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Delivering Stable Timing Under Harsh Real-World
Environmental Conditions with High-Precision 100-ppb
TCXO
Sassan Tabatabaei
Nazar Tshchynskyy
Outline
• Introduction
• Precision MEMS-Based Temperature Compensated Oscillator
(TCXO) architecture
• Performance data
• Conclusions
2
1 10 50 100 250 1000
Frequency
stability (ppb)
Introduction
3
Rubidium
Double oven
OCXO
Single oven
OCXO
Precision
TCXO TCXO
MEMS-based
TCXO
70°C
or
85°C
105°C MEMS-based
TCXO
Applications
• Stratum-3: GR1244 compliant
• SyncE
• IEEE1588
• Small Cell / Femtocell
• Industrial GPS
4
MEMS-Based TCXO Target: Precision
Timing Under All Conditions
5
Stability
• Temperature range
• Temperature ramp
• Activity dips
• Freq. slope over temperature
• VDD / Load
Short Term Stability
• ADEV
• Wander (MTIE / TDEV)
• Still air and in airflow
Robustness
• Vibration
• Shock
• PSNR
Flexible Feature Set
• Frequency range
• Differential output
• Digital frequency control
MEMS-Based Precision TCXO
Architecture
6
OE Vctrl
OSC-
TF
Temp
Sense
(TDC)
TempFlat™
Resonator Frac-N PLL
Frequency Synthesis
VCXO
ADC IO
Temp
Comp
CLK
I2C/SPI
Dividers
& Driver
MEMS CMOS
• Fractional-N synthesizer for frequency control
• High bandwidth low noise temperature sensing for fast temperature
tracking without phase noise degradation
Sustaining Circuit
Vibrating Resonator Principle
drivebiasVV
gap
areaForce
20
gap
Vbias
gm
Oscillation
Sense
Drive
Drive voltage
causes the beam
to move
Beam movement causes
capacitance change
Suspended
silicon beam
7
Electrostatic MEMS is free of activity dips
Single-Anchor MEMS Resonators:
Robust Against Shock & Vibration
• Resonator mass is extremely small and stiff Large acceleration
needed to move mass
• MEMS resonator mass is 1000 to 3000 times smaller than quartz
Silicon MEMS Resonator Mass
Independent of Package and frequency
8
F r e q u e n c y
R a t
o
M e a s
u r e
m e n t
< 60 ppm over 200°C
~ 1400 ppm over 200°C
Digital Temp Out
TF
TS
f
f T
CMOS
Resonator and
temp sensor
thermal coupled
tightly
Dual MEMS Resonator Temperature
Sensing
9
High bandwidth
low noise
• < 30 μK resolution in 100 Hz bandwidth
• Temp sensor phase noise < oscillator phase noise Noise-less
temperature compensation
Fre
qu
en
cy R
ati
o
Me
as
ure
me
nt
TempFlat™
Resonator
Temp Sensing
Resonator
High-BW Temperature Compensation: Key to
Precision TCXO in thermally noisy environment
• MEMS-based TCXO Temperature tracking BW up to 350 Hz, 40x faster than best-in-
class TCXO
• Tracks fast temperature fluctuations (fans, heating due to chipset proximity)
• Noiseless Temperature compensation: No additional phase noise from temp comp
11
Temperature
sensor
Digital
Temp Out
CMOS
Frac-N PLL
Polynomial Compensation
f TF f OUT T
T
T
TempFlat™
Resonator
Voltage Control for MEMS-Based VC-
TCXO
• PLL-based frequency pull
• Highly linear characteristics across full Vctrl range: < 0.1%
• Highly consistent VCXO frequency control gain: < 10%
• f-vs-T degradation due to Vcntrl: ~ ±10ppb across full Vctrl range
• Low noise ADC for low phase noise performance
• Wide pull range selection: ±6.25 ppm to ±50 ppm
• Factory programmable Vctrl bandwidth: 5 kHz to 40 kHz
13
OSC-
TF
Temp
Sense
