Atacama Large Millimeter/submillimeter Array

Expanded Very Large Array

Robert C. Byrd Green Bank Telescope

Very Long Baseline Array

Phased Array Feed Development

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

Acknowledgements2

• 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

Outline3

• 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

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

● 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● Beams can be individually steered on the sky.

– Not possible with traditional array receivers● 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!

NRAO PAF History10

Year Array tested Result1996 19-element sinuous antenna

arrayTsys/ηap unknown

2007 19-element, uncooled, thin dipoles 20-m antenna

Tsys/ηap = 96 K (Tsys ~ 66 K)

2011 impedance optimized, uncooled, single-pol dipoles, 20-m antenna

Tsys/ηap = 87 K (Tsys ~ 61 K)

2011 cryogenic, SiGe LNAs , dual-pol dipole array, 20-m antenna

Tsys/ηap = 50 K (Tsys ~ 35 K)

2013 cryogenic, SiGe LNAs , dual-pol dipole array, GBT 100-m antenna

???

NRAO PAF History11

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

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Row 3Row 4

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PAF Metrics

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Current NRAO PAF Receiver14

• Cooled LNA receiver-dewar, CTI-1020 refrigerator, 19 element dual polarization

• Receiver is cold and outdoor range testing is underway

Dipole Elements15

“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)

Dipole Elements16

Element Optimization (BYU)17

Dual-Polarization LNAs and Thermal Transition

18

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 channelBias 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 interface. LNA based on: S. Weinreb, J. Bardin, H. Mani, G. Jones, “Matched wideband

low-noise amplifiers for radio astronomy”, Rev. of Sci. Instr., vol. 80, 044702, 2009.

LNA Measured Performance19

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

GBT PAF Demonstration System20

Single channel only represented

Element Patterns21

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Cross Elevation (Degrees)E

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Cygnus-X Region Mosaic

Cross Elevation (Degrees)

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Canadian Galactic Plane SurveyConvolved to 20-m Beam

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

23

GBT PAF Future Directions24

• 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

AO-19 Phased Array Feed System25

AO-19 System Description26

• 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

AO-19 System27

AO-19 System LNA/Feed28

element

AO-19 System Dewar29

element

Wideband Backend Design Proposal30

Single channel only represented

Signal Processing Block Diagram31

Beamforming Calibration32

• 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.

Challenges for deploying the PAF for science

33

• 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