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EE290C - Spring 2004 Advanced Topics in Circuit Design High-Speed Electrical Interfaces Lecture #4 Communication Techniques Equalization & Modulation Jared Zerbe 1/29/04 2 Agenda Background Common Equalization Today Receive Linear Equalization Transmit Linear Equalization DFE Setting coefficients Modulation approaches Some results
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EE290C - Spring 2004Advanced Topics in Circuit DesignHigh-Speed Electrical Interfaces

Lecture #4Communication Techniques

Equalization & ModulationJared Zerbe1/29/04

2

AgendaBackgroundCommon Equalization Today

Receive Linear EqualizationTransmit Linear EqualizationDFE

Setting coefficientsModulation approachesSome results

2

3

History

Equalizing for loss in band-limited channels has been around ….a long timeWhat makes links unique

PerformancePeak power constraintPower & area limits

4

Equalization For Loss :Goal is to Flatten Response

Channel is band-limitedEqualization : boost high-frequencies relative to lower frequencies

+

=

3

5

The Single Bit Response (SBR)What is observed at the receiver when the transmitter sends an unequalized single-bit pulse

Usually normalized at TXEach dot is a symbol sample

Can be very helpful in understanding nature of the system

Attenuation : reduction in amplitude of main pulseDispersion : spreading of the narrow pulseReflections : ripples off of Z-discontinuities

0 0.2 0.4 0.6 0.8 1

x 10-8

0

0.2

0.4

0.6

0.8

1

sec

sbR

-- R

aw

Raw Single Bit Response

TX single pulse

RX waveform

6

Dispersion Causes ISIBand-limited channels mean dispersion

Our nice short pulse gets spread out

0 1 2 3

0

0.2

0.4

0.6

0.8

1

ns

puls

e re

spon

se

Tsymbol=160ps

Dispersion –short latency(skin-effect, dielectric loss) Reflections –long latency(impedance mismatches –connectors, via stubs, device parasitics, package)

4

7

ISI Leads to Bit Failures

Middle sample is corrupted by 0.2 trailing ISI (from the previous symbol), and 0.1 leading ISI (from the next symbol) resulting in 0.3 total ISIAs a result middle symbol is detected in error

0 2 4 6 8 10 12 14 16 180

0.2

0.4

0.6

0.8

1

Symbol time

Am

plitu

de

Error!

8

SBR : Before & After Equalization

0 0.2 0.4 0.6 0.8 1

x 10-8

0

0.2

0.4

0.6

0.8

1

sec

sbR

-- R

aw

Raw Single Bit Response

0 0.2 0.4 0.6 0.8 1

x 10-8

0

0.2

0.4

0.6

0.8

1

sec

sbR

-- E

qual

ized

Equalized Single Bit Response

sbRsampled sbRTxEqRxEq

ISI significantlyimproved

5

9

Reflections & Worst-Case Sequences

0

-200

-100

200

0 100ps

100

200

mV

PRBS

Worst-casepattern

Distribution

10

AgendaBackgroundCommon Equalization Today

Receive Linear EqualizationTransmit Linear EqualizationDFE

Setting coefficientsAlternate approachesSome results

6

11

Receive Linear Equalizer

Amplifies high-frequencies attenuated by the channelPre-decisionDigital or Analog FIR filterIssues

Also amplifies noise!PrecisionTuning delays (if analog)Setting coefficients

Adaptive algorithms such asLMS

WL-1

DDDWLW1

+

H(s)

freq

12

Transmit Linear Equalizer

Attenuates low-frequenciesNeed to be careful about output amplitude : limited output power

If you could make bigger swings, you wouldEQ really attenuates low-frequencies to match high frequencies Also FIR filter : D/A converter

Can get better precision than RXIssues

How to set EQ weights?Doesn’t help loss at f

H(s)

freq

7

13

Transmit Linear Equalizer : Single Bit

0.0 0.3 0.6 0.9 1.2-0.3

-0.1

0.1

0.3

0.5

0.7UnequalizedEqualization PulseEnd of Line

time (ns)

