cryocooled sapphire oscillator frequency standards for the shortest vlbi wavelengths

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.