Power Matters.TM
Fundamentals of Frequency
Reference Oscillators
Paul R. Gerry
Senior Product Manager, Clocks BU
Paul.Gerry@microsemi.com
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For Frequency Generation
You need…
The Earth
rotating
Electronic
Oscillator
A pendulum
(mechanical oscillator)
Atomic resonance
… a method of generating a repeatable event
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What is Frequency
Frequency = the number of cycles per second
Ideal frequency source generates a pure, repeatable sine wave
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Related Frequency Definitions
 Offset – the frequency error from the ideal (fast or slow)
 Accuracy – refers to frequency offset of a device
 Stability – how well an oscillator produces time or
frequency over a given time interval
 Aging – change of frequency over time (also called drift)
 Temperature Stability – the change of frequency over
temperature
 Accumulated Time Error – total of all the above
characteristics acting on a clock
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Performance Definitions
 Short Term Stability – the change of frequency over 1-100
seconds from noise and vibration. Sometimes called flicker
or jitter
 Long Term Stability – the change of frequency over hours,
days, or months. Result due to age and temperature
 Phase Noise – The rapid, short-term, random fluctuations in
the phase of a sine wave due to oscillator quality, semiconductor and white noise
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What is Frequency Stability & Accuracy
Courtesy John Vig
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Oscillator Stability Over Time
Frequency stability typically improves in the short term,
stabilizes, then becomes less predictable in the long term
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Short and Long Term Oscillator Stability
Frequency Stability Chart
1.00E-06
1.00E-07
1.00E-08
Stability
1.00E-09
OCXO
1.00E-10
1.00E-11
Hi-Stab OCXO
1.00E-12
Rb
1.00E-13
1.00E-14
Cs-High Perf
1.00E-15
1 Sec
10 Sec
100 Sec
1K Sec
10K Sec
100K Sec
Week
Month
H-Maser
GPS
Time
Some oscillators perform better short term… others long term
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What is Phase Noise
Unwanted noise sidebands on either side of the output signal center
frequency caused by random variations of the frequency and the
phase of the carrier
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What is Single Side Band Phase Noise
The SSB is defined as the ratio of power in one phase
modulation sideband to the total signal power
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What is Oscillator Phase Noise
Noise Level (dBc/Hz)
Oscillator Phase Noise
-80
-90
-100
-110
-120
-130
-140
-150
-160
1Hz
OCXO
Rb
H-Maser
10Hz
Hi-Stab OCXO
Cs
100Hz
1KHz
10KHz
Offset Frequency
Phase noise is a short term noise component indicative of the
spectral purity of an oscillator signal
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Putting the Fundamentals into Perspective…
 What is one part in 1.0E-10 ? (As in 1 x 10-10/day aging)
• ~1/2 cm out of the circumference of the Earth.
• ~1/4 second per human lifetime (of ~80 years).
 Power received on Earth from a GPS satellite, -160 dBW, is
as “bright” as a flashlight in Los Angeles when viewed in
New York City, ~5000 km away
 What is -170 dB? (As in -170 dBc/Hz phase noise)
• -170 dB = 1 part in 1017 ~thickness of a sheet of paper out
of the total distance traveled by all the cars in the world in a
day
The second is the most precise SI unit of measure!
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Frequency Sources Types
Quartz Crystal Oscillators
Atomic Frequency Standards
Emerging Clock Technologies
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Crystal Oscillator Types
f
f +10 ppm
Voltage
Tune
Output
-450C
Crystal Oscillator (XO)
Temperature
Sensor
Compensation
Network or
Computer
250C
+1000C
T
-10 ppm
-450C
f
f
+1 ppm
+1000C
T
XO
-1 ppm
Temperature Compensated (TCXO)
Oven
Oven
control
XO
Temperature
Sensor
-450C
f
f
+1 x 10-8
-1 x 10-8
+1000C
T
Oven Controlled (OCXO)
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Atomic Frequency Standards
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Atomic Clock Technologies
 Rubidium Gas Cell:
 Cesium Beam:
 Hydrogen Maser:
6,834,682,610.904 Hz
9,192,631,770 Hz
1,420,405,751.768 Hz
 Fountains use cesium, rubidium
 Stored Ions use mercury, ytterbium
 Optical Clocks use mercury, calcium
The resonant frequency of atoms does not age… the apparatus
to interrogate or confine atoms can in some atomic clocks
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Atomic (Passive) Clock Basics
Synthesizer
Atoms
Detector
RF Output
Oscillator
Servo
Divider
Clock Output
 Stimulate an energy state change in the atoms
 Detect when resonant frequency is achieved
 Servo the oscillator to maintain optimal performance
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Cesium Technology Applications
 Cesium Technology is considered the most
comprehensive holdover option against GNSS
vulnerabilities
• Exhibit no frequency drift
• Maintains 5x10-15 accuracy over the life of the
instrument
 Critical for long-term autonomous operation
 No on-going calibration required
 More expensive than Rubidium and OCXO
• Consumes more power and space
 Typical applications
• Fixed wireline communications infrastructure
• Under sea (Submarine)
• Satellite ground stations
• Metrology and Time Keeping
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Rubidium Frequency Standards (Lamp)
Magnetic Shield
Lamp Oven
Lamp
Lamp
Exciter
Rb-87
Lamp
Coil
Filter Oven
Cavity Oven
Filter
Absorption
Rb-85
Rb-87
Cell
Cell
C-Field Coil
RF
Excitation
PhotoDetector
Signal
Out
C-Field
Current
(3) Oven Temperature Sensors and Heaters
W
Interrogation
Physics
Package
RF Chain
Servo Modulation
Discriminator
Signal
Frequency Lock Loop
O/P
Servo
Amplifier
Crystal
Oscillator
O/P
Amp
Control
Voltage
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Rubidium Frequency Standards (CPT)
The MAC uses Coherent Population Trapping (CPT)
 Coherent Population
Jinquan Deng, Peter Vlitas, Dwayne Taylor, Larry Perletz, and Robert Lutwak, "A
COMMERCIAL CPT RUBIDIUM CLOCK“ EFTF 2008 Toulouse, France.
