OC-192 communications system block diagram

EECS 270C / Spring 2014 Prof. M. Green / U.C. Irvine 1

NetworkProcessor

TX

RX

O E

NetworkProcessor

O E

RX

TX

E O

E O

16

16

16

16

622Mb/s 10 Gb/s 622Mb/s

Photo Diode

TIA + Preamp

10 Gb/sModLaser

10 Gb/s

10 GHz

• OC-192 (10 Gb/s) transceiver• 0.18 µm CMOS process

OC-192 communications system block diagram


Transceiver block diagram:

10 GHz

10 Gb/s


ReadPointer

FIFOControlRESET

CLK16IP

CLK16IN

DI0P

DI0N

DI15P

DI15N

REFCLKP

REFCLKN

REF155EN

INP

UT

RE

GIS

TE

R

10/10.7 GHzCMU

SELFECB

16

X 1

0 F

IFO

WritePointer

16

:1 M

UX

DIVIDE-BY-16

OutputRetime

IFSEL

VCP

VCN

TSDP

TSDN

TSCKP

TSCKN

CLK16OP

CLK16ON

LCKDET

RB_LD

LVDS Parallel

Input Bus

LVPECLRef.

Clock

CMLHigh- SpeedOutputs

LVDSOutput Clock

OVF

Transmitter Block Diagram


Low-Frequency Input signals

Reference clock

Input clock

Input data

Input data aligned to input clock (usually jittery)

Very low jitter (~10 ppm)reference clock; used in CMUto generate 10 GHz internal clock

Reference clock and input clock are not synchronized.

T

tsh

Maximum allowable variation between Input clock & Reference clock is T − tsh


Input clock timing domain

Reference clock timing domain

input clock

input data

16

Reference clock

High-frequency clock

16

16

Connection could exhibit varying delay Variable phasing between

input & reference clock domainscan cause bit errors in MUX

Illustration of Input Timing Regimes

16:1 MUX


We require an intermediateblock to resolve timing variationsbetween input & reference clock

First-In/First-Out (FIFO) Circuit (1)

16:1 MUX


First-In/First-Out Circuit (2)

Din_0

Din_n

k

k

Readclock

Writeclock

Readclock

Write clock

Dout_0

Dout_n

Synchronizedwith input clock

To serializer(signals synchronizedwith reference clock)

Since these signals have period k times longer than the input period, the circuit can tolerate k times larger variation between input & reference clocks.

Ref clock

Ref clock

Read clock based on input clockWrite clock based on reference clock


FIFO approach:• Large amount of hardware (many latches)• Significant power dissipation unless static CMOS is used• Can handle arbitrarily large delay variations

DLL approach:• Less hardware• Can handle modest delay variations• Better choice for BJT or GaAs processes

Appropriate phase chosen


staticCMOS

CML

16:1 Multiplexer Tree Structure

10 Gb/s5 Gb/s

2.5 Gb/s1.25 Gb/s

5 GHz2.5 GHz1.25 GHz


2:1 MUX cell details

D flip-flop with extra latch


Assume all blocks have:• Tail current ISS

• Resistor R• Diff pair transistor sizes W/L

ISS5ISS2ISS10ISS Total current: 18ISS


We can take advantage of gain/bandwidth tradeoff by appropriate scaling:

Design parameters:• ISS

• R• W/L CL

Idea:

Lower bit rate allows lower power!


Cp ≈ 10 fF GSCALE=3 ISS = 1.2 mA

MSCALE=1/8 MSCALE=1/2 MSCALE=1/2 MSCALE=1

Itotal = 5.75ISS

= 6.9 mA


Clock Dividers

The operation of “real” high-speed clock dividers is more complex …


Clock divider based on CML D flip-flop:

Divider sensitivity curve:

Vmin = minimum input clock amplitude required for correct operation.

fso = self-oscillation frequency

Vmax = maximum dc differential voltage that can be applied to the input clock for which the circuit self-oscillates.

Vmax


Desired frequency divider operationQuasiperiodic operationSlew-rate limited operation

Region I:Region II:

Region III:

Sensitivity Curve Analysis


Region II: Quasiperiodic behavior

self-oscillating

lockedfin = 11GHz


Region III: Slew-rate limited Behavior

Sine-wave input

Square-wave input


Effect of Transistor Sizes on Sensitivity Curve

Latch transistors

Driver transistors

Clock transistors


Alternatives to DFF-Based Clock Dividers

• Latches present large capacitive load slow


At very high frequencies, latch transistors are not necessary and only add capacitance to the circuit:


Ring-Oscillator-Based Divider

Behaves like a 4-stage ring oscillator with injection of full-rate frequency.


