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M bius
Microsystems
A 9.2mW 528/66/50MHz Monolithic Clock
Synthesizer for Mobile µP Platforms
Custom Integrated Circuits Conference (CICC) 2005
Michael S. McCorquodale, Ph.D.
Mobius Microsystems, Inc.
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Outline
 Introduction
 Background
 Clock synthesizer reference oscillator and architecture
 Experimental results
 Conclusions and future work
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Introduction
Slide 3 of 21
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Microsystems
Introduction
Much recent work exploring alternative technologies to
XTALs for clock generation and frequency synthesis
 MEMS microresonators
 FBAR
Insufficient exploration of all-Si CMOS approaches
Build on recent work in free-running and open-loop
compensation of LC oscillators as frequency references for
clock generation
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Introduction
Goals
 Develop an accurate and stable clock synthesizer without an external
frequency reference (i.e. XTAL or ceramic resonator)
 Develop a clock synthesizer with very low frequency scaling latency
 Develop a clock synthesizer with very low start-up latency
 Characterize performance over PVT
 Demonstrate in a multi-chip module
Approach
 Explore free-running RF LC oscillators as frequency references
 Utilize a “top-down” synthesis architecture
Slide 5 of 21
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Background
Slide 6 of 21
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Architecture
Reference oscillator
 Free-running high-Q LC oscillator at a high frequency
 Simple frequency trimming interface
 Open loop compensation to stabilize over PVT
 Very low phase noise
 Very low start-up latency
Clock synthesis
 Divide down to target clock frequencies
 Decrease phase noise by 20log10(N) for divide by N
Slide 7 of 21
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Background
Resonant frequency
+
v
_
+
_
Ro
 o
RC
ic
i
-gm
_+
RL
Ro
L
1  CRL2  L 
 2
 
LC  CRC  L 
1  CRL2 
1 

LC 
L 
Sources of frequency drift
C
 Real losses: RL and RC
ic(t)
i(t)
gm0
t
t
 Frequency modulation from
harmonic content of driving
amplifier
 Filter response of LC network
and amplifier output resistance
Slide 8 of 21
Mobius Microsystems
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Background
fo
fmax
No
oscillation
fmin
gmo
gm
fo vs. gm relationship
 gmo → minimum gm for start-up
 fo → decreases as gm increases (harmonic content increases)
 fmin → approached as harmonic content approaches square wave
Can utilize harmonic modulation to self-compensate drift
by modulating gm through bias current
Slide 9 of 21
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Clock Synthesizer Reference Oscillator
and Architecture
Slide 10 of 21
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Reference Oscillator
R
 Complementary cross-coupled
architecture with PMOS tail for
low phase noise
 Bias current, temperature
dependent and scaled by
~10x in mirror
 Resistor divider self-biases
control voltage and reduces
VDD sensitivity
MRp
250
0.5
2.5m
0.5
R
430
0.53
MRn
430
0.53
6.1nH
50kW
+out
outvcal
300mA
0.8-2.5pF
 vcal trims frequency
 Reset transistors disable
oscillator
Slide 11 of 21
50kW
0.8-2.5pF
3pF
215
0.53
215
0.53
MRn
Mobius Microsystems
R
M bius
Microsystems
Architecture
1.056GHz
528MHz
BUF
R
÷2
÷10
50MHz
vcal
1
Out
0
S
÷8
66MHz
EN0
Out
EN1
 “Top-down” or divisive architecture reduces phase noise and
period jitter of reference oscillator by 20log10(N) and sqrt(N)
 RF reference oscillator can be started with low latency
 Any available frequency can be selected asynchronously: low
scaling latency
Slide 12 of 21
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Experimental Results
Slide 13 of 21
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Die Micrograph
 Fabricated in IBM’s 0.18mm
7RF-CMOS process
 Core macro size: <0.4mm2
 Test macros populate periphery
 Output drivers drive 10pF with
100ps rise/fall times at 20mArms
 Wire-bonded and characterized
in 16-pin ceramic DIP
 Au studs for flip-chip module
assembly
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Temperature and
Voltage Drift
Microsystems
VDD±10%
 25°C: ±0.17%
0.8
1.5
 100°C: ±0.33%
0.75
0.5
0
0.7
VDD = 1.98V
VDD = 1.80V
-0.5
VDD = 1.62V
0.65
-1
0.6
-1.5
vcal Required to keep fo Constant (V)
Normalized Frequency Drift, f/fo (%)
1
Temperature
 0 – 70°C: ±0.75%
 -40 – 100°C: ±1.5%
PVT Total
 Best: <±1%
 Worst: ~±1.5%
Temp. compensation
-2
-40
-20
0
40
20
Temperature (C)
60
80
0.55
100
 Under-compensated
 1.6mV/°C, R2 = 0.9984
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Start-up Latency
 Measured 3.2ms
start-up latency from
leakage only power
state
3.2ms
 Latency originates
primarily from bias
start-up time
 Bias circuitry can be
modified to reduce
latency to ~ns
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Period Jitter
 Measured with
Agilent Infinium
4GSa/s scope
 250k samples per
edge
 66MHz clock
measurement shown
 RMS jitter
determined by
removing trigger jitter
 J  40.32  34.42  21 ps
rms
Slide 17 of 21
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Performance
Summary
Parameter
Microsystems
Measured
Unit
Power supply voltage (nom./min.)
1.8/1.12
V
Power supply current (VDD = 1.8V/1.12V)
5.1/3.5
mA
Standby power supply current (VDD = 1.8V)
300
nA
Power dissipation (VDD = 1.8V)
9.2
mW
49.5 – 56.2
Output frequencies
61.9 – 70.2
MHz
495.2 – 561.6
Frequency calibration (tuning) range
±6.2
%
RMS period jitter (528/66/50 MHz output)
7.4/21/33
ps
Temperature frequency drift (-40 to 100°C)
±1.5
%
Power supply frequency drift (VDD ±10%)
±0.33
%
Total freq. accuracy (process, voltage, temp.)
±1.8
%
Start-up latency
3.2
ms
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Conclusions and Future Work
Slide 19 of 21
Mobius Microsystems
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Conclusions and
Future Work
Microsystems
Demonstrated a self-referenced LC clock
synthesizer with no external reference
 Low jitter and scaling/start-up latency
 Low overall drift, though drift under-compensated
 Temperature compensation correction linear
Alternative compensation techniques already in Si
 Very high total accuracy over PVT to be reported soon
 Potentially an all-Si approach to stable and accurate clock
synthesis
 Never underestimate what can be done with CMOS alone
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Questions welcome
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