Sascha Schediwyschediwy@physics.ox.ac.uk D-PAD Sparse Aperture Array.

transcript

Sascha Schediwy schediwy@physics.ox.ac.uk

D-PAD Sparse Aperture Array

Presentation Overview

D-PAD Aims What is D-PAD? Advantages of this design Recent results Future work

D-PAD Aims

Broadband SKA test system which will work in RFI environment

Develop an SKA1 AA-low compatible digital back-end processing system

Experimentally quantify the effect of side lobes on imaging dynamic range

Investigate novel direct imaging correlator algorithms

Compare calibration issues of aperture arrays with dishes of similar frequency

What is D-PAD?

D-PAD = Danny’s PhD Aperture-array Demonstrator

Key Values: f = 1000-1500MHz, 8 stations/tiles, sparse high-gain antennas

the first tile

What is D-PAD?

Least complex instrument possible that replicates most key aspects of an SKA aperture array

Flexible and reconfigurable hardware and software

Testbed for comparing measurements with simulations

Digital system can be recycled forAA-low and AA-mid test systems

Science with AA-high feasibility study;transients, solar, local HI, pulsars?

D-PAD High Gain Antenna Antenna element beam pattern@1300MHz)

Half power beam width: ±24°, directivity: 8.45dBi, sidelobe: -15dB, front-to-back: -18dB , ellipticity: 8%

D-PAD Analogue System

D-PAD Analogue SystemY-Polarisation

antenna blade

LNA LNA2-way 0° combiner

X-Polarisation

antenna blade

filterfilter

gain block

16-way 0° beamformer 16-way 0° beamformer

gain block

coax coax

filter

coax coax

gain block

gain blockfilter

D-PAD Digital System

16-port 10GbE switch

data acquisition computer

X2 Y2X1 Y1

F F F F

X4 Y4X3 Y3

F F F F

X6 Y6X5 Y5

F F F F

X8 Y8X7 Y7

iADC iADC

F F F F

10GbE 10GbE 10GbE

iADC iADCiADC iADCiADC iADC

Image credits CASPER

D-PAD Digital System

F Design Type Continuum 21-cm Line FPGA Clock 250MHz 75MHz Band Pass 500MHz 150MHz Nyquist Zone 3rd 9th Frequency Range 1000-1500 1350-1500Spectral Channels 2048 4096Spectral Resolution 488kHz 36.6kHz Velocity Resolution 51km/s 7.7km/s

Complimentary spectrometer designs

Milli-second, fast transient spectrometer design will be incorporated shortly

Image credits CASPER

Advantages of this Design

High-gain antenna results in greater sensitivity also reduces impact of sparse array grating lobes higher imaging dynamic range than sparse arrays with omni-

directional antennas

Sparse aperture arrays have faster survey speed small diameter stations = larger intrinsic Field-of-View than dishes

of same total collecting area: FoV = π (1.22*λ/d)2

Wide radio frequency bandwidth (500MHz) greater continuum sensitivity, greater flexibility

(compare with LOFAR and MWA; 32MHz )

Greater sensitivity for line surveys at higher redshift full collecting area over entire bandwidth: Aeff = G λc

Analogue beamformer reduces cost fewer receiver chains, less power, less computation

Fast ADCs means no complicated down conversion Direct sampling in 3rd Nyqusit zone

(same digital hardware as 30-470MHz SKA AA-low system)

Novel correlators reduce computational cost direct imaging correlators MOFF or FFTT

(close-packed tiles can use Fourier transform on a spatial grid ) MOFF can use FFT while traditional FX correlator must use DFT

Higher operating frequency, less demanding calibration lower sky brightness temperature, fewer bright in foreground

subtraction, less complex polarisation calibration

D-PAD Frequency Spectrum

1000 1100 1200 1300 1400 1500

Frequency (MHz)

Continuum Observations

20,000 spectra per polarisation over 5 days with 20s integration time

21cm Neutral Hydrogen Line

10,000 spectra per polarisation over 3 days with 17s integration time

Future Work

Detailed analysis of observations Millisecond transient spectrometer Array beam pattern measurement Construction of 8-tile system

Supplementary Slides

Greater sensitivity for line surveys at higher redshift full collecting area over entire bandwidth

effective area: Aeff = G λc

D-PAD Digital SystemX2 Y2

Finite Impulse Response Band-Pass Filter

Analogue to Digital Analogue to Digital

Fast Fourier Transform Real

Convert to Power

Vector Accumulate

BRAM BRAM

Vector Accumulate

BRAM BRAM

Cast/Slice

Packetise to 10GbE

Cast/Slice

Convert to Power

Radio Frequency Interference

Sidelobe Mitigation Techniques

Noise Figure Measurements

D-PAD Analogue Components LPDA Antenna (at boresight)

D-PAD Analogue Components Receiver Board (noise temperature ≈ 35K)

D-PAD Analogue Components Gain Amplifier

D-PAD Analogue Components Band-Pass Filter

D-PAD Analogue Components Beam-Forming Combiner

D-PAD Analogue Components Coaxial Cable A and C

D-PAD Analogue Components Coaxial Cable B

D-PAD Analogue System Total Gain

SKA Key Science Drivers

SKA Document Parameters

Grating Lobes

Station beam pattern

Antenna Element Separation

Station Beam Pattern

Antenna separation:dense (f < 1)nominal (f = 1)sparse (f > 1)random (f > 1)

[SKA Memo 87]

BeamPower

Instantaneous Field of View Fully Digital Dense A. Array 120deg

10,000deg2

18deg 250deg2

Hybrid Dense A. Array 120deg

10,000deg2

28deg 625deg2

18deg 250deg2

60m Dish + Phased Array Feed 120deg+

10,000deg2+ 18deg

250deg2

Sparse High Gain A. Array 36deg

1,000deg2

18deg 250deg2

100 200 300 500 700 1000 20000.01

Frequency (MHz)

Effective Area Effective Area: Aeff (θ,φ) = G (θ,φ) λc

sparse limit

Effective Area per Element

dense up to fc = 700MHz

dense up to fc = 1000MHz

[SKA Memo 100]

fc = 300MHz

Sky Brightness Temperature Tsky = 5e8 * f -2.861 + 4

SKA Memo 95 by Germán Cortés Medellín.

Sensitivity SKA Phase 1 Sensitivity

D-PADY-PolarisationX-Polarisation

filter

gain block

2-way 0° combiner

antenna blade

gain block

filter

Sascha Schediwyschediwy@physics.ox.ac.uk D-PAD Sparse Aperture Array.

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