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 Wettzell 2006Wettzell 2006

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

CARMA

+ SZASMA IRAM ALMA

elevation 2200 m 4200 2500 5000

antennas 23 8 6 50+

baselines 253 28 15 1225+

diameter 10, 6, 3.5 6 15 12, 7

area 850 m2 226 1060 5600+

max baseline

1900 m 500 m 400 m 14 km

E, D configurations

Now

1.6 km

baselines 8–150 m

1mm beam: 2”

E, D, C configurations

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, A configurations

for Winter 2008

1.6 km

baselines 8–1900 m

1mm beam: 0.13”

225 GHz zenith opacity

% tau mm H2O

SSB Tsys

25 <.12 <1.8 <290

50 <.16 <2.4 <350

75 <.28 <4.3 <520

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 GHz

hot 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 ~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 /20 goal is ~60 mm

Block diagram

x2

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

cryo amp (x4)

Controled temp. load

Magic-tee

hybrid

Thermal clamp to 12

K stage

Input (to

horn)

Input (to

horn)

K band cryo amp

(x4)

Thermal clamp to 12

K stageControled temp. load

Magic-tee

hybrid

The Controlled Temperature Load

Cernox sensor chip on top of inverted 50 Ω

alumina resistor

10 mil 50 Ω quartz 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/

downconverter

LO/downconverter

IF/MultiplierIF/Multiplier

MicrocontrollerMicrocontroller Signal conditioning

Signal conditioning

The Complete SystemWVR

dewar “inserts”

LO chain and downconvert

er

IF chain, AGC,

multiplier, phase

switching, and filters

Signal conditionin

g 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 wg adaptor

– 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.