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

Dual MEMS Resonator Yields Highest

Figure of Merit

10

Res. FOM = (Energy/Conversion)×Resolution2

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

MEMS-Based TCXO Phase Noise

12

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