Holdover Requirement: High Stability OCXO Challenges · 2019-10-23 · This presentation contains...
Transcript of Holdover Requirement: High Stability OCXO Challenges · 2019-10-23 · This presentation contains...
International Telecom Sync Forum (ITSF), November 2014
Holdover Requirement: High Stability OCXO Challenges
© 2014 Rakon. This presentation contains confidential information.
Presenter: Cyril Datin (R&D Engineer – Technical Leader)
International Telecom Sync Forum (ITSF), November 2014
OCXO Solutions for Holdover Requirements
© 2014 Rakon. This presentation contains confidential information.
Presenter: Cyril Datin (R&D Engineer – Technical Leader)
2
What is time and phase alignment?
What is Accuracy or stability?
Holdover mathematical definition
Holdover graphical approach
How to measure holdover?
Phase Noise (PN) over time (long term)
Short/mid/long term stability –mathematical tool
Agenda
Frequency instability contributor ADEV translation
Thermal compensation limitation
Ageing prediction limitation:
Ageing prediction limitation:
Summary
Conclusion: The next generation requirement?
Typical clock stability σy(τ)
ADEV interpretation medium / high stability comparison
OCXO fundamentals
OCXO fundamentals
Telecom system requirement:
High end digital compensation OCXO topology:
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-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0 2 4 6 8 10 12 14
mag
nit
ud
e
time
What is Time and Phase Alignment?
Frequency holdover: Ability to maintain a frequency alignment over a time period (Stratum 2, 3 3E requirement)
Time holdover: Ability to maintain a phase accuracy over time (Time division requirement)
Ideal reference
Signal same frequency AND
phase offset
t0
Signal same frequency without phase offset / time
aligned
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6
Accuracy or Stability ?
Accurate but…
ACCURATE AND STABLE
Stable but …
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9 3
6
12
9 3
6
12
9 3
6
12
9 3
6
12
9 3
6
12
9 3 time
time
time
Day 1H1m1
Day NH1m1
NOT STABLE
NOT ACCURATE
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The frequency drift is cumulating phase error x(t)This is also called time Holdover
Holdover Mathematical Definition
Fractional frequency fluctuation
Phase time
Ideal harmonic signal is represented
6 Holdover Graphical Approach
Phase error is cumulating gradually over time following frequency evolution
Linearif frequency constant (ageing free)
Quadraticif frequency linear (constant ageing)
Cubicif frequency quadratic (linear ageing)
Holdover (phase time)Is the area underneath the curves
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As an oscillator manufacturer we deal predominantly in the frequency domain, and convert to the time domain…
y(t) fractional frequency deviationx(t) phase time… => time Holdover
How to Measure Holdover?
OscillatorFrequency f0
τ0 counterintegration time
Each frequency sample has a τ0 duration
8 Phase Noise (PN) Over Time (Long Term)
Graphical approach – Phase Noise (PN) over time (long term)
f0 f0
f0 f0’
f0f0
’’
PN L(f)
poor PN
Caesiumclock
Rubidiumclock
OCXOclock
Excellent accuracy10-12
medium PNVery high accuracy
10-11
excellent PN Medium accuracy10-9
Holdover
Day 1 Day 10 Day 100
time
time
time
Absolute frequency slightly shifting
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Allan Deviation ADEV
τ : counter integration time (frequency sampling)
Most common time domain metric of frequency stability
ADEV remains convergent for most noise types
Another common metric is the modified Allan deviation mod σy(τ) which may be converted easily to TDEV
Short/mid/long term stability – mathematical tool
Short/Mid/Long Term Stability
y(t)M samples
yi yi+1
τ
time
10 Typical clock stability σy(τ)
1.00E-13
1.00E-12
1.00E-11
1.00E-10
10 100 1,000 10,000 100,000
AD
EV σ
y(τ)
τ [s]
Typical Clock Performance – Overlapping ADEV
Caesium clock
Rubidium clock
High stability OCXO
Medium stability OCXO
High stability
High accuracy
11 10MHz OCXO ADEV Interpretation
5.E-14
5.E-13
5.E-12
5.E-11
10 100 1,000 10,000 100,000
Ove
rlap
pin
gA
DEV
σy(
τ)
τ (s)
High stabilityDouble oven equivalent
Medium stability SMD
State of the art USO
10MHz OCXO ADEV Interpretation Medium / High Stability Comparison
12 OCXO Fundamentals
Highest Q resonator oscillator (>1000 000)
Environmental insulation construction
Low transient response (heating element thermal inertia)
But…
Long term stability limited due to inherent ageing effect…
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Good short term stability
Telecom System Requirement
Excellent time keeping ability
• Need for digital compensation implementation
OCXO Short Term Stability RubidiumMid/Long Term Stability
14 High End Digital Compensation OCXO Topology
Keep very short term stability performance
Upgraded TIME
keeping OCXO
High Stability OCXO
THERMAL Compensation Algorithm
Ageing Compensation Algorithm
Overcome unwanted THERMAL and TIME effect
15 Frequency Instability Contributor ADEV Translation
1.E-12
1.E-11
1.E-10
10 100 1,000 10,000 100,000
Ove
rlap
pin
gA
DEV
σy(
τ)
τ (s)
High stability OCXO
Ageing compensation algorithm
Thermal compensation algorithm
16 Thermal Compensation Limitation
Apply temperature profile
Evaluate frequency variation
-20
-16
-12
-8
-4
0
4
8
12
16
20
-100
-80
-60
-40
-20
0
20
40
60
80
100
0 20 40 60 80 100 120 140
y(t)
[p
pb
]
Tem
pe
ratu
re [
°C]
time
Temperature
-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
y(t)
[p
pb
]
Temperature [°C]
Frequency Response Over Temperature
Then apply compensation algorithm
Compute the ideal mathematical response
But…Oscillator hysteresis limiting residual error
Frequency vs. Temperature response
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Ideal ageing behaviour is predominantly logarithmic over time
Ageing prediction limitation
0
10
20
30
40
50
60
70
80
90
0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540
df/
f [p
pb
]
Number of days
The ideal curve fit may be computed
18 Ageing prediction limitation
Frequency gap between expected and actual ageing response
-1E-10
-8E-11
-6E-11
-4E-11
-2E-11
0
2E-11
4E-11
6E-11
-8E-11
-7E-11
-6E-11
-5E-11
-4E-11
-3E-11
-2E-11
-1E-11
0
1E-11
2E-11
3E-11
Prediction limited to extrapolation accuracy
19 Summary
High stability OCXOs are not able (by themselves) to maintain very tight holdover specification such as 1.5 µs over 24 hours
Additional distinct digital compensation is required to a target specification
Although mathematically easy to implement, actual OCXO behaviour requires a perfect understanding of Piezoelectric crystal phenomena
OCXO manufacturers are able to assess each of oscillator’s behaviour by monitoring over significant time periods and environmental conditions
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Time keeping is 1.5 µs over 24 hours
Conclusion
What about the next generation requirement?
Time keeping is 1.5 µs over 72 hours
Current Tomorrow
It means 3,5 x 10-11 (35ppt) stability for all causes over 24 hours
A wristwatch disciplined by such a stable clock would be off one second after 913 years!
A wristwatch disciplined by such a stable clock would be off one second after 8219 years!
Holdover is not linear
A clock 3² is needed = 9 times more stable to target this specification
3,8 x 10-12 stability (3,8ppt)