Comparison of Sensitivity Curves

Conventional divider:Dynamic divider:

Wider frequency range; lower self-oscillation frequencyNarrow frequency range; higher self-oscillation frequency


I1 I2

I2(t)

0

ISS

I1(t)

Vout-(t)

Vout+(t)

VDD

VDD – ISSR

VDD – ISS(R+R)

0

Vout+(t) – Vout-(t)

ISSR

ISS(R+R)

Effect of Non-Ideal Clock Signals

Offset resistance causes deviation from50% duty cycle in clock signal.


Half-rate clock

MUX output

ideal

with offset

with offset ideal

Result of nonideal half-rate clock is Periodic Jitter.


retimer

10 GHz clock

10 Gb/s retimed data5 Gb/s

5 GHz

10 Gb/s data

Full-rate clock(could be non-50% duty cycle)

retimed output

Retimer eliminates this problem:


10 Gb/s data output

10 GHz clock

2.5 Gb/s data input

5 Gb/s

5 GHz2.5 MHz

tp1

tp2

tp1 & tp2 are “clock-to-Q” delays.Because the clock & data flow in opposite directions, alignment between 5 Gb/s data & 5 GHz clockis determined by the sum: tp1 + tp2

(High sensitivity to processing / temp. corners)

Internal MUX Timing


• 50 back termination used to reduce reflections.

• CML blocks scaled up so that last stage drives ac load of 25

• Shunt-peaking used in second stage.

Serial Output 50 Line Driver


Receiver Block Diagram


DMUX Architecture


1:4 DMUX Tree Structure

10 Gb/s data input

2.5 Gb/s data outputs

10 GHz clock 5 GHz 2.5 GHz

5 Gb/s


1:2 DMUX cell details:


10 Gb/s data input

2.5 Gb/s data output

10 GHz clock 5 GHz 2.5 GHz

tp1 & tp2 are “clock-to-Q” delays.

Because the clock & data flow in the same direction, alignment between 5 Gb/s data & 2.5 GHz clockis determined by the difference: tp1 – tp2

(Low sensitivity to processing/temp. corners)

tp1

tp2

Internal DMUX Timing

5 Gb/s


Crosstalk in Transceivers

f1 f2

• Capacitive coupling between VCO’s can cause “frequency pulling”

• Momentary differences in frequencies between 2 VCO’s can give rise to additional jitter.


Crosstalk Measurement

Serial input data10 Gb/s

CMU reference clock

recovered clock10 GHz

output data10 Gb/s + 100ppm

output clock10 GHz + 100ppm

Low-frequencyinputs/outputs

Low-frequencyinputs/outputs

Jitter is measured at TX output clock (or data) and RX recovered clock.


• Sufficient physical separation between VCO’s

• Separate supply connections to package for each block (e.g., CMU, CDR, MUX, DMUX, FIFO, etc.)

• Ample guard rings to minimize substrate coupling

Techniques for Reducing Transceiver Crosstalk

Very difficult to simulate & predict!


SONET Jitter Specifications

1. Jitter Generation (transmitters)

2. Jitter Tolerance (receivers)

3. Jitter Transfer (repeaters)


Jitter Generation (1)

• DJ always specified in peak-to-peak

• RJ rms jitter well-characterized

• RJ peak-to-peak jitter dependent on measurement time (increases without bound)

SONET:

JPP usually measured over a specified frequency range.

Wideband jitter (p-p or rms) can be measured directly from serial output data signal

Gigabit Ethernet & Fiber Channel:

Equivalent JPP determined by measured BER.


SONET jitter generation is specified within a certain jitter frequency range.For OC-192: 50 kHz – 80 MHz

To measure narrowband jitter generation, we can:

A. Measure the recovered clock from a “golden” CDR:

TXCDR

(low jitter generation)

Ref. clock

output data

recovered clock

to jitter analyzer

Should have jitter bandwidth > 80MHz

SONET OC-192 bandpass filter


10 GHz


B. Measure the TX output clock directly(assuming its jitter is the same as the data):

TX

Ref. clock

output data

TX output clock

to jitter analyzer

Note: ISI is usually measured separately (peak-to-peak only).


10 GHz


9.95328 GHz 10.6642 GHz

Measured at output clock; 231-1 PRBS serial data applied to input

Phase noise: -100 dBc/Hz @ 1MHz offsetJitter generation (SONET filter): 5.6mUI rms / 60mUI p-p

Phase noise: -100 dBc/Hz @ 1MHz offsetJitter generation (SONET filter): 6.2mUI rms / 65mUI p-p



Jitter Generation (231-1 PRBS):6.44 ps pp (wide band) 0.38ps rms (within SONET band)

Closed-loop VCO phase noise (231-1 PRBS): –107 dBc/Hz @ 1 MHz offset


Jitter measurements from clock:


10.6642GHz clock Wideband jitter: 7.5ps p-p / 1.2ps rms

10.6642Gb/s data

Wideband jitter: 10.7ps p-p / 1.8ps rms

231-1 PRBS input data applied:



Serial data in

recovered clock

retimed data out

To DMUXretimer

Experiment: Apply serial data to CDR with jitter at a certain frequency. Increase the jitter amplitude until a bit error occurs.

