6.976 high speed communication circuits and …. perrott mit ocw future goals low cost, low power,...
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6.976High Speed Communication Circuits and Systems
Lecture 1Overview of Course
Michael PerrottMassachusetts Institute of Technology
Copyright © 2003 by Michael H. Perrott
M.H. Perrott MIT OCW
Wireless Systems
Direct conversion architecture
sin(wot)
90o
D/A
D/A
DigitalProcessing
Block
DigitalProcessing
Block
sin(wot)
90o
A/D
A/D
Transmit IC Receive IC
LNA
PowerAmp
Transmitter issues- Meeting the spectral mask (LO phase noise & feedthrough,
quadrature accuracy), D/A accuracy, power amp linearityReceiver Issues- Meeting SNR (Noise figure, blocking performance, channel
selectivity, LO phase noise, A/D nonlinearity and noise), selectivity (filtering), and emission requirements
M.H. Perrott MIT OCW
Future Goals
Low cost, low power, and small area solutions- New architectures and circuits!
Increased spectral efficiency- Example: GSM cellphones (GMSK) to 8-PSK (Edge)
Requires a linear power amplifier!Increased data rates- Example: 802.11b (11 Mb/s) to 802.11a (> 50 Mb/s)
GFSK modulation changes to OFDM modulationHigher carrier frequencies- 802.11b (2.5 GHz) to 802.11a (5 GHz) to ? (60 GHz)
New modulation formats- GMSK, CDMA, OFDM, pulse position modulation
New application areas
M.H. Perrott MIT OCW
High Speed Data Links
A common architecture
DEMUXDigital
ProcessingBlock
Receive IC
AmpClock
and DataRecovery
ClockDistribution
10 Gb/sData Link
MUX DriverDigitalProcessing
Block
Transmit IC
Clock Generation
Transmitter Issues- Intersymbol interference (limited bandwidth of IC
amplifiers, packaging), clock jitter, power, areaReceiver Issue- Intersymbol interference (same as above), jitter from
clock and data recovery, power, area
M.H. Perrott MIT OCW
Future Goals
Low cost, low power, small area solutions- New architectures and circuits!
Increased data rates- 40 Gb/s for optical (moving to 120 Gb/s!)
Electronics is a limitation (optical issues getting significant)- > 5 Gb/s for backplane applications
The channel (i.e., the PC board trace) is the limitationHigh frequency compensation/equalization- Higher data rates, lower bit error rates (BER), improved
robustness in the face of varying conditions- How do you do this at GHz speeds?
Multi-level modulation- Better spectral efficiency (more bits in given bandwidth)
M.H. Perrott MIT OCW
This Class
Circuit AND system focus- Knowing circuit design is not enough- Knowing system theory is not enough
Circuit stuff- RF issues: transmission lines and impedance transformers- High speed design techniques- Basic building blocks: amplifiers, mixers, VCO’s, digital
components- Nonidealities: noise and nonlinearity
System stuff- Macromodeling and simulation- Wireless and high speed data link principles- System level blocks: PLL’s, CDR’s, transceivers
M.H. Perrott MIT OCW
The Goal – Design at Circuit/System Level
1. Design architecture with analytical modelsMay require new circuits – guess what they look like
2. Verify architectural ideas by simulating with ideal macro-models of circuit blocks
Guess macro-models for new circuits 3. Add known non-idealities of circuit blocks
(nonlinearity, noise, offsets, etc.) Go back to 1. if the architecture breaks!
4. Design circuit blocks and get better macro-modelsGo back to 1. if you can’t build the circuit!Go back to 1. if the architecture breaks!
5. Verify as much of system as possible with SPICE6. Layout, extract, verify
Do this soon for high speed systems - iteration likely!
