Atomic Clock Technology ITSF 07 13 November 2007 Dr. R. Michael Garvey
[email protected]
Outline Goals and definitions of terms Atomic resonance properties and interrogation techniques Basic atomic clock architecture Cesium beam atomic clocks Hydrogen maser atomic clocks Rubidium gas cell atomic clocks Contrast cesium cesium, hydrogen hydrogen, and rubidium performance Other clock technologies 2
Goals Understand different clock technologies so that intelligent decisions can be made to optimize system performance, cost, lifetime and reliability.
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Terms: Normalized Frequency Normalized frequency: Also called “fractional fractional frequency offset” offset
y
( 0
Example: -1 1 Hz offset at 5 MHz (4 999 999 Hz) is -2e-7 2e 7
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Terms: Allan Deviation The Allan deviation is a statistical measure (analogous to the well known standard deviation) of frequency stability. 1 y ( ) 2( M 1)
1/ 2
( y i 1 y i ) i 1
i M 1
2
Allan deviation can be predicted from basic atomic resonance parameters (more on this later)
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Why Atomic Clocks? Atoms of a given element and isotope are identical Properly designed apparatus can interrogate atomic resonances to form precise and stable frequency q y references The best atoms are “hydrogen like” in their atomic structure: 1H, 133Cs, 87Rb. While cesium defines the SI second, stored ions and optical transitions in other atoms (199Hg+, 171Yb+, 88Sr+, Ca) may be candidates for evolutionary laboratory standards. 6
Basic (Passive) Atomic Clock
Synthesizer
Atoms
Detector
RF Output Oscillator Servo Divider Clock Output
Cesium: 0 = 9 192 631 770 Hz (definition) Hydrogen: 0 = 1 420 405 751 751.770 770 (3) Hz Rubidium: 0 = 6 834 682 610.904 29(9) Hz 7
Atomic Energy Levels and Resonance
E2
E2-E1 = hv0
E1
--Atoms can reside only in well defined energy states --Transitions Transitions between energy states define a resonance, usually in the microwave region 8
Accuracy and Stability
Accuracy vs Stability
..........
....... ... 2
3
1
4
5
8
6
10
7
9
Stable-not accurate
. . . ... . .. . Accurate-not stable (noisy)
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12
Stable-linear drift-not accurate
........... Stable and Accurate 10
Design Considerations Accuracy: preserve the intrinsic accuracy of the atomic resonance Short Term Stability: extract information with highest signal-to-noise from the atoms Characterized with Allan deviation Long Term Stability: control the effects of time (aging) and environment (e (e.g. g temperature) on the atomic interrogation? Accuracy and stability are usually interrelated. How do we achieve reasonable size, cost, and operational reliability? 11
Accuracy Presumption of ensemble of identical unperturbed atoms at rest at 0 K Realities of practical devices:
Atoms are in motion: Doppler pp effects Confinement effects Interrogation effects Environmental effects
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Calibration vs Accuracy vs Stability Calibration does not guarantee accuracy indefinitely The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom 13th Conférence Général des Poids et Mesures (1967)
Clarification of 1997: …cesium atoms at rest at a temperature of 0 K. Stability implies absence of noise or change change—says says nothing about accuracy
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Frequency Offsets Clocks operate with frequency offsets with respect to the atomic resonance frequency; q y understanding g and controlling these offsets is essential “Primary” frequency standards either have no offsets or we can precisely calculate and control offsets ff Offsets we can calculate and stabilize are OK Some S offsets ff t can’t’t be b exactly tl calculated l l t d and d require i calibration
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Atomic Resonance Requirements Narrow resonance – we seek resonances with high quality factor
vo Q W
0 is resonance frequency and W is atomic linewidth
High signal-to-noise ratio – good short term stability
1 y ( ) S : N *Q
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Atomic State Populations --Atoms can reside only in discrete energy states --States populations are essentially equal for microwave resonances in th thermal l equilibrium ilib i
E2 hv0
E2-E1 = hv0
--Signal-to-Noise considerations require q we alter the population distribution --Common techniques to alter the population distribution include “optical pumping” and “state selection”
h 0 hv
E1
Goal: deplete one state, induce transitions back into this state, detect the results 16
Cesium Beam Frequency Standards
Cesium Beam Frequency Standards Cesium (caesium) resonance forms the i t internationally ti ll acknowledged k l d dd definition fi iti off th the SI second interval of time M t Mature technology,excellent t h l ll t reliability li bilit and d stability t bilit and performance at reasonable cost Two levels of performance in commercial products today
Device of choice when superior long term stability (and environmental immunity) or when autonomy is required Intrinsic accuracy--calibration not required No “aging” of frequency 18
Cesium Beam Tube Cartoon 9192 MHz
Detector
F=3 + F=4
F=3
S F=4
F=3 + F=4
F=3
S F=4
N
N
"A" M Magnett
"B" M Magnett
Magnetically-Selected CBT 19
Cesium Beam Tube
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Cesium Spectrum
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Cesium Spectrum (high resolution)
Linewidth 450 Hz Atomic line Q 20 million
0 = 9 192 631 770 Hz 22
Cesium Beam Block Diagram
Servo including DACs, ADCs, microprocessor
Direct Digital Synthesizer
RS 232 RS-232
5 MHz Oven Controlled Quartz Oscillator
Frequency Mulitplier/Synthesizer
9192631770 Hz
User Output
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Atomic Beam Pluses and Minuses + Cesium beam is a primary standard and does not require calibration + Beam has no interaction with its “confinement” Most accurate and most stable atomic clock, in long term
