Recent PAF Developments at BYU/NRAOcsas.ee.byu.edu/PAFSKA2011/Presentations/BYU Phased Arrays... ·...
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Transcript of Recent PAF Developments at BYU/NRAOcsas.ee.byu.edu/PAFSKA2011/Presentations/BYU Phased Arrays... ·...
Recent PAF Developments at BYU/NRAO
Karl F. Warnick, Brian D. Jeffs, Jonathan Landon, Michael Elmer,David Carter, Taylor Webb, Vikas AsthanaDepartment of Electrical and Computer EngineeringBrigham Young University, Provo, UT
J. Richard Fisher, Roger Norrod, and Anish RoshiNational Radio Astronomy ObservatoryGreen Bank, West Virginia
German CortesCornell University National Astronomy and Ionosphere Center (NAIC)Arecibo Observatory
June 2011
BYURadio Astronomy Systems
Research GroupCurrent Projects
PAFs:– 19 element low loss, high effficiency active impedance matched array
– 19 x 2 element dual-polarized array
– Cryogenic 19 x 2 – elements (BYU), dewar (R. Norrod, NRAO), LNAs (S. Weinreb, Caltech), back end (BYU, NRAO)
Signal processing– Multichannel downconverter boards
– 40 channel narrowband data acquisition system
– 64 channel ROACH-based 50 Msample/sec real time spectrometer/correlator/beamformer
Algorithms– Controlled beam shape, RFI mitigation, polarimetric calibration
Arecibo PAF feasibility study (G. Cortes) Focal L-band Array for Green Bank Telescope (FLAG)
National Radio Astronomy Observatory
Arecibo ObservatoryNational Astronomy and Ionosphere CenterCornell University
BYURadio Astronomy Systems
Research GroupDesign Challenges for PAFs
Broadband Near term single reflector PAFs: 300 to 500 MHz bandwidth at L-band
SKA goal: 500-1500MHz (3:1)
High sensitivity80% aperture efficiency, >99% radiation efficiency
System temperature below 50 Kelvin at L-band
Mutual coupling
Low SNR (-30 to -50 dB) Stable gain for radiometric detection High dynamic range - weak fields near bright sources
Stable, well characterized sidelobes
High polarimetric accuracy Immunity to radio frequency interference (RFI) Modeling, optimization, and characterization of large, complex system
BYURadio Astronomy Systems
Research GroupHeritage of the BYU/NRAO PAF Design
90’s: R. Fisher’s array of sinuous elements– Complex impedance behavior, poor matching
2000’s: Goal was to move to the opposite extreme: a simple, well-modeled, low-loss design
Dipole array effort began in 2005 – thin dipoles Emphasis on low noise above all else 2009 – fat dipole effort to improve bandwidth and matching 2010 – dual pol dipole (feed port was challenging) 2011 – dipole element for cryogenic PAF
BYURadio Astronomy Systems
Research GroupSystem Noise Budgets
Component 2008 ThinDipoles
(Measured)
2010 Thick Dipoles, Active
Matched(Target)
Cryogenic PAF(Target)
Sky 4 4 4
Spillover 5 5 5
Antenna Loss 4 1 5
LNA Tmin 33 33 5
Mutual Coupling 20 3 1
Total 66 K 46 K 20 K
BYURadio Astronomy Systems
Research GroupCygnus X Region at 1600 MHz
Cross Elevation (Degrees)
Ele
vatio
n (D
egre
es)
-4 -2 0 2 4-4
-2
0
2
4
5 x 5 mosaic of PAF pointingsCircle indicates half power beamwidthRequired antenna pointings:
Single-pixel feed: ~600PAF: 25Imaging speedup: 24x
Canadian Galactic Plane SurveyConvolved to 20-Meter beamwidth
BYURadio Astronomy Systems
Research GroupThin Dipole Array System Noise Budget (2008)
Measured Model
LNA Tmin 33 K 33 K
Mutual coupling 20 K 23 K
Spillover 5 K 5 K
Sky 3 K 3 K
Loss 5 K 5 K
Tsys: 66 K 69 K
Amplifier noise coupling
BYURadio Astronomy Systems
Research GroupDesign Optimization Process
Computationally challenging!
Single Element
7 x 2 Element
Array
19 x 2 Element
Array
Forward EMModel
Sensitivity Cost Function
(System model -Reflector, LNAs, Receiver chains,
Beamformer)
Infinite Array
Unit Cell
Challenges:
Beamformer weights are required to compute efficiency and active impedances, but the weights are not known until the array it designed
The antenna design optimization couples the full system – array, receivers, beamforming algorithm!
BYURadio Astronomy Systems
Research GroupMeasured Noise Performance
Modeled On-Reflector Beam
Modeled/Measured Single Channel
Expected improvement due to active impedance matched array design - coupled reverse noise partially cancels forward noise
Hot load (absorber) and cold load (sky) can be used to characterize single-channel array noise performance, but measuring the beam equivalent noise temperature requires that array be mounted on-reflector (calibrated beamformer coefficients are needed)
BYURadio Astronomy Systems
Research GroupArecibo PAF Feasibility Study
Evaluate feasibility and capability of PAF arrays for the Arecibo Telescope.
