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OC-192 communications system block diagram
Laser
10 Gb/s
Mod
TIA + Preamp
10 Gb/s
Photo Diode
10 GHz
16
TX
E
O
Network
Processor 622Mb/s
16
O
E
RX
10 Gb/s
RX
E
O
16
622Mb/s
O
E
TX
Network
Processor
16
• OC-192 (10 Gb/s) transceiver
• 0.18 µm CMOS process
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
1
Transceiver block diagram:
10 GHz
10 Gb/s
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
2
Transmitter Block Diagram
FIFO
Control
CLK16IP
CLK16IN
DI15P
DI15N
16:1 MUX
LVDS
Parallel
Input Bus
Write
Pointer
INPUT REGISTER
DI0P
DI0N
OVF
16 X 10 FIFO
RESET
Output
Retime
TSDP
TSDN
TSCKP
TSCKN
CML
HighSpeed
Outputs
Read
Pointer
SELFECB
RB_LD
REF155EN
LVPECL REFCLKP
Ref.
REFCLKN
Clock
DIVIDE-BY-16
10/10.7 GHz
CMU
LVDS
Output
Clock
CLK16OP
CLK16ON
LCKDET
IFSEL
VCP
VCN
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
3
Low-Frequency Input signals
Input clock
Input data aligned to input clock
(usually jittery)
Input data
T
Reference clock
Very low jitter (~10 ppm)
reference clock; used in CMU
to generate 10 GHz internal clock
tsh
Reference clock and input clock are not synchronized.
Maximum allowable variation between
Input clock & Reference clock is T − tsh
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
4
Illustration of Input Timing Regimes
Reference clock
High-frequency clock
input clock
16
16
16:1 MUX
16
input data
Input clock
timing domain
Connection could
exhibit varying delay
EECS 270C / Spring 2014
Reference clock
timing domain
Variable phasing between
input & reference clock domains
can cause bit errors in MUX
Prof. M. Green / U.C. Irvine
5
First-In/First-Out (FIFO) Circuit (1)
16:1 MUX
We require an intermediate
block to resolve timing variations
between input & reference clock
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
6
First-In/First-Out Circuit (2)
Read clock based on input clock
Write clock based on reference clock
Read
clock
Write
clock
k
Din_0
Dout_0
Ref
clock
Synchronized
with input clock
Read
clock
Din_n
Write
clock
To serializer
(signals synchronized
with reference clock)
k
Dout_n
Ref
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.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
7
FIFO approach:
• Large amount of hardware (many latches)
• Significant power dissipation unless static CMOS is used
• Can handle arbitrarily large delay variations
Appropriate phase chosen
DLL approach:
• Less hardware
• Can handle modest delay variations
• Better choice for BJT or GaAs processes
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
8
16:1 Multiplexer Tree Structure
5 Gb/s
10 Gb/s
2.5 Gb/s
1.25 Gb/s
1.25 GHz
2.5 GHz
static
CMOS
EECS 270C / Spring 2014
5 GHz
CML
Prof. M. Green / U.C. Irvine
9
2:1 MUX cell details
D flip-flop with extra latch
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
10
1
f
4
10ISS
1
f
2
2ISS
1
f
2
5ISS
f
ISS
Total current: 18ISS
Assume all blocks have:
• Tail current ISS
• Resistor R
• Diff pair transistor sizes W/L
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
11
We can take advantage of gain/bandwidth tradeoff by appropriate scaling:
W
L
W
L
W
L
Design parameters:
• ISS
• R
• W/L  CL
W
L
1
f
2
Idea:
t = 2RCg
1
1
1
ISS ,2R, W, Cg
2
2
2
f
f
t = RCg
ISS ,R,W,Cg
ISS ,R,W,Cg
Lower bit rate allows lower power!
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
12
1
f
4
MSCALE=1/8
1
1
ISS ,8R, W
8
8
t = 4RCg
10
ISS
8
EECS 270C / Spring 2014
1
f
2
MSCALE=1/2
1
1
ISS ,2R, W
2
2
t = 2RCg
ISS
1
f
2
MSCALE=1/2
1
1
ISS ,2R, W
2
2
t = 2RCg
5
ISS
2
Prof. M. Green / U.C. Irvine
f
MSCALE=1
ISS ,R,W
Cp ≈ 10 fF
GSCALE=3
ISS = 1.2 mA
t = RCg
ISS
Itotal = 5.75ISS
= 6.9 mA
13
Clock Dividers
The operation of “real” high-speed clock dividers is more complex …
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
14
Clock divider based on CML D flip-flop:
æW ö
ç ÷
è L øD
æW ö
ç ÷
è L øD
æW ö
ç ÷
è L øC
æW ö
ç ÷
è L øL
æW ö
ç ÷
è L øL
æW ö
ç ÷
è L øD
æW ö
ç ÷
è L øC
æW ö
ç ÷
è L øD
æW ö
ç ÷
è L øL
æW ö
ç ÷
è L øC
æW ö
ç ÷
è L øL
æW ö
ç ÷
è L øC
Divider sensitivity curve:
Vmin = minimum input clock amplitude required for
correct operation.
fso = self-oscillation frequency
Vmax
EECS 270C / Spring 2014
Vmax = maximum dc differential voltage that can be
applied to the input clock for which the circuit
self-oscillates.
