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Phased Array Feed Development

Bill Shillue, Anish Roshi, Bob Simon, Steve White, John Ford

Atacama Large Millimeter/submillimeter Array Expanded Very Large Array Robert C. Byrd Green Bank Telescope Very Long Baseline Array


Acknowledgements
· Rick Fisher, Roger Norrod...many others at NRAO · Karl Warnick, Brian Jeffs, Jonathan Landon, David Jones, Jacob Waldran, and other students at Brigham Young University


Outline
· Introduction and Motivation · PAF History · PAF Hardware Front-ends (Analog stuff) Digital Signal Processing · Some Problems and Solutions


Introduction


What is a Phased Array Feed?


A feed system for a dish antenna that is made of elements that spatially sample the focal plane Consists of antennas, amplifiers, and signal processing hardware




How does the operation of a phased array feed differ from a multi-pixel focal plane array?


Introduction

Basically, by using different materials to form beams
­

Silicon vs Aluminum


Introduction

From Jeffs, et al.


Motivation


So what's the big deal? -- Digital Beamforming!


Beams can be formed that fully sample the sky
­

Not possible with traditional array receivers Not possible with traditional array receivers



Beams can be individually steered on the sky.
­



More beams with less weight on the front-end Possible to place a null on an interfering source




Introduction and Motivation


Great! But what's the catch?


Beamforming is complicated.
­ ­

More front-end hardware More back-end hardware



Current state of the art in computing limits bandwidth to much less than can be done with traditional feeds A radio astronomy PAF is much different than a Phased array antenna for a communication system




Characteristics of RA PAF


Radio Astronomy signals are very weak


SNRs of -50dB or lower are routine



Ultra wide band, as compared to communications applications of phased array technology Need extreme stability so that signals may be integrated over long time periods to beat down the noise Since RA signals are so weak, RFI excision and removal desired







Achieving low noise, wide bandwidths, extreme stability is very challenging!


0

NRAO PAF History
Year
1996 2007 2011

Array tested
19-element sinuous antenna array 19-element, uncooled, thin dipoles 20-m antenna impedance optimized, uncooled, single-pol dipoles, 20-m antenna cryogenic, SiGe LNAs , dual-pol dipole array, 20-m antenna cryogenic, SiGe LNAs , dual-pol dipole array, GBT 100-m antenna

Result
Tsys/ap unknown Tsys/ap = 96 K (Tsys ~ 66 K) Tsys/ap = 87 K (Tsys ~ 61 K) Tsys/ap = 50 K (Tsys ~ 35 K)

2011 2013

? ??


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NRAO PAF History
Sinuous elements 140-ft 1996 Thin Dipoles 20-meter 2007

Thick, impedance-optimized dipoles, 20-meter, 2011

Cryogenic, Weinreb SiGe LNAs, 20-meter, 2011


Phased Array Feeds
12 10 8 6 4 2 0 Ro w 2 Ro w 1 Ro w 3 Ro w 4 Co lumn 1 Co lumn 2 Co lumn 3


3

PAF Metrics


4

Current NRAO PAF Receiver

· ·

Cooled LNA receiver-dewar, CTI-1020 refrigerator, 19 element dual polarization Receiver is cold and outdoor range testing is underway


5

Dipole Elements
"Kite" Element, BYU design for 20-m telescope 2009

New GBT2 Element, BYU design 2013, optimized for best efficiency on GBT (over seven dimension parameters)


6

Dipole Elements


7

Element Optimization (BYU)


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Dual-Polarization LNAs and Thermal Transition
NXP SiGe transistors. Surface mount components. Thin-wall SS tubular coax. Quartz beads for vacuum seal and center conductor heat sink. Est. input coax heat load 150 mW per channel Bias power 17 mW / channel Pair of two-channel LNAs with integrated low-loss coaxial lines for transition from 15 to 300K, vacuum seal, and antenna base LNA based on: S. Weinreb, interface. low-noise amplifiers for rad
044702, 2009.

J. Bardin, H. Mani, G. Jones, "Matched wideband io astronomy", Rev. of Sci. Instr., vol. 80,


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LNA Measured Performance

· Noise Y-factor measured with LN2 cold load at room temperature SMA connector.


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GBT PAF Demonstration System

Single channel only represented


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Element Patterns
1 Elevat ion (Degrees) -1 0.8 0.6 0 0.4 1 -1 0 1 Cross Elevat ion (Degrees ) 0.2 0

1 Elevat ion (Degrees)

1 Elevat ion (Degrees) Elevat ion (Degrees) -1 0.8 0.6 0 0.4 1 -1 0 1 Cross Elevat ion (Degrees ) 0.2 0 -1 0 1 Cross Elevat ion (Degrees ) -1

1 0.8 0.6 0 0.4 1 0.2 0

-1

0.8 0.6

0 0.4 1 -1 0 1 Cross Elevation (Degrees ) 0.2 0


2

Cygnus-X Region Mosaic

4 Elevation (Degrees) 2

150

100 0 -2 -4 -4 50

-2 0 2 4 Cross El evation (Degrees)

0

Canadian Galactic Plane Survey Convolved to 20-m Beam


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First test on GBT
· · Scheduled June July 2013 Completed tasks ­ New fiber installations between GBT tape room, GBT, and outdoor test facility ­ Receiver integration with fiber transmitters ­ Repair of receiver vacuum leaks ­ Downconverters, fiber receivers, signal sources, ADCs installed in GBT "tape" room ­ Software for data acquisition, monitoring, GBT grid offset pointing, software correlator, and interfaces for BYU backend ­ GBT fiber cabling ­ amplifier rework ­ PAF receiver installed and tested in outdoor test building


4

GBT PAF Future Directions
· Wider dipole spacing optimized for GBT F/D · Increase number of elements from 19x2 to 37x2 · Verification of electromagnetic modeling · New, lower noise LNAs · Development of real time wideband digital backend · Demonstration of improvement over L-Band single pixel feed · GBT Science instrument · Cooling of elements? · Improvements in receiver size/cost/weight


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AO-19 Phased Array Feed System


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AO-19 System Description
· 19 element test system for the Arecibo antenna · Cooled system Including the feeds LNA noise temperature of < 10k Feeds and LNAs cooled to 18K · 1200 to 1800 MHz


7

AO-19 System


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AO-19 System LNA/Feed

element


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AO-19 System Dewar

element


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Wideband Backend Design Proposal

ingle channel only represented


1

Signal Processing Block Diagram


2

Beamforming Calibration
· Optimize Ta/Tsys at each beam position in the FOV
­ Use strong radio source ­ Form cross-correlation matrix of all elements on and off source ­ Solve for complex weights

· Experience thus far is that calibration is stable for hours and possibly days in the experimental setup. · Calibration method does not distinguish between various noise and efficiency factors.


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Challenges for deploying the PAF for science
· Tsys ­ Mutual coupling increases the noise at the antenna, reducing PSS · System Complexity and cost Analog Systems Amplifiers Mixers ADCs Digital Systems Beamformer Correlator · System integration, training, and acceptance by users