Use observed BYU PAF data to directly estimate:
– Achievable field of view– Beam sensitivity and shape– Number of usable independent beams
– Required arrays size and element spacing– Focal surface shape– Optimal array placement for wide FOV
– Gregorian optics effects on PAFs– Calibration performance and stability
Study results will inform the design of a permanent PAF instrument
National Astronomy and Ionosphere Center
BYURadio Astronomy Systems
Research GroupThick Dipole “Carter” PAF on Arecibo Telescope
BYURadio Astronomy Systems
Research GroupPreliminary Results – Beam Patterns
National Astronomy and Ionosphere Center
BYURadio Astronomy Systems
Research GroupPreliminary Results - Sensitivity Map (m2/K)
National Astronomy and Ionosphere Center
Cross Elevation (Degrees)
Ele
vatio
n (D
egre
es)
-1 0 1
-1.5
-1
-0.5
0
0.5
1
1.5
0.
1
1.
2
2.
3
Thin dipole array on20-Meter (2008)
BYURadio Astronomy Systems
Research GroupNoise Temperature and Sensitivity Figure of Merit
~500 MHz 1 dB Sensitivity Bandwidth
1 1.2 1.4 1.6 1.8 20
50
100
150
200
250
300
Frequency (GHz)
Noi
se T
empe
ratu
re (K
)
Tlna (Modeled S-parameters)
Tsys (Modeled S-parameters)
Tsys/ηap (Modeled S-parameters)
Tsys/ηap (Measured)
BYURadio Astronomy Systems
Research GroupThick Dipole PAF on Green Bank 20-Meter
BYURadio Astronomy Systems
Research GroupThick Dipole PAF on Green Bank 20-Meter
BYURadio Astronomy Systems
Research Group40 Channel Data Acquisition System
BYURadio Astronomy Systems
Research GroupCold Sky / Warm Absorber Noise Tests
BYURadio Astronomy Systems
Research GroupSingle-channel Noise Temperatures
BYURadio Astronomy Systems
Research GroupSensitivity – Single Pol Thick Dipole PAF
1.2 1.4 1.6 1.8 20
1
2
3
4
5
Frequency
Sen
sitiv
ity
Sensitivity vs. Frequency
Model, max-SNRModel, center el. onlyMeasured, max-SNRMeasured, center el. only
BYURadio Astronomy Systems
Research GroupIssues / Problems Encountered
Many long struggles with data acquisition system Higher Tsys/efficiency than expected Formed beams showed higher noise variance than single
element powers (!)– Solution: correct for ADC card misalighment using delays rather than
phase shifts (even over < 1 MHz bandwidth) Difficult to image weak sources (!)
– How to precisely calibrate many formed beams?
BYURadio Astronomy Systems
Research GroupCrygenic PAF (April-May, 2011)
BYURadio Astronomy Systems
Research GroupFinished Dewar
BYURadio Astronomy Systems
Research GroupSingle Element Tests
BYURadio Astronomy Systems
Research GroupCro PAF
BYURadio Astronomy Systems
Research GroupCryo PAF in Ground Shield
BYURadio Astronomy Systems
Research GroupExperimental Results
BYURadio Astronomy Systems
Research GroupPreliminary Sensitivity
ModeledTsys/Efficiency
MeasuredTsys/Efficiency
Room Temp PAF 68 K 87 K
Cryo PAF (May 2011) 31 K 49.6 K
GBT L-band 29 K
BYURadio Astronomy Systems
Research GroupPAF “Calibration”
Absolute source intensity calibration (electronic cal) Radiometric calibration (on – off) Array calibration
– Initial array calibration - form beams with proper gains/phases– Correction for varying relative receiver chain gains/phases
Polarimetric calibration
BYURadio Astronomy Systems
Research GroupPolarimetric Calibration and Beamforming
One calibrator source observation provides a raw (uncalibrated) beam pair for each beam steering direction
Calibrate each using a “standard” method – multiple polarized source snapshots or one polarized source for several hours – produce a new beam pair with identity Jones matrix / Mueller matrix
One unpolarized source + two polarized sources One polarized source + one polarized source? – restricted
Mueller matrix How to efficiently calibrate all beams?
BYURadio Astronomy Systems
Research Group
Focal L-Band Array for Green Bank Telescope (FLAG)
X Engine: Correlator/Beamformer, Spectrometer, Pulsar
Array aperture, Antenna elements,LNAs, Cryo system,Down converters
•••
Sample clockfunction gen
and distribution
Ch. 1 ADC1 Gsps
ROACHFPGA
•••
1 TB SATARAID 0 Disk Array
Rack Mount PC
1 TB SATARAID 0 Disk Array
Rack Mount PC
20 p
ort 1
0Gb
Ethe
rnet
Sw
itch
Fujit
su X
G20
00 S
erie
s XF
P
F Engine:Direct RF sampling, digital down conversion and FFT
•••
•••
(× 10)
Attached PCs:System control and data storage(1/2 existing)
(×10)
(×10)
(× 40)
ADC1 Gsps
Ch. 4
ADC1 Gsps
ROACHFPGA
ADC1 Gsps
Ch. 40
ROACHFPGA
ROACHFPGA
(existing)
CX4 copper 10 Gbethernet links
LNA
Ant.
BPF
×
LO
Back End (Jansky Lab)Front End (GBT)
LNA
Ant.
BPF
×
LO
Signal Transport:Optical fiberand modems
BYURadio Astronomy Systems
Research GroupNext Steps
Finish characterization of cryogenic PAF Transition from 20-Meter to GBT demonstrator Complete 64 channel x 20 MHz back end After summer 2012:
– Formal funding for L-band work ends
– New funding for mm-wave GBT PAF back end (collaboration with UMASS)– Dual purpose back-end to serve L-band and mm-wave efforts– FLAG on GBT
– AO40 PAF on Arecibo– Support PAFSKA R&D– Commercial satcom phased arrays