M.H. Perrott MIT OCW
Key System Level Simulation Needs
You need a fast simulator- To design new things well, you must be able to iterate- The faster the simulation, the faster you can iterate
You need to be able to add non-idealities in a controlled manner- Fundamental issues with architectures need to be
separated from implementation issuesAn architecture that is fundamentally flawed should be quickly abandoned
You need flexibility- Capable of implementing circuit blocks such as filters,
VCO’s, etc.- Capable of implementing algorithms- Arbitrary level of detail
M.H. Perrott MIT OCW
A Custom C++ Simulator Will Be Used - CppSim
Blocks are implemented with C/C++ code- High computation speed- Complex block descriptions
Users enter designs in graphical form using Cadence schematic capture- System analysis and transistor level analysis in the
same CAD frameworkResulting signals are viewed in Matlab- Powerful post-processing and viewing capability
Note: Hspice used for circuit level simulations
CppSim is on Athena and freely downloadable athttp://www-mtl.mit.edu/~perrott
M.H. Perrott MIT OCW
HW1 – Transmission Lines and Transformers
High speed data link application:
VoutC1 RL
L1Delay = xCharacteristic Impedance = Ro
Ideal Transmission Line
Ro
Vin
Two-Port Model
C2
Ei1
Er1
Ei2
Er2
dieAdjoining pinsConnector
Controlled ImpedancePCB trace
package
On-ChipDrivingSource
High Speed Trace (RF Connector to Chip Die)
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HW2 – High Speed Amplifiers
M4
M1 M2
M3
IbiasVin+
R1
Vin-
R2
Vo+Vo-
50 Ω
Vin
M1
M2
Ls
Ld
Lg
Cbig
Ibias = 1mA
M3
Vout
CL=1pF5 kΩ
Zin
x
Broadband
Narrowband
M.H. Perrott MIT OCW
HW3 – Amplifier Noise and Nonlinearity
Amplifier circuit
ModelM1
Ibias
Vout
100.18
20.18
M2
RL
Vin
Cbig2
Cbig1
RT
50 Ω
Vin
50 Ω
50 Ω
Nonlinearity
Vout
Vout = co + c1x + c2x2 + c3x3
Noise
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HW4 – Low Noise Amplifiers and Mixers
Narrowband LNA
Passive Mixer
Vin
CL
RL/2 RL/2
RS/2 Cbig
RS/2 Cbig
LO
LO LO
LO
Vout Vout
0 V
0
Vdd
0
Vdd
50 Ω
Vin
M1
M2
Ls
Lg
Cbig
Ibias = 1mA
M3
Vout
CL=1pF
5 kΩ
Zin
Rps
RpgRps
Cbig
Ld
Rpd
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HW5 – Voltage Controlled Oscillators
Differential CMOS
Colpitts
Vbias=1.2V
M1
Ld=4nH
Vout
C1=2pF
Ibias=100 µAC2=8pF
Rd=10kΩ
M1M2
M3
Vout
Ctune
3 nH
100/0.18
50/0.18
M4100/0.18
50/0.18
VoutVin
1.8 V
0 V
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Project 1 - High Speed Frequency Dividers
High speed latches/registers
High speed dual-modulus divider
Load
Load
ININ
OUT OUT
Φ Φ
22/3Core
ControlQualifier
CON
IN OUT2A B
2/3
INAB
OUTCON*
8 + CON Cycles
CON*
CON
M.H. Perrott MIT OCW
HW6 – Phase Locked Loop Design
Integer-N synthesizer
Phase noise simulation
PFD LoopFilter
ref(t) out(t)
Divider
T
T
e(t) vin(t)
div(t)
VCO
N[k] = Nnom
Icp
out(t) = cos(2π (fo+Kvvin(τ))dτ)
vin out
t
s1 + s/(2πfp)
vph2
SΦout(f)
foffset0
-20 dBc/Hz/dec
vspur = Asin(2πfst)
K2 dBc
K1 dBc/Hz
fpfs
M.H. Perrott MIT OCW
Project 2 – GMSK Transmitter for Wireless Apps
Kv = 30 MHz/Vfo = 900 MHz
GaussianLPF
DataGenerator
Digital I/Q Generation
out(t)
T
T
t
Td
t
T
Loop FilterReferenceFrequency
vin(t)PFD
N
RF TransmitSpectrum
0ffRF
Trans.Noise
PowerAmp
Kph
1 - z-1
cos(Φ)
sin(Φ)
D/A
D/A
Φfinst
90o
I
Q
Peak-to-PeakFrequencyDeviation
Td
t
Inst
anta
neou
sFr
eque
ncy
Data Eye
LimitAmp
(100 MHz)
= 11 MHz
IncludesZero-Order
Hold
Icp H(s)
M.H. Perrott MIT OCW
Project 2 – Accompanying Receiver
0
f
ffRF
fRF
ReceivedSpectrum
ReceiverNoise
f
BasebandSpectrum
cos(2πfRFt)
IR
QR
NR
ReceiverNoise
ModulationSignal
TransmitterNoise
S(IR+jQR)
Trans.Noise LNA
BandSelectFilter
ChannelSelectFilter
sin(2πfRFt)
M.H. Perrott MIT OCW
Example: A High Speed Backplane Data Link
Suppose we consider packaging issues at the receiver side (ignore transmitter packaging now for simplicity)
Vout
Delay = 110 psCharacteristic Impedance = 50 Ω
Ideal Transmission Line100 Ω
Vin
Two-Port Model
Ei1
Er1
Ei2
Er2
dieAdjoining pins
Controlled ImpedancePCB trace
package
On-ChipDrivingSource
55 Ω0.5 pF
M4
M1 M2
M3
Ibias Vin+ Vin-
Vo+
100 Ω100 Ω
ReceiverTransmitter
unintentionalmismatch
intentionalmismatch
0.5 pF
1 nH
M.H. Perrott MIT OCW
Modulation Format
Binary, Non-Return to Zero (NRZ), Pulse Amplitude Modulation (PAM)- Send either a zero or one in a given time interval Td- Time interval set by a low jitter clock- Ideal signal from transmitter:
0 0.5 1 1.5 2 2.5
x 10−8
−0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
in
TIME
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Receiver Function
