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Cryocooled Sapphire Oscillator Frequency Standards for the shortest VLBI Wavelengths
Maria Rioja, Richard Dodson Yoshiharu Asaki John Hartnett Steven Tingay
(or improving sensitivity by reducing coherence losses)
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1. Why need to improve frequency standard?
2. Description of Simulation Studies
3. Comparative Performance: Coherence losses for H-maser and CSO
4. Other Strategies to improve sensitivity: 4.1 WVR (co-located independent technique), 4.2 Frequency Phase Transfer (FPT) (simultaneous dual frequency observations)
Contents
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3
H-maser
Good Weather
Very Good=ALMA-type weather
VW=WVR@ALMA
Why? The Quest for Sensitivity…
MoreStable
Cryocooled Sapphire Oscilator
Trop phase fluctuations site with stable weather conditionsH-maser instabilities ultra stable Cryogenic Sapphire Oscilator (CSO)Clock
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4
Hartnett & Nand, 2010Hartnett et al. 2012
Ultra-stable Cryocooled Sapphire Oscillator (CSO)
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CLOCK only
CLOCK & TRP
TRP only
Generate Synthetic with ARIS
Dataset
GEOSource/antenna/errors
Trp/Ion Error
TRPFluctuation
CLOCK Frequency (GHz)
Source:PointStrong
Ion Fluct: Nominal errors
Single freq:86 175 350
Array:-VLBA-EHTEOP
Trp error: 3 cmIon error:6 TECU
- VW- V very good
- G good - T typical
- P poor
-CSO-H-maser
Dual freq: 43 / 86 87 / 175175 / 350
Simulations: Parameter Space
(Asaki+2007)
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Synthetic Datasets generated with ARIS
(86 GHz, Good Weather,) (Worse weather)
VisibilityPhases
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Analysis with AIPS
Self-Calibration (SC) X11
Frequency Phase Transfer (FPT) (Dual Freq.) X11FPT + SC = Hybrid X11
(x 11) Solint:
0.1, 0.2,… 6 minutes
MAP
MAP
MAP
(x 11)
Simulations: Data Analysis
Simulated Dataset
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MAPS
Figure of
Merit
Flux loss4%
Flux loss20%
Uncompensated residual phase fluctuations leads to Flux loss.Use Flux loss as a measure of coherence loss for comparative studies.
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RESULTS:CLOCK noise only, all freq. H-maser
CSO
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RESULTS:CLOCK noise only, all freq.
0%
0.5%
10%
40%
86 GHz175 GHz350 GHz
CSO0%
H-maser
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RESULTS:CLOCK noise only, all freq.RESULTS: ATM noise only, all weathers, all freq.
ASD_V=3*ASD_VWASD_G = 2*ASD_VASD_T = 2*ASD_GASD_P = 2*ASD_T
V
G
VW
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RESULTS: ATM noise only, all weathers, all freq.RESULTS: ATM noise only, all weathers, all freq.
20%
86 GHz80%
G
V
P
T
VW
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RESULTS: ATM noise only, all weathers, all freq.RESULTS: ATM noise only, all weathers, all freq.
175 GHz
50%
80%
20%
20%
86 GHz80%
G
V
P
T
VW
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RESULTS: ATM noise only, all weathers, all freq.RESULTS: ATM noise only, all weathers, all freq.
175 GHz
80%
20%
20%
86 GHz80%
G
V
P
T
350 GHz20%
80%
VW
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15
SUPERIMPOSED H-Maser vs. ATM noise, all weathers, all freq (zoomed).
10%
10%
10%
86 GHz
175 GHz
350 GHz
H-maser
H-maser
H-maser Significance of H-maser noiseExpected to increase at highestfrequency (350 GHz) and with best quality weather conditions (V,VW); the CSO noise remains negligible in all Circumstances.
VW
V
G
PT
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RESULTS: CLOCK (H-maser/CSO-100MHz) + ATM (Very Good), all freq.
2% change
20%
86 GHz
6% change
175 GHz
350 GHz
20% change
+ CSOx H-maser
Comparative Performance
CSO Significant Benefit (
i.e. increased sensitiv
ity)
@ 350 GHz with V quality weather conditio
ns.
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INTERPRETATION of RESULTS: SENSITIVITY
+ H-maser+ CSO
Thermal only
20% increasesensitivity withCSO wrt H-maser
@ 350 GHz, V weather
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86 GHz
20%
175 GHz
350 GHz
RESULTS: CLOCK (H-maser/CSO-100MHz) + ATM (VW), all freq.
2% change
10% change
40% change
+ CSOx H-maser
CSO Very Significant Benefit (
i.e. increased sensitiv
ity)
@ 350 GHz with VW quality weather conditio
ns.
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RESULTS: CLOCK (H-maser/CSO-100MHz) + ATM (G), all freq.
86 GHz 175 GHz
350 GHz
20%
1% change
3% change
8% change
+ CSOx H-maser
Comparative Performance
CSO moderate benefit (i.e. in
creased sensitivity)
@ 350 GHz with G quality weather conditio
ns.
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+CSO, 8%
IMPROVEMENTS WRT H-maser, G weather, @350 GHz (G trop. loss)
H-maser+WVR, 50%
+CSO+WVR, 70%
Other Strategy(1): WVR to “upgrade” weather quality
(G tropospheric loss)
(V tropospheric loss,H-maser loss)
(V tropospheric loss)
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0-5%
20%
Hybrid analysis: FPT @low freq (0.5’) + SC@high freq (3’, 6’).
FTP: Use Low Freq. Analysis to Guide High Frequency(“disciplined phases”).
FPT & Hybrid Analysis, Very Good Weather
FPT & Hybrid Analysis, Good Weather
(43x2) 86GHz
(87x2) 175GHz
(175x2) 350GHz
Other Strategy(2): Multi Frequency Observations + FPT analysis
86 GHz
175,350 GHz86,
175 GHz350 GHz
Extended (hours!) coherence Time at all frequencies also with GQuality weather conditions.
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Master Title22
Summary• The stability of typical H-masers introduce significant coherence losses at submm wavelengths.
• Most noticeable in very best weather conditions. • A CSO based frequency standard for submm VLBI benefits from superior stability
which results in Increased coherence time. • Our estimates are 20% increase in sensitivity at 350GHz with “Very Good” (i.e.
ALMA-type) weather conditions; along with WVR, 40% increase is possible.
• WVR have the potential to upgrade `Good’ sites into `VeryGood’ sites, ideal for submm observations (maximum benefits along with CSO).
• Including Freq. Phase Transfer has great potential to increase coherence time (i.e. sensitivity) at submm wavelengths
- requires dual frequency observations.