Prof. M. Green / U.C. Irvine
15
Sensitivity Curve Analysis
Region I: Desired frequency divider operation
Region II: Quasiperiodic operation
Region III: Slew-rate limited operation
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
16
Region II: Quasiperiodic behavior
self-oscillating
fin = 11GHz
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
locked
17
Region III: Slew-rate limited Behavior
Sine-wave input
Square-wave input
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
18
Effect of Transistor Sizes on Sensitivity Curve
Driver transistors
Latch transistors
Clock transistors
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
19
Alternatives to DFF-Based Clock Dividers
• Latches present large capacitive load  slow
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
20
At very high frequencies, latch transistors are
not necessary and only add capacitance to
the circuit:
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
21
Ring-Oscillator-Based Divider
Behaves like a 4-stage ring oscillator with injection of full-rate frequency.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
22
Comparison of Sensitivity Curves
Conventional divider: Wider frequency range; lower self-oscillation frequency
Dynamic divider: Narrow frequency range; higher self-oscillation frequency
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
23
Effect of Non-Ideal Clock Signals
I2(t)
ISS
0
VDD
I1
I1(t)
Vout-(t)
I2
VDD – ISSR
Vout+(t)
VDD – ISS(R+DR)
ISSR
0
ISS(R+DR)
Vout+(t)
– Vout-(t)
Offset resistance causes deviation from
50% duty cycle in clock signal.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
24
Result of nonideal half-rate clock is Periodic Jitter.
with offset
Half-rate clock
ideal
MUX output
EECS 270C / Spring 2014
with offset
ideal
Prof. M. Green / U.C. Irvine
25
Retimer eliminates this problem:
10 Gb/s data
10 Gb/s retimed data
5 Gb/s
retimer
5 GHz
10 GHz clock
retimed output
Full-rate clock
(could be non-50% duty cycle)
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
26
Internal MUX Timing
5 Gb/s
10 Gb/s data output
tp2
2.5 Gb/s data input
2.5 MHz
5 GHz
10 GHz clock
tp1
tp1 & tp2 are “clock-to-Q” delays.
EECS 270C / Spring 2014
Because the clock & data flow in opposite directions,
alignment between 5 Gb/s data & 5 GHz clock
is determined by the sum: tp1 + tp2
(High sensitivity to processing / temp. corners)
Prof. M. Green / U.C. Irvine
27
Serial Output 50 Line Driver
• 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.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
28
Receiver Block Diagram
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
29
DMUX Architecture
1:2
2:4
4:8
8:16
A
B
D
1:2
A
B
D
1:2
A
B
D
1:2
A
B
D
10Gb/s Data
1:2
A
B
D
1:2
A
B
D
1:2
A
B
D
622Mb/s
1:2
10GHz CLK
/2
/2
/2
/2
/2
311MHz
622MHz
CML
EECS 270C / Spring 2014
Static CMOS
Prof. M. Green / U.C. Irvine
30
1:4 DMUX Tree Structure
5 Gb/s
10 Gb/s data input
2.5 Gb/s data outputs
10 GHz clock
EECS 270C / Spring 2014
5 GHz
2.5 GHz
Prof. M. Green / U.C. Irvine
31
1:2 DMUX cell details:
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
32
Internal DMUX Timing
5 Gb/s
10 Gb/s data input
2.5 Gb/s data output
tp2
10 GHz clock
5 GHz
2.5 GHz
tp1
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 clock
is determined by the difference: tp1 – tp2
(Low sensitivity to processing/temp. corners)
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
33
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.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
34
Crosstalk Measurement
CMU reference clock
fref
(
10 GHz
=
× 1+10-4
16
)
Serial input data
10 Gb/s
Low-frequency
inputs/outputs
Low-frequency
inputs/outputs
output clock
recovered clock
10 GHz + 100ppm
10 GHz
output data
10 Gb/s + 100ppm
Jitter is measured at TX output clock (or data) and RX recovered clock.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
35
Techniques for Reducing Transceiver Crosstalk
• 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
Very difficult to simulate & predict!