Two operations- Recover clock and use it to sample data- Evaluate data to be 0 or 1 based on a slicer
SliceLevel
SamplingInstant
RecoveredClock
Detector
Data
ClockRecovery
Out
Data
Out0 1 1 111 10 0 0 0 0 0
RecoveredClock
M.H. Perrott MIT OCW
Issue: PC Board Trace is Not an Ideal Channel
Chip capacitance and inductance limits bandwidthTransmission line effects cause reflections in the presence of impedance mismatchExample: transmit at 1 Gb/s across link in previous slide (assume bondwire inductance is zero)- Signal at receiver termination resistor
0 0.5 1 1.5 2 2.5
x 10−8
−0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
out
TIME
M.H. Perrott MIT OCW
Eye Diagram for 1 Gb/s Data Rate
Wrap signal back onto itself every 2*Td seconds- Same as an oscilloscope would do
Allows immediate assessment of the quality of the signal at the receiver (look at eye opening)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 10−9
−0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Time (seconds)
out
Eye Diagram
M.H. Perrott MIT OCW
Relationship of Eye to Sampling Time and Slice Level
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 109
0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Time (seconds)
out
Eye Diagram
SliceLevel
SamplingInstant
Horizontal portion of eye indicates sensitivity to timing jitterVertical portion of eye indicates sensitivity to additional noise and ISI
M.H. Perrott MIT OCW
What Happens if We Increase the Data Rate?
Limited bandwidth and reflections cause intersymbolinterference (ISI)Eye diagram at 10 Gb/s for same data link
0 1 2
x 10−10
−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
out
Eye Diagram
M.H. Perrott MIT OCW
What is the Impact of the Bondwire Inductance?
Rule of thumb: 1 nH/mm for bondwire- Assume 1 nH
Impact of inductance here increases bandwidth- less ISI occurs
0 1 2
x 10−10
−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
out
Eye Diagram
M.H. Perrott MIT OCW
How High of a Data Rate Can The Channel Support?
Raise it to 25 Gb/s
However, we haven’t considered other issues- PC board trace attenuates severely at high frequencies
Bandwidth is < 5 GHz for 48 inch PC board trace (FR4)
0 2 4 6 8
x 10−11
−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
out
Eye Diagram
M.H. Perrott MIT OCW
Multi-Level Signaling
Increase spectral efficiency by sending more than one bit during a symbol interval- Example: 4-Level PAM at 12.5 Gb/s on same channel
Effective data rate: 25 Gb/s
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
x 10−10
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
Time (seconds)
out
Eye Diagram
M.H. Perrott MIT OCW
How Else Can We Reduce ISI?
Consider a system level view of the link- Channel can be viewed as having an equivalent
frequency responseAssumes linearity and time-invariance (accurate for most transmission line systems)
TransmitterDriver
ReceiverDetector
Channel
Transmitter Receiver
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Equalization
Undo channel frequency response with an inverse filter at the receiver- Removes ISI!- Can make it adaptive to learn channel
TransmitterDriver
ReceiverDetector
Channel
Transmitter Receiver
Equalization
M.H. Perrott MIT OCW
The Catch
Equalization enhances noise- Overall SNR may be reduced
Optimal approach is to make ISI and noise degradation about equal
TransmitterDriver
ReceiverDetector
Channel
Transmitter Receiver
EqualizationNoise
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Alternative – Pre-emphasize at Transmitter
Put inverse filter at transmitter instead of receiver- No enhancement of noise, but …- Need feedback from receiver to learn channel- Requires higher dynamic range/power from transmitter
TransmitterDriver
ReceiverDetector
Channel
Trransmitter Receiver
NoiseCompensation(Pre-emphasis)
M.H. Perrott MIT OCW
Best Overall Performance
Combine compensation and equalization- Starting to see this for high speed links
TransmitterDriver
ReceiverDetector
Channel
Trransmitter Receiver
EqualizationNoiseCompensation
(Pre-emphasis)
M.H. Perrott MIT OCW
What are the Issues with Wireless Systems?
Noise- Need to extract the radio signal with sufficient SNR
Selectivity (filtering, processing gain)- Need to remove interferers (which are often much larger!)
Nonlinearity- Degrades transmit spectral mask- Degrades selectivity for receiver
Multi-path (channel response)- Degrades signal – nulls rather than ISI usually the issue- Can actually be used to advantage!
We will look at BOTH broadband data links and wireless systems in this class