Trapping (CPT)
interrogation of
Rubidium
 Laser diode (VCSEL)
modulated to achieve
CPT resonance
 Photodiode detects the
CPT resonance
 10 MHz VCXO synthesizes 3.4 GHZ microwaves
 Microwave frequency is locked to CPT resonance signal
stabilizing the output to 10 MHZ
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Rubidium Gas Cell Frequency Standards
 Most widely used type of atomic clock
• Smallest, lightest, lowest power
• Least complex, least expensive, longest life
• Excellent performance, stability & reliability
 Device of choice when better stability is needed
compared to crystal oscillator
• Lower aging, lower temperature sensitivity
• Faster warm-up, excellent retrace
• Used as an inexpensive holdover technology
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Emerging Clock Technologies
 Fountain Clocks
• Atoms are cooled and “tossed” upward in Earth’s gravity
• Used for primary standards where ultimate accuracy is
desired
 Coherent Population Trapping Clocks: Miniature
Atomic Clocks (MAC) & CSAC
• Ultra miniature size and low power requirement-SWaP
 Optical clocks relying upon optical atomic transitions
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SA.31m Laser Pumped Rb &
Chip Scale Atomic Clock (CSAC)
 Rb Miniature Atomic Clock (MAC)
• Small form factor: 51mm x 51mm x 18mm (H)
• Lower power: 5W @ 25o C
• Stability
– 1s <3E-11; 100s <8E-12
• Aging: <3E-10/month
• Temp Stability: <1E-10 (–10o C to +75oC)
 CSAC (Chip Scale Atomic Clock)
•
•
•
•
Volume: <17 cc
Weight : 35g
Very Low power: <120 mW
Stability
– 1s <2E-10; 100s <2E-11
• Aging: <3E-10/month
• Temp Stability: <5E-10 (0 to +75o C)
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Hierarchy of Oscillator Types
Oscillator Type*
Accuracy**
Typical Applications
• Crystal oscillator (XO)
10-5 to 10-4
Computer timing
• Temperature compensated
crystal oscillator (TCXO)
10-6
Frequency control in tactical
radios
• Microcomputer compensated
crystal oscillator (MCXO)
10-8 to 10-7
Spread spectrum system clock
• Oven controlled crystal
oscillator (OCXO)
10-8 (with 10-10
per g option)
Navigation system clock &
frequency standard, MTI radar
• Small atomic frequency
standard (Rb, MAC, CSAC)
10-9
C3 satellite terminals, bistatic,
& multistatic radar, T&M
Communications
• High performance atomic
standard (Cs)
10-12 to 10-11
Strategic C3, EW
* Sizes range from <5cm3 for clock oscillators to > 30 liters for Cs standards
Costs range from <$5 for clock oscillators to > $50,000 for Cs standards
** Including environmental effects (e.g., -40oC to +75oC) and one year of aging
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Frequency and Time Relationship
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Time is Derived from Frequency
 Every clock ever made is an oscillator + a counter
 The 1 pulse-per-second (PPS) is the epoch or definition of the on-time
marker of a clock
 Further counting of the 1PPS is used to keep track of seconds, minutes,
hours, days, and years
OSC
Counter
1PPS
FAST / EARLY
SLOW / LATE
1PPS
Clock 1PPS rising edge is typically on-time:
HH:MM:SS.000000000
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Accumulated Time Error in Oscillators
Microseconds
Clock operating from an OCXO with an offset & aging of 5E-10 and
a temperature error of 1 degree C
Accumulated Time Error
400
350
300
250
200
150
100
50
0
1
5
9
13
17
21
Aging Error(uS)
Offset Err.(uS)
Temp Err.(uS)
Total Err.(uS)
25
29
33
37
41
45
49
53
57
Hours
Oscillator errors accumulate impacting clock performance
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Cesium Time Error Due to Offset
Cesium Time Error (Offset 2E-12)
6
Microseconds
5
Time error = Freq offset * seconds/day
Or
2E-12 x 86400s = ~0.2µs / day
4
3
2
1
0
1
4
7
Offset Err.(uS)
10
13
16
19
22
25
28
31
Days
Cesium atomic oscillators do not age
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GPS Disciplining Phase Plot
If we look at the phase error over time we can see just how
the Stability and Accuracy are linked
TCXO Disciplining
OCXO Disciplining
Rubidium Disciplining
Cesium Disciplining
Nanoseconds
+150
+100
+50
0
-50
-100
-150
TIME
Oscillator is key to improving GPS clock performance
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Thank You
Ramki Ramakrishnan
Director of Marketing & Business Dev, Clocks BU
ramki.ramakrishnan@microsemi.com
+1 707-636-1914
Paul R. Gerry
Senior Product Manager, Clocks BU
Paul.Gerry@microsemi.com
+1 978-522-5798
Steve Fossi
VP and General Manager, Clocks BU
Steve.Fossi@microsemi.com
+1 707-636-1810
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References
• NIST
http://www.nist.gov/index.html
• USNO
http://www.usno.navy.mil/USNO/about-us
• JPO
http://www.jpo.go.jp/
• Time and Frequency User's Manual, National
Bureau of Standards Special Publication 559,
U.S. Government Printing Office, Washington,
DC, 1979
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