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
36
SONET Jitter Specifications
1. Jitter Generation (transmitters)
2. Jitter Tolerance (receivers)
3. Jitter Transfer (repeaters)
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
37
Jitter Generation (1)
Wideband jitter (p-p or rms) can be measured
directly from serial output data signal
• 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.
Gigabit Ethernet & Fiber Channel:
Equivalent JPP determined by measured BER.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
38
Jitter Generation (2)
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:
Ref. clock
TX
CDR
output data
(low jitter generation)
recovered clock
Should have
jitter bandwidth > 80MHz
10 GHz
SONET OC-192
bandpass filter
to jitter analyzer
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
39
Jitter Generation (3)
B.
Measure the TX output clock directly
(assuming its jitter is the same as the data):
Ref. clock
TX
output data
TX output clock
10 GHz
to jitter analyzer
Note: ISI is usually measured separately (peak-to-peak only).
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
40
Jitter Generation (4)
Measured at output clock;
231-1 PRBS serial data applied to input
9.95328 GHz
10.6642 GHz
Phase noise:
-100 dBc/Hz @ 1MHz offset
Jitter generation (SONET filter):
5.6mUI rms / 60mUI p-p
EECS 270C / Spring 2014
Phase noise:
-100 dBc/Hz @ 1MHz offset
Jitter generation (SONET filter):
6.2mUI rms / 65mUI p-p
Prof. M. Green / U.C. Irvine
41
Jitter Generation (5)
Jitter measurements from clock:
Jitter Generation (231-1 PRBS):
6.44 ps pp (wide band)
0.38ps rms (within SONET band)
EECS 270C / Spring 2014
Closed-loop VCO phase noise (231-1 PRBS):
–107 dBc/Hz @ 1 MHz offset
Prof. M. Green / U.C. Irvine
42
Jitter Generation (6)
231-1 PRBS input data applied:
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
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
43
Jitter Tolerance (1)
Experiment: Apply serial data to CDR with jitter at a certain frequency.
Increase the jitter amplitude until a bit error occurs.
retimed data out
Serial data in
To DMUX
retimer
recovered clock
If data jitter & recovered clock jitter
could perfectly track, then retiming
would be error-free.
T
Recovered clock
tsh
Data in
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
44
Jitter Tolerance (2)
Given CDR open-loop characteristic
( )
G jw = K pd
^
K
×F jw × vco
jw
( )
fdata
fclock
G
=
fdata 1+G
fclock
fdata - fclock
1
=
fdata
1+G
fdata(max) (w) = 1+G( jw) × fdata - fclock
max
= 1+G( jw) ×2p
æ t ö
JTOL(w) = 1+G( jw) × ç1- sh ÷
è T ø
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
T - tsh
T
(expressed in UI)
45
Jitter Tolerance (3)
10000
rate: 10.7Gb/s
Bit rate:Bit
10.7
Gb/s
Pattern: 231-1PRBS
31
Pattern:
BER
2 threshold:
-1PRBS
10-12
Data in: 50 mV pp
BER threshold: 10-12
Jitter Tolerance [UIpp]
1000
100
10
1
0.1
0.01
10
100
1K
10K
100K
1M
10M
100M
Jitter
Frequency (Hz) [Hz]
Jitter
Frequency
Jitter Tolerance > 40 ps pp at high frequency
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
46
Jitter Transfer
repeater
OE
[
RX
TX
HRX ( jw )
HTX ( jw)
]
n repeaters: HRX ( jw) ×HRX ( jw)
EO
n
Jitter peaking should be minimized.
Jitter Transfer Mask:
0.1dB
-20 dB/decade
f0
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
47
Electrical-to-Optical Interfaces (1)
Electrical to optical (TX):
IL
MUX
laser
driver
optical
output
power
T
laser diode or
Vertical Cavity Surface Emitting Laser
(VCSEL)
Ith ~ 10mA IL
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
48
Electrical-to-Optical Interfaces (2)
Electroabsorption modulator
Pout
Pin
Pin
VM
Pout
Vswing~ 3V
VM
Operates by making optical material
more or less absorptive.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
49
49
Electrical-to-Optical Interfaces (3)
Mach-Zender modulator:
Mach-Zender interferometer:
Pout
Pin
• Invented in 1890s
• Used to precisely measure optical
phase shift of materials.
• By using constructive/destructive
interference, can be used as a laser
modulator.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
Vswing ~ 6V
VM
50
50
Electrical-to-Optical Interfaces (4)
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)
Results in DCD
Optical output
Additional circuitry to correct pulsewidth is often added to system...
