Lecture02 Link Over Environ 2up

7/28/2019 Lecture02 Link Over Environ 2up

1/15

EE290C Spring 2011

Lecture 2: High-Speed Link Overview and

Environment

Elad AlonDept. of EECS

EE290C Lecture 2 2

Keep in mind that your goal is to receive thesame bits that were sent

Most Basic Link


2/15

EE290C Lecture 2 3

Why Wouldnt You Get What You Sent?

EE290C Lecture 2 4

This is a 1

This is a 0

Eye Opening - space between 1 and 0

te

Ve

With voltage noise

With timing noise

With Both!

V0

V1

tb

Eye Diagrams


3/15

EE290C Lecture 2 5

BER

clk

BER = Bit Error Rate

Average # of wrong received bits / total transmitted bits

Simplified example:

(voltage only)

BER = 10-12: (Vin,ampl Voff) = 7n BER = 10-20: (Vin,ampl Voff) = 9.25n

,12

2

in ampl off

noise

V VBER erfc

=

EE290C Lecture 2 6

What About That Wire

Back plane connector

Line card trace

Package

On-chip parasitic(termination resistance and

device loading capacitance)

Line cardvia

Back plane trace

Backplane via

Packagevia

Back plane connector

Line card trace

Package

On-chip parasitic(termination resistance and

device loading capacitance)

Line cardvia

Back plane trace

Backplane via

Packagevia

[Kollipara, DesignCon03]


4/15

EE290C Lecture 2 7

ICs: usually use lumped models for wires

Capacitance almost always matters

Sometimes resistance

Less often inductance

Works because dimensions


5/15

EE290C Lecture 2 9

Links and Lengths

Cables connecting chips on two different

PCBs

Cables are lossy, but relatively clean if coax

Connector transitions usually the bad part

Distance: ~0.5m up to ~10s of m (Ethernet)

Data-rate: 1-10Gb/s

Wavelength in free space =

Wavelength on PCB (FR4) =

EE290C Lecture 2 10

Links and Lengths

High-speed board-to-board connectors

Daughtercard (mezzanine-type)

Backplane connectors

Distance: 8 up to ~40

Data-rate: 5-20Gb/s

Wavelength in free space =

Wavelength on PCB (FR4) =


6/15

EE290C Lecture 2 11

Transmission Lines Quick Review

Delay

Characteristic Impedance

Reflections

Loss

EE290C Lecture 2 12

Reflections

Sources of Reflections : Z - Discontinuities

PCB Z mismatch

Connector Z mismatch

Vias (through) Z mismatch

Device parasitics - effective Z mismatch

Z1 Z2

Z2 Z 1

Z 1 Z2+--------------------

2Z 2

Z 1 Z2+--------------------

DC via Conn via BP

(1) Energy conserved

(2) Voltages equal


7/15

EE290C Lecture 2 13

Skin Effect

At high f, current crowds

along the surface of the

conductor

Skin depth proportional to f-

Model as if skin is thick Starts when skin depth equals

conductor radius (fs)

Figure 2001 Bill Dally

EE290C Lecture 2 14

Skin Effect contd

100100MHz 500MHzMHz 500MHz 1GHz1GHz

W=210umt=28um

=6.6 um =2.08 um=2.95 um

Skin depth


8/15

EE290C Lecture 2 15

Dielectric Loss

High frequency signals jiggle

molecules in the insulator

Insulator absorbs energy

Effect is approximately linear

with frequency

Modeled as conductance term in

transmission line equations

Dielectric loss often specified

in terms of loss tangent

Transfer function =

Table 2001 Bill Dally

DLengthe

EE290C Lecture 2 16

Dielectric Loss contd

FR4 cheapest most widely used

Rogers is most expensive high-end systems

May not matter that much due to surface roughness

8 mil wide and 1 m long 50 Ohm strip line

-40.0

-30.0

-20.0

-10.0

0.0

1.E+06 1.E+07 1.E+08 1.E+09 1.E+10

Frequency, Hz

Attenuation

FR4

Roger 4350

Kollipara DesignCon03


9/15

EE290C Lecture 2 17

Skin + Dielectric Losses

Skin Loss f Dielectric loss f: bigger issue at high f

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.0 E+06 1. 0E+0 7 1. 0E+08 1.0 E+09 1. 0E+10

