cryocooled sapphire oscillator frequency standards for the shortest vlbi wavelengths
DESCRIPTION
Cryocooled Sapphire Oscillator Frequency Standards for the shortest VLBI Wavelengths. (or improving sensitivity by reducing coherence losses). Maria Rioja, Richard Dodson Yoshiharu Asaki John Hartnett Steven Tingay. Contents. Why need to improve frequency standard? - PowerPoint PPT PresentationTRANSCRIPT
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)
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
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
4
Hartnett & Nand, 2010Hartnett et al. 2012
Ultra-stable Cryocooled Sapphire Oscillator (CSO)
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)
Synthetic Datasets generated with ARIS
(86 GHz, Good Weather,) (Worse weather)
VisibilityPhases
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
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.
RESULTS:CLOCK noise only, all freq. H-maser
CSO
RESULTS:CLOCK noise only, all freq.
0%
0.5%
10%
40%
86 GHz175 GHz350 GHz
CSO0%
H-maser
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
RESULTS: ATM noise only, all weathers, all freq.RESULTS: ATM noise only, all weathers, all freq.
20%
86 GHz80%
G
V
P
T
VW
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
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
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
16
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.
17
INTERPRETATION of RESULTS: SENSITIVITY
+ H-maser+ CSO
Thermal only
20% increasesensitivity withCSO wrt H-maser
@ 350 GHz, V weather
18
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.
19
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.
20
+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)
21
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.
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.