SRC Review 9/10/03 W. Namgoong, USC 1

Design of High-Speed Serial-Links in CMOS

(Task ID: 930.001)

SRC Research ReviewSeptember 10, 2003

Won NamgoongUniversity of Southern California

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Design of High-Speed Serial-Links in CMOS

• Technical Thrust– Circuit Design

• Students– Kyongsu Lee– Lei Feng

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Accomplishments for 2003

• Developed adaptive/synchronization techniques for frequency channelized receivers.

• Designed a serial-link prototype based on frequency channelization.– Currently in fabrication.

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Outline of Talk

• Adaptive frequency channelized receiver.• Frequency channelized receiver

implementation.• Research plans for next year.

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Background in Signaling

• Transistor mismatches.• On-chip noise.• Inter-symbol interference.

– Wire losses and package parasitics.– Finite receiver/transmitter bandwidth.

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Existing Architecture – Time-Interleaved Receiver

• Sample at approx. Nyquist rate; 2-4 bit ADC’s (flash).• ADC sees the full bandwidth of the input signal.

– Sample/hold circuitry difficult to design.– Sensitive to sampling jitter and sample-time offsets.

• Large input capacitance.

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Frequency Channelized Receiver

• Achieves the same effective sampling frequency as time-interleaved receiver using the same number of ADCs.

• ADC input bandwidth reduced.– Sample/hold circuitry relaxed.– More robust to sampling jitter even with mixer phase noise present.

• Reduced input bandwidth.

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Adaptive Frequency Channelized Receiver Overview

• Adaptive synthesis filter bank.– Equalize distortion caused by the propagation channel

and reconstruct the channelized signal for detection.– Analog analysis filter bank not accurately known at

design time.– Error signal based on the detected symbol.

• Digital interpolators.– ADC sampling frequency generally not an integer

multiple of the symbol frequency. – Interpolation must occur after synthesis filter bank.

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Overall Receiver Structure

• Timing/detection module extracts timing information and detects transmitted symbol.

• Adaptive control module generates error signal used to update the adaptive synthesis filter bank.

AdaptiveFilterBank

TrainingSymbols

][ls ][ma

Delay

TimingController

ReferenceInterpolator

TimingRecovery

Backward TimingPrediction

DetectionInterpolator Detector

Adaptive Control Module

Timing/Detection Module

+-

][ˆ nd

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Adaptive Filter Bank (1)

ADC

ADC

ADC

2exp tfj sampleπ−

)1(2exp tfMj sample−− π

)()( tntx + ][0 ls

][1 ls

][1 lsM−

-][ne

+][nd

γ↑ )(0 zG

)(1 zG

)(1 zGM−γ↑

γ↑

][0 ny

][1 nyM−: :

Re ⋅

)( ΩjH

)( ΩjH

)( ΩjH][nd

samplef

samplef

samplef

• Effective sampling frequency , where γ = 2M – 1.• Re applied since transmitted signal is a baseband real signal.

sampleeff ff ⋅=γ

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Adaptive Filter Bank (2)][0 ls

][1 lsM−

][ lz γ

][ le γ

)(0,0 zW

)(0,1 zWM−:

-

]1[ −+ γγlz

+][nd

)(1,0 zW −γ

)(1,1 zWM −− γ

:

: :

Re ⋅

Re ⋅

]1[ −+ γγle][ne

code channel 0

code channel 1−γ

][ˆ nd

• Synthesis filter bank is LPTV system with period γ.• Each code channel estimates one of γ consecutive samples.• LMS adapts each code channel independently:

][][][]1[ ilelll ii ++=+ γεsww

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Timing/Detection Module

LoopFilterNCO

][meT][meTLmµ

][nη

][maDetection

Interpolator Detector][mu

M&MTiming Error

Detector

OverflowComputeFractionalIntervalTiming Controller

][ˆ nd

Timing Recovery

• Detection interpolator resamples to symbol rate.• Timing recovery determines timing error that

controls interpolator.

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Adaptive Control Module

ReferenceInterpolator

]'[nd]'[ne Symbols

rDnn −=′

+

-

rDDelay

NCO

n′ξ

][nηComputeFractionalInterval

Backward Timing Predictor

ComputePast

Overflow

][nwNCO

][ˆ nd

][ˆ nd ′ Overflow

• Backward timing predictor calculates timing information of delayed filter bank output based on current NCO state.– Delayed for used in DD mode.

• Reference interpolator generates desired reference signal.

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Simulation Results –Convergence Performance

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Simulation Results – Effect of Filter Taps

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Simulation Results – Effect of ADC Bits

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Summary of Adaptive Frequency Channelized Receiver Work

• Based on frequency channelized signals, an adaptive synchronization/detection scheme is described.

• Performance of proposed receiver is similar to that of a single channel receiver.– Convergence time is slightly longer.

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Implementation of Frequency Channelized Receiver

• A frequency channelized receiver excluding the digital back-end has been implemented in 0.25um CMOS.

• Symbol rate of 10Gsymbols/sec.• Three frequency subbands and 10 3-bit

ADC’s each operating at 1.24Gsamples/sec.• Currently in fabrication.

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System ArchitectureΩ50 fof 3dB- =

I/Q 2fo

I/Q 4fo

fofs =

LPF ADC

2fo 4fo

ADC

LPF ADC

ADC

LPF ADC

ADC

LPF ADC

ADC

LPF ADC

ADC

signalinput alDifferenti

driver Output &

decoderGray

2

1.24Gfo =

freq.

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Quad-Phase Mixing

• Conventional passive double balanced mixer.• 50 ohm matching allows wide input bandwidth: ~6.5GHz.

SIGV+LOI+

SIGV-LOI-

LOQ+

LOQ-

LOI+

LOI-

LOI+

I+

I−

Q+

Q−

Ω50

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Low Pass Filtering

• Vc adjusts gm of transistor M1 to control gain without affecting bandwidth.

• Achieves 20dB gain with 1.4GHz 3-dB bandwidth.

4-stage Feedback LPFBuffer

SR

FR

SR

FRVc

BV

CMFB1MMa

iR OR

Freq.

[dB]Gain

80dB/dec−

20dB/dec−

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ADC Architecture

LPF

1.24Gf 3dB =−

ADC

ADC

1.24Gf s =

• 2 Time-interleaved 3-bit ADC

• Each 3-bit ADC samples at 1.24Gsps.• Effective sampling rate/channel is 2.48Gsps.• No offset scheme necessary.

• ADC architecture

a6

PVNV

ref

a5 a4 a3 a2 a1 a0

comparator

sampler

latch

CLK

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Local Oscillator Frequency Generation

BPF LPF VCO

4fo

4fo phase-4

Doubler Freq.

phase-Quad mixer drive ⇒

2fo aldifferenti phase-Quad

DividerFreq.

fo phase-2

2fo phase-4 mixer drive ⇒

converter Code ADC, drive ⇒

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ADC & DECODER

LPF

MIXER &LO SIGNALINTERFACE

Freq. Doubler

Chip Layout

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Plans for Next Year• Fabricate and test design prototype to verify main

concepts.• Develop/analyze more sophisticated adaptive

reception algorithms based on maximum-likelihood sequence estimation (MLSE).– Faster convergence speed and higher performance.

• Design transmitter/receiver to support higher rates.– Multi-level signaling.– OFDM based communication systems.