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


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


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


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


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


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


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!


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)


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


Research GroupThick Dipole “Carter” PAF on Arecibo Telescope


Research GroupPreliminary Results – Beam Patterns



Research GroupPreliminary Results - Sensitivity Map (m2/K)


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)


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)


Research GroupThick Dipole PAF on Green Bank 20-Meter


Research Group40 Channel Data Acquisition System


Research GroupCold Sky / Warm Absorber Noise Tests


Research GroupSingle-channel Noise Temperatures


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


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?


Research GroupCrygenic PAF (April-May, 2011)


Research GroupFinished Dewar


Research GroupSingle Element Tests


Research GroupCro PAF


Research GroupCryo PAF in Ground Shield


Research GroupExperimental Results


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


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


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?


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


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

recent paf developments at byu/nraocsas.ee.byu.edu/pafska2011/presentations/byu phased arrays... ·...

Documents