of Effect for Differential Signal with Rise/Fall Time Mismatch

2 Gb/s Square Wave (Rise/Fall = 50 & 100 ps)

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.0E+00 2.0E-10 4.0E-10 6.0E-10 8.0E-10 1.0E-09 1.2E-09 1.4E-09 1.6E-09 1.8E-09 2.0E-09

Time (Seconds)

Vo

ltag

e

Channel 1

Channel 2

T/R=50/100ps


September, 2013 30

Common Mode Voltage on Differential Pair Due to Rise/Fall Time Mismatch

2 Gb/s with Differential Signal +/- 1.0 Volts

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0 5E-10 1E-09 1.5E-09 2E-09 2.5E-09 3E-09 3.5E-09 4E-09 4.5E-09 5E-09

Time (seconds)

Level

(vo

lts)

T/R=50/100ps

T/R=50/150ps

T/R=50/200ps

Eye opening and Jitter

Measures time domain performance of link

Measured using PRBS or application-specific data pattern (e. g., CJTPAT)

Eye opening -

–vertical "black space" in middle of many overlaid bits

–minimum opening needed for receiver to distinguish between "1" and "0"

Jitter - horizontal width of zero crossing of overlaid waveforms

eye opening

jitter

September, 2013 31

Eye Opening and Jitter – test setup

September, 2013 32

Asynch. Crosstalk Source

Test

card

PRBS7, 9, ..31 pattern

Vout ~= 1 Vpp

Trise ~= 30 ps

xx Gb/s

Color-graded display

Infinite persistence

x Histogram hits

(terminate unused ports

with 50 Ohms to Ground)

Clock

Test

card

Cable

Pattern or BERT Gen. Sampling or real-time oscilloscope

Sources of EMI in Cables

September, 2013 33

• Skew in system coupled to cable shield, due to• Asymmetric differential pairs• Unequal rise/fall time of signals• Common mode in signals

• Cable construction• Common mode conversion in bulk wire• Poor connection from Chassis to Cable plug backshell• Leaky backshell• Skew in plug/paddle card/bulk wire• Poorly shielded bulk wire

Common mode conversion

September, 2013 34

Cable Assembly Construction Influence on EMC

September, 2013 35

• Shielding• Pair shields – foil in or out? Shielded or not? Drain wire handling• Bulk shield

• Foil (high freq.)• Braid (low freq.) – shield coverage (typ. 80-90%), weave angle, etc.

• Backshell design• Seams, leakage potential• Latches, jack screws

• Grounding

• Backshell-chassis connection – springs, gaskets, drain wires

• Don’t forget the system influence!• In-pair skew• Mismatched rise/fall tmes• Common mode

Cable EMI sources

September, 2013 36

From Bergey and Altland, “EMI Shielding of Cable Assemblies”, DesignCon 2008

HDMI cable shield connection

Cable EMI sources

September, 2013 37

USB cable shield connection (or not!)


Cable EMI sources

September, 2013 38


Measuring Cable EMI

September, 2013 39

• Key parameters• Transfer Impedance• Shielding Effectiveness

• Measurement methods• EM 52022 (CISPR 22) – semi-anechoic chamber• Tube fixture (IEC 62153-4-7)

• Measures transfer impedance• Max. frequency ~1 GHz

• Reverb chamber (no standard yet)• Measures shielding effectiveness• Usable ~300 MHz – 20 GHz

Tube Fixture

September, 2013 40

Tube Fixture Sample Results

September, 2013 41

Reverb Chamber

September, 2013 42

• Closed, conductive-walled room• Usable frequency range ~300 MHz-20 GHz, depending on

room size and antennae used• Don’t dampen resonance, celebrate it!• CUT is driven with differential or common mode signal,

radiated energy is measured• No system hardware required• “Tuner” used to stir resonances, either stepped or

continuously from external controller• Much work on reverb chambers at OK State Univ.

(C. Bunting, et. al.)