+ No first order Doppler frequency offsets in properly designed and built apparatus g for minimal self-interaction + Beam densityy is low enough - Relatively complex and expensive apparatus - Linewidth limited by time-of-flight through the apparatus +/- Lifetime: 6 years for high performance; 12+ years for standard performance
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National Company Atomichron circa 1958
Symmetricom TimeCesium circa 2007
Symmetricom 5071A Circa 2007
Active Hydrogen Maser Frequency Standards
Active Hydrogen Masers Active masers provide the best short term frequency stability for averaging times less than 1 day Mature technology gy with g good operating p g lifetime and reliability Relatively large, complex and expensive Design of choice when the ultimate frequency stability is required
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Hydrogen Maser Maser Signal (-105 dBm)
Magnetic Shields
Cavity
Hydrogen Maser Physics Package
Tuning Varactor
Switching Varactor
Q t Bulb Quartz B lb
State Selector
Hydrogen Dissociator
Source Discharge Oscillator
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Maser Block Diagram MPG IF Amp A
28.75 Hz
Varactor Control
L.O. Multiplier
Receiver Amplifier
-105 105 dB dBm
5.751 KHz
Phase Detector & 10 MHz VCXO
5 751 KHz 5.751
14 digit Synthesizer
10 MHz 10 MHz
10 MHz
Tuning Varactor
Maser Physics Package
Switching Varactor
Signal Buffers 10 MHz
Cavity Register
405 KHz 28.75 Hz
24 V Battery 110 VAC 110 VAC 24-29 24 29 VDC
Synchronous Detectors
Power Module
U/D Source Pressure Control
Hydrogen Source
H2 supply Palladium Purifier
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Maser Accuracy and Stability Thermal motion of the atoms induces a secondorder Doppler effect of approximately - 5e-11. Confinement (“wall shift”) of hydrogen atoms induces a frequency offset of approximately -3e e-11. 11 Cavity pulling—dependent on cavity tuning error Spin exchange frequency shift Spin-exchange Magnetic field in the atomic environment To maintain aging of 2e-16/day requires that these effects be constant to better than 10 pp ppm/day y 30
Active Hydrogen Maser
--Frequency Stability is +40X superior to high performance f cesium i --Requires frequency calibration pp for most applications
MHM 2010
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Passive Hydrogen Maser
Passive Hydrogen Masers Smaller size achieved by dielectric loading of maser cavity Lower cavity Q precludes maser oscillation—operates in a passive mode using traditional frequency lock loop servo
Short term frequency stability better than cesium E hibit ffrequency d Exhibits drift/aging ift/ i iinferior f i tto cesium i Small installed base; little long term experience Two manufacturers in Russia
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Passive Hydrogen Maser Cavity Tuning
Magnetic Shields
Passive Hydrogen Maser Signal Input Physics Package Cavity
Dielectric Signal Output
Storage Bulb
State Selector
Hydrogen Dissociator
Source Di S Discharge h Oscillator
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Rubidium Gas Cell Frequency Standards
Rubidium Gas Cell Frequency Standards
Most Widely Used Type of Atomic Clock Smallest, Lightest, Lowest Power, Least Complex, Least Expensive, Longest Life, Good Performance Stability & Reliability Performance,
Device of Choice When Better Stability Than a Crystal Oscillator is Needed Lower Aging, Lower Temperature Sensitivity, Faster Warm-up, Excellent Retrace
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Gas Cell Confinement Nitrogen Rubidium
--Nitrogen atoms immobilize the rubidium atoms, slowing their velocity and minimizing wall collisions --Interaction between rubidium and buffer gas introduces large frequency offset (which must be calibrated) 37
Optical Pumping --Incident pumping photons stimulate transitions into an excited optical state
Excited Optical State
--Atoms in the excited state d decay tto the th ground d states t t with equal probability
h Optical pumping photons
--Continued Continued pumping out of one ground state effectively moves the atoms into the other state
h
E2 E1
h
h
E2-E1 = hv0
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Rubidium Gas Cell Magnetic Shield
Lamp Oven Lamp p 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
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Rubidium Gas Cells The cells in the latest commercial i l RFS d designs i have (along with their gotten much cavities)) g smaller. While this comes at the expense of a broader line, line lower Q and poorer short-term stability, good performance e.g. y()= ( ) 1e-11 1 11 att 1 second d can still be realized.
Comparison between classic 1” long (LPRO) and latest ultra-miniature (X72) integrated Rb gas cells
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Gas Cell Atomic Clock Pluses and Minuses + Buffer gas confinement allows small size and economical construction + Rubidium has a fortuitous isotope overlap to allow optical pumping - Buffer gas introduces a large frequency shift which results in poor accuracy, is difficult to perfectly stabilize over time (aging) and environment (temperature, barometric, etc) Buffer gas mixtures can reduce thermal effects, one gas with ith a positive iti ttemperature t coefficient ffi i t (N2) and d one with ith a negative temperature coefficient (Ar) p ) are small and Barometric effects ((1e-10/atmosphere) non-cumulative 41
Modular Rubidium Standard Height g less than 0.72”
0.7”
3.0” 3.5”
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Rubidium Standard Instruments
Defense Customization Rack Mount Instrumentation
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Rubidium Operational Accuracy Rubidium gas cell devices are secondary standards which have great utility once calibrated Buffer gas offset is ~1e-6; to achieve aging of