If data jitter & recovered clock jitter could perfectly track, then retiming would be error-free.

Data in

Recovered clock

T

tsh

Jitter Tolerance (1)


Given CDR open-loop characteristic

fdata fclock

(expressed in UI)

^



Bit rate: 10.7Gb/sPattern: 231-1PRBSBER threshold: 10-12

Jitter Frequency (Hz)

Jitt

er T

ole

ran

ce [

UIp

p]

Jitter Frequency [Hz]

Bit rate: 10.7 Gb/sPattern: 231-1PRBSData in: 50 mV ppBER threshold: 10-12

100 1K 10K 100K 1M 10M 100M0.01

10

0.1

1

10

100

1000

10000

Jitter Tolerance > 40 ps pp at high frequency



O E RX TX E O

n repeaters:

repeater

Jitter peaking should be minimized.

Jitter Transfer

f0

0.1dB

-20 dB/decade

Jitter Transfer Mask:

EECS 270C / Spring 2014 Prof. M. Green / U.C. Irvine

Electrical-to-Optical Interfaces (1)

Electrical to optical (TX):

MUXlaserdriver

IL

IL

optical output power

T

laser diode orVertical Cavity Surface Emitting Laser(VCSEL)

48



Electroabsorption modulator

Operates by making optical material more or less absorptive.

Pin

Pout

VM

VMVswing~ 3V

49


Mach-Zender modulator:


Mach-Zender interferometer:

• Invented in 1890s• Used to precisely measure optical

phase shift of materials.• By using constructive/destructive

interference, can be used as a laser modulator.

VMVswing ~ 6V

50



Optical pulsewidth distortion commonly occurs due to:• Unequal turn-on/turn-off times of laser diode• Non-ideal bias voltage in modulators.

Electrical signal(IL or VM)

Optical output

Additional circuitry to correct pulsewidth is often added to system...

Results in DCD

51



VM

Vref

IB

laser diode monitor diode

R

Optical output control circuit:

Feedback sets

52


Optical Receiver Block Diagram

O E

LA CDREQ DMUX

≈ -18 dBm ≈ 10 mV p-p≈ 10 µA ≈ 400 mV p-p

TIA

53


Optical-to-Electrical Interfaces (1)

p-i-n photodetector structure: circuit model:

+

_

+

_

n

p

i

applied optical signal

resulting electrical

current

VR~5V

54


Eye diagram of PRBS resulting from 96 km of single-mode fiber

and photodetector.

Optical-to-Electrical Interfaces (2)

• DCD & ISI are evident.

• Noise is higher at logic 1 than at logic 0.

Photodetector noise:

55


Transimpedance Amplifier (TIA)

R

Cd

A0

Vref

Iin

from photodetector

Vout

Used to convert photodetector current into voltage.

low-impedance node maintains nearly constant detector voltage good linearity.

56


Transimpedance Amplifier (2)

R

Cd

A0

Vref

Iin

photodetector

Vout

Transimpedance:

Input impedance:

Loop gain:

additional pole limits closed-loop BW

Cg

57



Noise analysis:

Good sensitivity requires:• Large Rf

• Large Cg

• Large gm

Tradeoff with BW

58



• Cd decoupled from feedback network

• Common-gate device increases noise

Cd

Cg

RCd

Cg

LB

R

• LB provides decoupling (series peaking); could be realized by bondwire.

59


Limiting Amplifiers

Requirements:

• Amplify input signal with variable amplitude (~10-30 mV) to a fixed-amplitude (~450 mV) output.

• Sufficiently high bandwidth

• Sufficiently low noise

• Low offset voltage

+Vin

−

+Vout

−

n stages

Single stage: n-stage amplifier:

Overall gain:

Overall bandwidth:

60


7-Stage Limiting Amplifier Example (1)

Each stage uses shunt-peaked CML buffer with:

A0 = 5.5 dBBW = 10 GHz

100 MHz 1 GHz 10 GHz 100 GHz

1st stage output

7th stage output

61



Input amplitude = 20 mV p-p Input amplitude = 40 mV p-p

1st stage output

7th stage output

6th stage output

1st stage output

7th stage output

6th stage output

62


Input amplitude = 20 mV p-p

1st stage output

7th stage output

6th stage output

Input-referred offset of 5 mV applied


Offset-cancellation circuitry required!

63


Limiting Amplifier Offset Compensation

n-stageamplifier core

lowpass filter

+vin

−

offset compensation

RL RL

M1 M1 M1 M1RF

CF

RF

CF

VOS

Vout+−

+−

V1

+Vout

−

H.-Y. Huang et al., “A 10-Gb/s inductorless CMOS limiting amplifier with third-order interleaving active feedback,” JSSC, May 2007, pp. 1111-1120.

compensation circuit:

amplifier circuit:

64

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OC-192 communications system block diagram