September, 2013 43

Reverb Chamber

Reverb Chamber

September, 2013 44

MeasurementAntennas

CUT support (non-conductive)

CUT

Tuner

Stepper motor

September, 2013 45

• Many paths to EMC cleanliness• Reduce system in-pair skew• Match signal rise/fall times• Reduce common mode energy coupling to cable shield• Improve cable shield connection to cable backshell

• Reduce connection inductance• Better shield coverage

• Utilize absorbing material in cable jacket• Utilize Band Gap devices on host card

EMI Mitigation in Cables

September, 2013 46

EMI Absorbing Material

September, 2013 47

• Available from ARC Technologies, Inc. for• extrusion in cable jacket• Molded enclosures (replace metal can)• Covers over connectors

• Frequency selective – suppression range depends on formula used• Doesn’t need to be used on whole cable – just ends are enough


September, 2013 48

Motivation - Eliminate Ferrite Cores on Cables


September, 2013 49

Ethernet Cable Emission Reduction (When Drive Signal at Same End of Cable)

ARC Lossy Material Covers Partial Length

0

2

4

6

8

10

12

14

16

18

20

0.0E+00 1.0E+09 2.0E+09 3.0E+09 4.0E+09 5.0E+09 6.0E+09 7.0E+09 8.0E+09 9.0E+09 1.0E+10

Frequency (Hz)

Re

du

cti

on

in

Em

issio

ns

(d

B)

Ethernet Sample #1 w/ 11" Covered



Ethernet Sample #1 Full Cable Covered

References

September, 2013 50

• Diepenbrock, J.: Measurement and Analysis of Shielding Effectiveness and Transfer Impedance of High Speed Data Cables, DesignCon 2012

• Archambeault, B., Connor, S., Diepenbrock, J., and Knight, A.: Developing Limits for Common Mode Noise on High Speed Differential Signals, DesignCon 2011

• Hill. D.: “Electromagnetic Theory of Reverberation Chambers,” Natl. Inst. of Standards and Technology Tech Note 1506, 1998

• Vignesh Rajamani, Charles F. Bunting and James C. West, “Calibration of a Numerically Modeled Reverberation Chamber,” IEEE Symposium on Electromagnetic Compatibility 2009

• Archambeault, B., Chikando, E., Connor, S., and Diepenbrock, J.: “High SpeedCables with Lossy Material Coating,” IEEE 2010 Symposium on Electromagnetic Compatibility 2010

Other ReferencesStandards• Code of Federal Regulations Title 47, Telecommunications, part 15 (US)• EN 55022, Information Technology Equipment – Radio Disturbance Characteristics – Limits and

Methods of Measurement (Europe)• ANSI/EIA/ECA 364-66A EMI Shielding Effectiveness of Electrical Connectors• IEC 61000-4-21 Reverb chamber test methods• IEC 61276 Screening attenuation measurement by the reverberation chamber method• IEC 62153-4-7 Transfer impedance and screening, tube in tube method• IEC 62153-4-9 Coupling attenuation of screened balanced cables, triaxial method• IEEE 802• InfiniBand Specification, volume 2• PCI-Express Cabling Specification

Other• Agilent Technologies: Understanding the Fundamental Principles of Vector Network

Analysis," AN 1287-1, available at http://www.agilent.com

• Bogatin, E: "Differential Impedance Finally Made Simple,“ available at

http://www.ewh.ieee.org/r5/denver/rockymountainemc/archive/2000/diffimp.pdf

• Carey, Scott, and Weeks: "Characterization of Multiple Parallel Transmission Lines,"

IEEE Trans. Instr. and Meas., Sept. 1969

• Deutsch, A., "Electrical Characteristics of Interconnections for High-Performance

Systems," IEEE Proceedings vol. 86 No. 2, Feb. 1998

September, 2013 51

IEEE10/30/2013 52

Conferences

• DesignCon – February, in Santa Clara, CA• IEEE Electrical Performance of Electronic Packaging (EPEP)• IEEE EMC Symposium (EMCS)

• in Raleigh, NC in August, 2014• Embedded SI conference• http://www.emcs.org

• IEEE ECTC, ED, ISSCC• IEEE SPI workshop (Europe)

high performance (copper) cable technology

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