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EE290C - Spring 2004Advanced Topics in Circuit DesignHigh-Speed Electrical Interfaces
Lecture #4Communication Techniques
Equalization & ModulationJared Zerbe1/29/04
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AgendaBackgroundCommon Equalization Today
Receive Linear EqualizationTransmit Linear EqualizationDFE
Setting coefficientsModulation approachesSome results
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History
Equalizing for loss in band-limited channels has been around ….a long timeWhat makes links unique
PerformancePeak power constraintPower & area limits
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Equalization For Loss :Goal is to Flatten Response
Channel is band-limitedEqualization : boost high-frequencies relative to lower frequencies
+
=
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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
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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)
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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!
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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
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Reflections & Worst-Case Sequences
0
-200
-100
200
0 100ps
100
200
mV
PRBS
Worst-casepattern
Distribution
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AgendaBackgroundCommon Equalization Today
Receive Linear EqualizationTransmit Linear EqualizationDFE
Setting coefficientsAlternate approachesSome results
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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
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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
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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
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Example : 5Gbps Over 26” of FR4With No Equalization
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Example : 5Gbps Over 26” of FR4Under Equalized
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Example : 5Gbps Over 66cm FR4Over Equalized
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Example : 5Gbps Over 66cm FR4Correct Tx Equalization
Maximum SNR at the sample point
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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
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FIR filter
Decision (slicer)
FIR filter
Feed-forward EQ
Feed-back EQ
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AgendaBackgroundCommon Equalization Today
Receive Linear EqualizationTransmit Linear EqualizationDFE
Setting coefficientsAlternate approachesSome results
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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
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Tx Adaptation Example (animation)
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AgendaBackgroundCommon Equalization Today
Receive Linear EqualizationTransmit Linear EqualizationDFE
Setting coefficientsAlternate approachesSome results
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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
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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
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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
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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!
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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
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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?
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AgendaBackgroundCommon Equalization Today
Receive Linear EqualizationTransmit Linear EqualizationDFE
Setting coefficientsAlternate approachesSome results
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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
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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
