Post on 16-Feb-2020
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.