High Performance (Copper) Cable Technology

Jay Diepenbrock October, 2013

High Performance

(Copper) Cable

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

High Performance Cables

• 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

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 Active Copper 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

connector Cu bulk wire

connector connector Half active (Tx or Rx end) Cu bulk wire Full active connector

connector Optical fiber

Single-conductor Cable (coax)

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 𝑒_𝑟 𝐷 𝑑 shield d Center Cond. dielectric

Differential Pair Cables

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)

Differential Pair Bulk wire

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

• 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

• 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

Shielded Parallel Pair (“Twinax”)

Advantages

Pitfalls

• Common Mode generation

• Skew

• System asymmetries

• Manufacturing

good

Shielded Parallel Pair shield topology

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 D 2 1 d B / 1 0 m e te r thru fixture EXD 1 EXD 2 EXD 3 EXD 4

Optimized for High Frequency 1 Optimized for High Frequency 2 Optimized for High Frequency 3 Optimized for High Frequency 4

EXD

Spiral 10 meter data, fixture not removed

Longitudinal shield

Quad

Construction

• Four signal lines (two pairs), but smaller • Dielectric – PE, PTFE

• Unshielded quad • Bulk shield • Jacket

Applications

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

• Hard to terminate

1+

1-2+

Connectors

7-pin Serial ATA right-angle 7-pin Serial ATA straight

SFF-8482 SAS 29 pin w /power SFF-8470 SAS

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

Connectors

PCI-Express x16

SFF-8038 (SFP+) (QSFP)

Raw Cable Spring Cover Shell Screw PCBA Latch Base Insert Molding Spacer Cover

Tear-down – QSFP (SFF-8088)

Wire termination

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 wire

Differential Impedance

• “Modes" are now possible

• Case 1 L/C C₁₁ C₁₂ L₁₁ L₁₂ L, C, Z L, C, Z common mode

Differential Impedance

• “Modes" are now possible

• Case 1 L/C C_C11 C12 21 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!

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

Z₁₁ Z₁₂ Z₂₂

see Carey, Scott, and Weeks: "Characterization of Multiple Parallel Transmission Lines," IEEE Trans. Instr. and Meas., Sept. 1969

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

Skew

• 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

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 V olt ag e Channel 1 No Skew 10 ps 20 ps 50 ps 100 ps 150 ps 200 ps

Skew

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) A mpl it ud e ( vo lt s) 10 ps 20 ps 50 ps 100 ps 150 ps 200 ps

Rise/fall time mismatch

• Small amounts of mismatch create significant

CM noise

• Not as significant as skew, but harder to control!

• Telltale is significant 2

_{harmonic content}

Rise/fall time mismatch

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) V olt ag e Channel 1 Channel 2 T/R=50/100ps

Rise/fall time mismatch

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 Le ve l (vo lt s) T/R=50/100ps T/R=50/150ps T/R=50/200ps

Eye opening and Jitter

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 and Jitter – test setup

(terminate unused ports with 50 Ohms to Ground) Clock

Test card Cable

Sources of EMI in Cables

• 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

Cable Assembly Construction Influence on EMC

• 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

Cable EMI sources

Cable EMI sources

USB cable shield connection (or not!)

Cable EMI sources

Measuring Cable EMI

• 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

Reverb Chamber

• 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.)

Reverb Chamber

• 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 Absorbing Material

• 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

EMI Absorbing Material

EMI Absorbing Material

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) R ed uc ti on i n Emi ss ion s ( dB )

Ethernet Sample #1 w/ 11" Covered Ethernet Sample #1 w/ 23" Covered Ethernet Sample #1 w/ 37" Covered Ethernet Sample #1 Full Cable Covered

References

• 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 Speed Cables with Lossy Material Coating,” IEEE 2010 Symposium on

Other References

Standards

• 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

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