Delivering OCXO-grade PTP Performance with MEMS Precision …

Delivering OCXO-grade PTP

Performance with MEMS Precision TCXO

Nazariy Tshchynskyy

WSTS 2017

2

Introduction

• Ethernet is becoming increasingly popular method for transferring data in a

mobile backhaul, but it’s asynchronous in nature

• LTE BS (eNodeB) requires 1.5 us time/phase synchronization

• PTP can be used to achieve sub-microsecond time synchronization over a

packet network, like Ethernet

• High quality local oscillator is required for best PTP performance

• OCXO’s are considered higher performance than TCXO’s because they are

less sensitive to airflow and offer tighter frequency stability

• Do you need a frequency stability of OCXO if you can get the same

level of ΔF/ΔT performance with precision MEMS TCXO?

3

Factors affecting PTP Accuracy

• Local oscillator quality

• Sensitivity to external conditions (for example, slope of frequency over

temperature, sensitivity to VDD change)

• Wander

• Control loop design

• Control loop bandwidth/transfer function

• Packet delay variation

• Network load

• Network architecture

• Use of network devices with PTP support (Transparent Clocks, Boundary Clocks)

• Time stamping accuracy

• Typically hardware based time stamping used to avoid software delays

• Hardware timestamping resolution is usually in nanosecond range

4

Servo Loop BW is a Tradeoff between

PDV filtering and Oscillator Noise

PDV Servo Loop is LPF for PDV

Filtered time

+tau

ADEV

Dominated by Temperature

sensitivity

Servo Loop is HPF for OSC

Time_error (s)

time (s)

• Time Error is a combination of Network Performance and Oscillator Noise

• Tradeoff between PDV filtering and Oscillator Noise is defined by Servo

Loop Bandwidth

5

Oscillator performance determines the

limits of PDV filtering capability

• Lower servo loop bandwidth → better PDV filtering

• Requires oscillator with good ADEV at long tau (dominated by temperature effects)

• ΔF/ΔT of an oscillator (slope) is a Temperature to ADEV conversion factor

• For best PTP performance ΔF/ΔT of an oscillator should be minimized

Oscillator characteristics

determine the limits of

PDV filtering capability

Note: servo loop algorithm may use higher bandwidth in unlocked state to ensure fast lock time and reduce the bandwidth

once locked to improve filtering performance

Packet selection and time stamping

Low pass filter (eliminates rapid

transients)

Servo Loop(Low BW to filter

PDV jitter)

TCXO

Tuning

CLK

GMII

6

Properties of the Oscillator that affect PTP

Performance

• Sensitivity to temperature changes (defined as Frequency Slope)

• Dominating contributor to Time Error

• Ambient temperature variations translate to oscillator output frequency change

• Short term aging (1-day aging)

• Has little impact on PTP performance if 1 ppb/day or better

• Native oscillator wander

• In good quality TCXO’s is small enough and doesn’t impact µs-level Time Error

performance

• Important for achieving <100 ns Timer Error performance level

7

Simulation methodology of Local

Oscillator impact on PTP performance

Time_error (s)

time (s)

Time error (t)

1.5us

-1.5us

PN (dBc/Hz)

f offset (Hz)

Phase noise and ADEV

Phase Noise → Time Jitter

+

f (ppb)

time (s)

TCXO frequency in time

∫0

tΔf(t) Δφ(t) /(2πfc)

PLL responsemodel

f

H(f)

f (ppb)

T (°C)

f (ppb)

time (s)

1 ppb/day

f (ppb)

time (s)

Frequency vs Temperature 1 day aging

Temperature ProfileT (°C)

time (s)

T1 T2

T1

T2

Frequency change in time (due to temperature)

Dwell time = 15 minRamp rate = 0.5 °C/min

+

PLL responsemodel

f

H(f)

8

PTP Performance with 1 ppb/°C and

10 ppb/°C TCXO (Time Constant 10 min)

9

PTP Performance with 1 ppb/°C and

10 ppb/°C TCXO (Time Constant 1 min)

10

Frequency Slope over Temperature

• Frequency Slope over Temperature is a measure of frequency change due

to temperature change by 1°C and is typically expressed in ppb/°C

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

ΔT

ΔF

Slope = ΔF/ΔT [ppb/°C]

11

Which part is better? 50 ppb or 100 ppb?

-100

-50

0

50

100

-50 -10 30 70 110

Fre

qu

en

cy S

tab

ility

(p

pb

)

Temperature (C)

± 50 ppb

± 100 ppb

12

MEMS Precision TCXO delivers OCXO-

level Frequency Slope Performance

1 ppb

- 1 ppb Better than 1ppb/°C

OCXO-level performance

13

Time Error Measurement Setup

Master

Oregano Systems syn1588® VIP

Ethernet Slave

Oregano Systems syn1588® VIP

Slave

Oregano Systems syn1588® VIP

Ethernet

Ethernet

Unmanaged Switch

Frequency CounterAgilent 53230A

Frequency CounterAgilent 53230A

1pps output

CH 1 CH 1

CH 2 CH 2

- -CH 1 – CH2 CH 1 – CH2

1pps output

1pps output

OCXO

MEMS TCXO

Quartz TCXO or OCXO

14

Time Error Measurement Data

(Temperature Transient)

Hot air 50°C applied to

oscillators

15

Short Term Holdover is a Reflection of the

Oscillator Performance

• Short term holdover may range from few seconds to few hours

• During holdover servo loop freezes the TCXO tuning at the last

known good value

• Holdover performance is a reflection of the oscillator characteristics

• Possible causes may include

• Master change – few seconds to few minutes

• Equipment failure or reconfiguration – up to few hours

• During the holdover clock should maintain Time Error within the

specified limits while running of the local oscillator

16

Short term Holdover Simulation

f (ppb)

T (°C)

f (ppb)

time (s)

1 ppb/day

f (ppb)

time (s)

Frequency vs Temperature 1 day aging

Temperature ProfileT (°C)

time (s)

T1 T2

T1

T2

Frequency change in time (due to temperature)

Dwell time = 15 minRamp rate = 0.5 °C/min

f (ppb)

time (s)

+

TCXO frequency in time

∫0

tΔf(t) Δφ(t) /(2πfc)

PN (dBc/Hz)

f offset (Hz)

Phase noise

Phase Noise → Phase Jitter

+

Time_error (s)

time (s)

Time error (t)

1.5us

tholdover

17

Short term Holdover Simulation

18

Short term Holdover Measurement

Slave lost network

connection

19

Conclusions

• PTP devices require high quality oscillators to achieve good accuracy

• Better stability oscillators allow tuning servo loops for better PDV filtering

• Frequency Slope impacts PTP performance not Frequency Stability

over full operating temperature range

• TCXO’s with the same Frequency stability spec may have significantly

different Slope over Temperature

• MEMS-based precision TCXO’s have been designed to minimize

Frequency Slope over Temperature (5x to 20x improvement comparing to

Quartz TCXO’s) and can be used to replace OCXO’s in PTP applications

• SFP modules is an example of an application where MEMS-based precision

TCXO’s provide OCXO-level performance while saving critical space and

power

20

Thank You!

Questions?