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
51
51
Electrical-to-Optical Interfaces (5)
Optical output control circuit:
laser diode
VM
monitor diode
IB
Vref
R
Feedback sets IB =
EECS 270C / Spring 2014
Vref
R
Prof. M. Green / U.C. Irvine
52
52
Optical Receiver Block Diagram
OE
TIA
≈ -18 dBm
≈ 10 µA
EECS 270C / Spring 2014
LA
≈ 10 mV p-p
EQ
CDR
DMUX
≈ 400 mV p-p
Prof. M. Green / U.C. Irvine
53
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Optical-to-Electrical Interfaces (1)
p-i-n photodetector structure:
resulting
electrical
current
circuit model:
+
n
+
i
VR~5V
_
CD
ID = r ×Popt
p
applied
optical
signal
_
r = 0.6 ~ 0.9 A W
CD ~ 400 fF
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
54
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Optical-to-Electrical Interfaces (2)
• DCD & ISI are evident.
• Noise is higher at logic 1 than at logic 0.
Photodetector noise:
Eye diagram of PRBS resulting
from 96 km of single-mode fiber
and photodetector.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
55
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Transimpedance Amplifier (TIA)
Used to convert photodetector current into voltage.
R
A0
Iin
Vout
Cd
Vref
from photodetector
low-impedance node maintains
nearly constant detector voltage
 good linearity.
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
56
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Transimpedance Amplifier (2)
R
Transimpedance: ZT º
Cg
Iin
A0
Cd
Vout
Input impedance: Zin º
Vout
1
= -Rf ×
Iin
1+ 1
A0
VRf
=
Iin 1+ A0
Vref
photodetector
Loop gain: A(s) ×f (s) =
A0
1
×
(1+ s p1) ×(1+ s p2 ) ××× 1+ sRf (Cd +Cg )
additional pole limits
closed-loop BW
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
57
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Transimpedance Amplifier (3)
Noise analysis:
i nR
R
v out
Good sensitivity requires:
• Large Rf
Tradeoff with BW
• Large Cg
• Large gm
v ni
2
2
v out
= v ni2 + i nR
× Rf2
2
= i eq
× Rf2
Þi
2
eq
v ni2
2
= 2 + i nR
Rf
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
58
58
Transimpedance Amplifier (4)
R
R
Cd
Cd
LB
Cg
Cg
• Cd decoupled from feedback network
• Common-gate device increases noise
EECS 270C / Spring 2014
• LB provides decoupling (series peaking);
could be realized by bondwire.
Prof. M. Green / U.C. Irvine
59
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
A(s)
A(s)
+
Vout
A(s)
−
−
n stages
Single stage:
A0
A(s) =
1+ s p
EECS 270C / Spring 2014
n-stage amplifier:
n
æ
ö
A
0
An (s) = ç
÷
è1+ s p ø
Prof. M. Green / U.C. Irvine
n
Overall gain: A0
Overall bandwidth: p × 21 n -1
60
60
7-Stage Limiting Amplifier Example (1)
Each stage uses shunt-peaked CML buffer with:
A0 = 5.5 dB
BW = 10 GHz
( )
A jw (dB)
7th stage output
1st stage output
100 MHz
EECS 270C / Spring 2014
1 GHz
Prof. M. Green / U.C. Irvine
10 GHz
100 GHz
61
61
7-Stage Limiting Amplifier Example (2)
7th stage output
7th stage output
6th stage output
6th stage output
1st stage output
Input amplitude = 20 mV p-p
EECS 270C / Spring 2014
1st stage output
Input amplitude = 40 mV p-p
Prof. M. Green / U.C. Irvine
62
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7-Stage Limiting Amplifier Example (3)
7th stage output
6th stage output
1st stage output
Input amplitude = 20 mV p-p
Input-referred offset of 5 mV applied
Vout » A0n ×VOS
Offset-cancellation circuitry required!
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
63
63
Limiting Amplifier Offset Compensation
RL
n-stage
amplifier core
RL
+
V
− 1
+
V
− out
VOS
M1
+
vin
−
M1
M1
M1
+
Vout
−
RF
RF
CF
offset compensation
compensation circuit:
[(
)
]
CF
lowpass filter
(
V1 = gm1R × vin + VOS -Vout Þ V1 = gm1R × VOS -Vout
v1 = gm1R × vin
)
Þ Vout
amplifier circuit:
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.
gm1RA0n
=
×VOS » VOS
1+ gm1RA0n
vout = gm1RA0n × vin
Vout = A0n ×V1 Þ Vout = A0n ×V1
vout = A0n × v1
EECS 270C / Spring 2014
Prof. M. Green / U.C. Irvine
64
64