Vol

tage

14

Example : 5Gbps Over 26” of FR4With No Equalization

8

15

Example : 5Gbps Over 26” of FR4Under Equalized

16

Example : 5Gbps Over 66cm FR4Over Equalized

9

17

Example : 5Gbps Over 66cm FR4Correct Tx Equalization

Maximum SNR at the sample point

18

Decision Feedback Equalization

Don’t invert channel…just remove ISI

Know ISI because already received symbolsDoesn’t amplify noiseHas error accumulation problem

Less of an issue in linkswhere random noise small

Requires a feed-forward equalizer for precursor ISI

Reshapes pulse to eliminateprecursor

-

FIR filter

Decision (slicer)

FIR filter

Feed-forward EQ

Feed-back EQ

10

19

AgendaBackgroundCommon Equalization Today

Receive Linear EqualizationTransmit Linear EqualizationDFE

Setting coefficientsAlternate approachesSome results

20

Setting Equalizer CoefficientsTwo basic techniques

Set and forgetBased on manual channel measurement or…Calculate on basis of a single-bit-response

AdaptationUse an optimizing algorithm to find ‘minimum’Optimize multiple variables at onceAdapt once or continuously adaptive

11

21

Tx Adaptation Example (animation)

22

AgendaBackgroundCommon Equalization Today

Receive Linear EqualizationTransmit Linear EqualizationDFE

Setting coefficientsAlternate approachesSome results

12

23

Alternate Approaches : Multi-Level Signaling

Binary (NRZ) is 2-PAM2-PAM uses 2-levels to send one bit per symbolSignaling rate = 2 x Nyquist

4-PAM uses 4-levels to send 2 bits per symbolEach level has 2 bit valueSignaling rate = 4 x Nyquist

00

01

11

10

1

0

1

0

Note : both can be either single-ended or differential

24

When Does 4-PAM Make Sense?

First order : slope of S213 eyes : 1 eye = 10dbloss > 10db/octave : 4-PAM should be considered

0.0 1.0 2.0 3.0 4.0

Nyquist Frequency (GHz)

|H(f)

|

-20db

-40db

-60db

13

25

Receive signal after 100m CAT-6 cable Receive signal after analog equalization (no echo)

• Addition of another level : eyes are 75% of 4-PAM eyes• Adds some more bandwidth, but << 1 bit

• Can be used for coding• Inclusion of a ‘null’ in differential version valuable• 4 pairs @ 125Ms/s used in 1000-T to get 1Gb/s

5-PAM?

Figure K. Azadet, 1999

26

Multi-PAM ChallengesReceiver offsets

Most receivers have untrimmed offsets of >10mVYou just added more receivers

Setting slicer levelsYou’re not just comparing + & - any moreYou need to know where the ‘sweet spot is’

HousekeepingMore levels, more samplers, more clocks, more complicated

But it can all be run at ½ the clock speed!

14

27

Alternate Approaches : Simultaneous BiDirectional

Two signals at halfspeed

Makes sense if b/w need equal in both directions

IssuesGetting ideal timingbetween TX & RX is tough

Vlinedrv

VrefVrefH (shared)VrefL (shared)

rcvr

receive signal

transmit signal

VlineVref

(Vline - Vref)+ve

-ve

VrefH

VrefL

Fixed VrefL= Vdd – 1.5*Vswing

28

Simultaneous BiDi ChallengesTransmit noise injection can ruin your day

So can TX clock injectionNo matter what you do margins will change relative to TX position.

Building and distributing referencesHow do you make these quiet?What are they bypassed to?

Limited to mesochronous systems?

15

29

AgendaBackgroundCommon Equalization Today

Receive Linear EqualizationTransmit Linear EqualizationDFE

Setting coefficientsAlternate approachesSome results

30

Rx Equalization Simulation

Simulations show how Rx equalization greatly compresses eye distribution including worst-case sequences

0 1 2

x 10-10

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

sec0 1 2

x 10-10

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

sec

No Rx EQ With Rx EQ

16

31

Measured Equalization Effectiveness

Equalization very effective in band-limited systemsDFE & 4-PAM are powerful tools when used together

TX + RX EQ

No EQ

TX only EQ

50cm of FR4, two 7.5cmLC, two connectors


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