Attenuation

Frequency, Hz

FR4 dielectric, 8 mil wide and 1m long 50 Ohm strip line

Total loss

Conductor loss

Dielectric loss

Kollipara DesignCon03

EE290C Lecture 2 18

Everything Together: S21

S21: ratio of received vs. transmitted signals

Breakdown of a 26" FR4 channel with 270 mil stubs

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 .0E+0 0 5.0E +08 1.0E+09 1 .5E+0 9 2.0E +09 2.5 E+09 3 .0E+0 9 3.5E +09 4.0 E+09

Frequency, Hz

Transferfunction

PCB traces

PCB traces & connectors

PCB traces, connectors & vias

Entire channel


10/15

EE290C Lecture 2 19

Real Backplane

EE290C Lecture 2 20

Practical PCB Differential Lines

Differential signaling has nice properties

Many sources of noise can be made common-mode

Differential impedance raised as f(mutuals) between

wires

Strong mutual L, C can improve immunity

W S

H

H

SW

r

H

+ -

- Strip Strip-line


11/15

EE290C Lecture 2 21

Coupling Crosstalk

Near-end xtalk: NEXT (reverse wave)

Far-end xtalk: FEXT (forward wave)

NEXT in particular can be very destructive

Full swing TX vs. attenuated RX signal Good news: can control through design

NEXT typically 3-6%, FEXT typically 1-3%

EE290C Lecture 2 22

NEXT: What Not To Do

X

X

X

X

X

X

X

X

Tx Rx Tx

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Time, ps

Voltage,

V Tx

Rx

XTX


12/15

EE290C Lecture 2 23

NEXT: Better Design

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Time, ps

Voltage,

V Tx

Rx

XTX

X

X

X

X

X

X

X

X

Tx

Rx

EE290C Lecture 2 24

Connectors Particularly Tough

NEXT FEXT

55 ps (20-80%) 55 ps (20-80%)

80ps (10-90%) 80ps (10-90%)

AB 4.4% 3.7%

DF 3.3% 2.6%

GH 3.3% 2.6%

JK 4.3% 3.5%

Tight footprint constraints

Hard to match pairs and even individual lines

May compensate skew on line card

Also big source of impedance discontinuities


13/15

EE290C Lecture 2 25

Skew Within Link

Need very tight control to maintain constant % of bit

time

1% skew on 30 line 50ps skew Half of a bit time at 10Gb/s

Good news: connectors relatively short (~200ps)

EE290C Lecture 2 26

Reflections RevisitedTX

DATA

RX

DATA

AT

AR

CR

CT

D

B

-8

-6

-4

-2

0

2

4

6

8

10

gh-ghconn. (baseline):NormalizedRaw andeq pulse response:PRlength after

main60

A T,R

A2 T,R

B

C T,R D

-8

-6

-4

-2

0

2

4

6

8

10


main60

-8

-6

-4

-2

0

2

4

6

8

10


main60

A T,R

A2 T,R B

C T,R D

T

Connector-BP

transitions


14/15

EE290C Lecture 2 27

Reflections Due To Via Stub

0 2 4 6 8 10

-60

-50

-40

-30

-20

-10

0

frequency [GHz]

Attenuation[dB]

9" FR4,via stub

26" FR4,via stub

26" FR4

9" FR4

Stub: extra piece of T-line hanging off main path

Usually leads to resonance (notch) Especially on thick backplanes, vias are a big culprit

EE290C Lecture 2 28

Minimizing Via Stubs Thinner PCB?

counter-

boredblind

via

All expensive: 1.1-2x

Better vias?


15/15

EE290C Lecture 2 29

Summary

Packaging, chip connection, etc. can all have an effect

Entire conferences dedicated to signal integrity (SI)

EE290C Lecture 2 30

Implications

Need to know range of channels you will face Drives design of the link circuitry

Start diving in to that next lecture

Dont be a pure circuit weenie

Simple fixes to channel may go a long way

FR-4 BP, Length: 20", T/S: 30/270 mil

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 .0 0 0 .5 0 1 .0 1 1 .5 1 2 .0 1 2 .5 2 3 .0 2 3 . 5 2 4 .0 2 4 .5 3 5 .0 3 5 .5 3

frequency, GHz

Transferfunction

meas

sim

Roger BP, Length: 1.5", T/S: 30/270 mil

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0 0 .7 8 1.56 2.33 3.11 3 .89 4. 67 5. 45 6.22 7 .0 0

Frequency, GHz

Transferfunction(s21)

meas

sim

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Lecture02 Link Over Environ 2up