Transcript Document

WVR workshop
Wettzell 2006
CARMA, and the CARMA
WVR effort
Alberto Bolatto
Associate Research Astronomer
U.C. Berkeley Astronomy
Radio Astronomy Lab
Dick Plambeck (UCB/RAL), Dave Woody (Caltech),
Leslie Looney, Yu-Shao Shiao (UI), Douglas Bock
(CARMA)
Outline
•
•
•
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What is CARMA?
The OVRO experience
The RAL correlation radiometer
What next?
Berkeley-Illinois-Maryland array
10 6.1-m diameter antennas
CEDAR
FLAT
Caltech array
6 10.4-m antennas
+ UChicago SZA
8 3.5-m antennas
Cedar Flat – elevation 2200m
August
June 2004
2005
21 Jul 2004 – lifting
off the first reflector
panel adjustment
surface error determined from holography
before adjustment: 127 μm rms
after adjustment: 28 μm rms
→ 75% loss at 225 GHz
→ 7% loss at 225 GHz
all antennas assembled
10 Aug 2005
Comparison with other arrays
CARMA
+ SZA
SMA
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
Now
E, D
configurations
baselines 8–150 m
1mm beam: 2”
1.6 km
for Winter 2005
E, D, C
configurations
baselines 8–350 m
1mm beam: 0.8”
1.6 km
for Winter 2006
E, D, C, B, B+
configurations
baselines 8–1700 m
1mm beam: 0.2”
1.6 km
for Winter 2008
E, D, C, B, A
configurations
baselines 8–1900 m
1mm beam: 0.13”
1.6 km
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 data
Y.-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
D0612LA
9-13.5 GHz YIG OSC.
WR42 th. gap +
window
NARDA 4317B-2
BIMA dewar
180 hybrid/magic T
40 K stage
4-q multiplier
22 GHz optics
NARDA 4017C-10
180° PHASE SWITCH
HMC 281 cryo amps
MARKI M1R-0726L
DITOM D3I1826
DUAL HMC281
MCL SLP-550
CTL
12 K stage
DETECTOR
CANbus uP + DAQ
18-26 GHz
CARMA X-band
ASTRONOMY IF
The Receiver
K band cryo
amp (x4)
Magic-tee
hybrid
Controled
temp. load
K band cryo
amp (x4)
Input
(to horn)
Thermal
clamp to 12 K
stage
Magic-tee
hybrid
Controled
temp. load
Input
(to horn)
Thermal
clamp to 12 K
stage
The Controlled Temperature Load
10 mil 50 Ω
quartz μstrip
• Load + sensor mass is 3 mg:
fast temperature response
• Once mounted, sensors are
calibrated against standards
•S11~-20 dB
Brass
pedestal
Heater
biasing wire
Cernox sensor
chip on top of
inverted 50 Ω
alumina resistor
The Amplifiers
Hittite
HMC 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
LO/downconverter
Microcontroller
Other Hardware
IF/Multiplier
Signal conditioning
The Complete System
WVR dewar
“inserts”
LO chain and
downconverter
Signal
conditioning
and control
electronics
IF chain, AGC,
multiplier, phase
switching, and
filters
XAC uP,
ADC, DAC,
and CANbus
Nice idea, but it has proven difficult
to make it work
Status May 2005
• 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
Nice idea, but it has proven difficult
to make it work
Status May 2005
• 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?
What next?
• Revert to basics – Simple is beautiful
• Implement a Dicke-switch radiometer
– Room temperature: use noise diode
– Cryogenic: use controlled temperature load
CTL
LO
DET
e.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.