(TDC)
TempFlat™
Resonator Frac-N PLL
Frequency Synthesis
ADC IO
Temp
Comp
CLK
I2C/SPI
Dividers
& Driver
Vctrl
Temperature Stability: ±100 ppb up to
110ºC
14
Specification MEMS TCXO Quartz TCXO
Frequency Stability ±100 ppb ±100 ppb
Temperature range -40C to +105C -40C to +85C
Max temperature ramp rate 10C/min 1C/min
MEMS-Based TCXO Frequency Stability – Measured
Results
-100
-50
0
50
100
-50 -10 30 70 110
Fre
qu
en
cy S
tab
ility
(p
pb
)
Temperature (C)
20 ppb
100 ppb
-6
-4
-2
0
2
4
6
-50 -30 -10 10 30 50 70 90 110
Fre
qu
en
cy S
lop
e (
pp
b/
C)
Temperature (C)
Frequency Slope vs. Temperature for
IEEE 1588
15
2 ppb/C
1 ppb/C
MEMS-Based TCXO Freq. vs Temperature slope – Measured Results
ADEV in Airflow Conditions
16
MEMS-Based TCXO: 20 MHz, ±100 ppb, -40ºC to 85ºC
Quartz TCXO: 10 MHz, ±50 ppb, -5ºC to 70ºC
ADEV in Still Air, 25ºC ADEV with Oven Airflow, 25ºC
2x
2.5x
38x
MEMS TCXO maintains
ADEV under airflow
ADEV
8e-11 at 1000 s MEMS TCXO ADEV
3e-11 at 10 s
ITU-T G.8262-Compliant TDEV
Under Still Air and Airflow Conditions
17
Measured on SiLabs demo board Si5328-EVB
with 88 mHz PLL setting
20x
2x
Still Air, 25ºC Oven Airflow, 25ºC
MEMS-Based TCXO: 20 MHz, ±100 ppb, -40ºC to 85ºC
Quartz TCXO, 40 MHz
Compliance Mask
Improved margin
with MEMS TCXO
ITU-T G.8262-Compliant MTIE
Under Still Air and Airflow Conditions
18
7x
Still Air, 25ºC Oven Airflow, 25ºC
MEMS-Based TCXO: 20 MHz, ±100 ppb, -40ºC to 85ºC
Quartz TCXO, 40 MHz
Improved margin
with MEMS TCXO
Activity Dips / Micro Jumps
19
Temperature: Swept from -40ºC to 85ºC
-15
-10
-5
0
5
10
15
0 50 100 150 200 250
Fre
qu
en
cy s
tab
ility
(p
pb
)
Time (minutes)
No activity dip / micro-jumps
MEMS Resonators Deliver Vibration
Immunity
20
0.10
1.00
10.00
100.00
10 100 1000Vib
rati
on
Se
ns
itiv
ity
(pp
b/g
)
Vibration Frequency (Hz)
Performance in Presence of VibrationSiTime Epson Kyocera SiLabs
SiTime30x Better
MEMS Quartz 1 Quartz 2 Quartz 3
MEMS-Based VC-TCXO Freq-vs-Vctrl
Linearity
21
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
0 0.5 1 1.5 2 2.5 3 3.5
Freq
uen
cy (
pp
b)
Control Voltage (V)
VCMO pull characteristic Linear fit
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
0 0.5 1 1.5 2 2.5 3 3.5
Res
idu
al e
rro
r (%
)
Control Voltage (V)
-100
-80
-60
-40
-20
0
20
40
60
80
100
-40 -20 0 20 40 60 80
Fre
qu
en
cy s
tab
ility
(p
pb
)
Temperature (°C)
0.33 0.66 0.99 1.32 1.65 1.98 2.31 2.64 2.97Control Voltage (V):
MEMS-Based VC-TCXO: Vctrl Tuning
Skew
22
Extended Frequency Control Features
23
Specification MEMS-Based TCXO Quartz TCXO
Frequency Range
SE: 1 - 220 MHz
DE: 10 - 720 MHz
Programmable
SE: 10 - 52 MHz
DE: 10 – 200 MHz
Fixed frequencies
Outputs LVPECL / LVDS / HCSL /
LVCMOS / Clipped-Sine LVPECL
Temperature Readout Yes No
High Resolution Digital
Frequency control (DCXO)
< 0.1ppb resolution at
10kupdate/s NA
Conclusions
• Novel Dual MEMS resonator + PLL MEMS-based TCXO
architecture enables new class of precision oscillators
• ±100 ppb over -40°C to +110°C
• Enable applications in harsh environmental conditions
• Wide temperature support up to 105C
• High temperature ramp rate or airflow
• Shock and Vibration robustness
• No activity dips
24
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