CARMA, and the CARMA CARMA, and the CARMA WVR effortWVR effort

Alberto BolattoAlberto BolattoAssociate Research AstronomerAssociate Research Astronomer

U.C. Berkeley AstronomyU.C. Berkeley AstronomyRadio Astronomy LabRadio Astronomy Lab

Dick Plambeck (UCB/RAL), Dave Woody (Caltech), Leslie Looney, Yu-Shao Shiao (UI), Douglas Bock

(CARMA)

WVR workshop WVR workshop WettzellWettzell 20062006

Outline

• What is CARMA?• The OVRO experience• The RAL correlation radiometer• What next?

+ UChicago SZA 8 3.5-m antennas

Berkeley-Illinois-Maryland array 10 6.1-m diameter antennas

Caltech array 6 10.4-m antennas

CEDAR FLAT

Cedar Flat – elevation 2200m

June 2004August 2005

21 Jul 2004 – lifting off the first reflector

panel adjustmentsurface error determined from holography

before adjustment: 127 µm rms

→ 75% loss at 225 GHz

after adjustment: 28 µm rms

→ 7% loss at 225 GHz

all antennas assembled10 Aug 2005

Comparison with other arrays

1225+1528253baselines

500 m

226

6

8

4200

SMA

14 km400 m1900 mmax baseline

500025002200 melevation

5600+

12, 7

50+

ALMA

1510, 6, 3.5diameter

1060

6

IRAM

850 m2area

23antennas

CARMA+ SZA

Comparison of u,v coverage6 hr track on source at decl +10º

OVRO E, 15 baselines CARMA D, 105 baselines

Synthesized beams5% contours

E, D configurations

Now

1.6 km

baselines 8–150 m

1mm beam: 2”

E, D, Cconfigurations

for Winter 2005

1.6 km

baselines 8–350 m

1mm beam: 0.8”

E, D, C, B, B+configurations

for Winter 2006

1.6 km

baselines 8–1700 m

1mm beam: 0.2”

E, D, C, B, Aconfigurations

for Winter 2008

1.6 km

baselines 8–1900 m

1mm beam: 0.13”

225 GHz zenith opacity

<520<4.3<.2875

<350<2.4<.1650

<290<1.8<.1225

SSB Tsys

mm H2O

tau%

Tsys computed for 1.5 airmasses, Trcvr(DSB) = 45 K

OVRO WVR

Sample phase improvement

It can work, but…• Can it work

reliably?• It’s easy to

improve very bad tracks, but good tracks can be worsen

• Only works for ~40% of the dataY.-S. Shiao et al., SPIE, (2006)

Correlation WVR at 22 GHz• Correlation receiver: less sensitive to amplifier gain

variations, no moving parts, built-in absolute calibration. Fast control of temperature of reference for nulling: ultimate stability.

• Weak points: complexity, sensitive to spurious correlations

Expected performance

• Measured amplifier performance based on Hittite commercial HMC 281 GaAs mmic ($40):Tnoise ~55 K, G ~23 dB, BP ~16-36 GHz

• Expect Tsys ~ 140 K, or RMS ~5 mK in 1s in 1 GHzhot spill~3% (9 K), input w.g. loss~0.5 dB (32 K), hybrid+w.g./coax loss~0.3 dB (4 K), 2nd amp stage~5-10 K

• Assuming canonical δλ~4.5 mm/K @ 22.2 GHz expect path RMS ~20 mm in 1s

• Performance will be degraded by control of load temperature, thermometry, spectral baseline removal, etc, but there is a safe margin – λ/20 goal is ~60 mm

Block diagramx2

CARMA X-band

CTL

CANbus uP + DAQ

22 GHz optics

180 hybrid/magic T

BIMA dewar

HMC 281 cryo amps

DITOM D3I1826DUAL HMC281

NARDA 4017C-10

MARKI M1R-0726L

18-26 GHz

ASTRONOMY IF

MCL SLP-550

NARDA 4317B-2D0612LA

9-13.5 GHz YIG OSC.

DETECTOR

4-q multiplier

180° PHASE SWITCH

WR42 th. gap + window

12 K stage

40 K stage

The ReceiverK band cryoamp (x4)

Controledtemp. load

Magic-tee hybrid

Thermal clamp to 12 K

stage

Input (to horn)

Input (to horn)

K band cryoamp (x4)

Thermal clamp to 12 K

stageControledtemp. load

Magic-tee hybrid

The Controlled Temperature Load

Cernox sensor chip on top of inverted 50 Ω

alumina resistor

10 mil 50 Ωquartz µstrip

Heater biasing wire

• Load + sensor mass is 3 mg: fast temperature response

• Once mounted, sensors are calibrated against standards

•S11~-20 dB

Brass pedestal

The Amplifiers

HittiteHMC 281

12 amps put together by Dusty Madison, a freshman summer student who learned

to assemble and wirebond them

The Dewar “Insert”• Minimum impact on existing BIMA dewar

• No internal screws/electrical connections: just plugs in

• Special 2-port test dewar designed and fabricated

Other HardwareLO/downconverterLO/downconverter IF/MultiplierIF/Multiplier

MicrocontrollerMicrocontroller Signal conditioningSignal conditioning

The Complete SystemWVR dewar

“inserts”

LO chain and downconverter

IF chain, AGC, multiplier, phase switching, and

filters

Signal conditioning and control electronics

XAC uP, ADC, DAC,

and CANbus

Nice idea, but it has proven difficult to make it work

• Tests looking into heated cryogenic waveguide load in 2nd dewar

• Non flat passband– Slope is caused by

imperfect hybrid– Central feature is

from CTL wgadaptor

– Edges not quite understood

– A few K of “extra” correlation, probably reflections in hybrid

Status May 2005Status May 2005

Nice idea, but it has proven difficult to make it work

• Spurious correlation due to internal coherent reflections– Could be mitigated

with input isolators• Even without moving

parts, calibration is not repeatable enough– Difficult to attain the

mK calibrability goal• Further work?

Status May 2005Status May 2005

What next?• Revert to basics – Simple is beautiful• Implement a Dicke-switch radiometer

– Room temperature: use noise diode– Cryogenic: use controlled temperature load

LOCTL

DETe.g. AD8309

Dicke-WVR assembly using CTL

Conclusions• Phase correction schemes improve correlation

for a fraction of the tracks, but not all the time. Atmosphere or engineering?

• Nulling correlation radiometers are nice in theory, very difficult in practice. Large part count and complexity makes them unattractive for (university based) interferometers.

• Dickey-switch type schemes are considerably simpler, and more attractive if stability of 1:10,000 can be attained. Partial successes at PdBI, VLA, and ALMA/SMA suggest they are viable.