Sascha D-PAD Sparse Aperture Array

Sascha Presentation Overview  D-PAD Aims  What is D-PAD?  Advantages of this design  Recent results  Future work

Sascha D-PAD Aims  Broadband SKA test system which will work in RFI environment  Develop an SKA 1 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

Sascha What is D-PAD?  D-PAD = Danny’s PhD Aperture-array Demonstrator  Key Values:  f = MHz, 8 stations/tiles, sparse high-gain antennas the first tile

Sascha 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 for AA-low and AA-mid test systems  Science with AA-high feasibility study; transients, solar, local HI, pulsars?

Sascha D-PAD High Gain Antenna  Antenna element beam  Half power beam width: ±24°, directivity: 8.45dBi, sidelobe: -15dB, front-to-back: -18dB, ellipticity: 8%

Sascha D-PAD Analogue System

Sascha D-PAD Analogue System Y-Polarisation antenna blade LNA 2-way 0° combiner X-Polarisation amp2 coax antenna blade LNA 2-way 0° combiner amp2 coax filter gain block 16-way 0° beamformer gain block coax filter coax gain block filter

Sascha 10GbE D-PAD Digital System 16-port 10GbE switch data acquisition computer 10GbE X 2 Y 2 X 1 Y 1 ROACH FFFF X X 4 Y 4 X 3 Y 3 ROACH FFFF X 6 Y 6 X 5 Y 5 ROACH FFFF X 8 Y 8 X 7 Y 7 iADC ROACH FFFF 10GbE iADC Image credits CASPER

Sascha D-PAD Digital System F Design TypeContinuum21-cm Line FPGA Clock250MHz75MHz Band Pass500MHz150MHz Nyquist Zone3 rd 9 th Frequency Range Spectral Channels Spectral Resolution488kHz36.6kHz Velocity Resolution51km/s7.7km/s  Complimentary spectrometer designs  Milli-second, fast transient spectrometer design will be incorporated shortly Image credits CASPER

Sascha 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: A eff = G λ c 2 /4π

Sascha Advantages of this Design  Analogue beamformer reduces cost  fewer receiver chains, less power, less computation  Fast ADCs means no complicated down conversion  Direct sampling in 3 rd Nyqusit zone (same digital hardware as MHz 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

Sascha D-PAD Frequency Spectrum Frequency (MHz) Arb. Power (dB)

Sascha Continuum Observations  20,000 spectra per polarisation over 5 days with 20s integration time

Sascha 21cm Neutral Hydrogen Line  10,000 spectra per polarisation over 3 days with 17s integration time

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

Sascha Supplementary Slides

Sascha Advantages of this Design  Greater sensitivity for line surveys at higher redshift  full collecting area over entire bandwidth effective area: A eff = G λ c 2 /4π

Sascha

Sascha D-PAD Digital System X 2 Y 2 Finite Impulse Response Band-Pass Filter Analogue to Digital Fast Fourier Transform Real Convert to Power Vector Accumulate BRAM Vector Accumulate BRAM Cast/Slice Packetise to 10GbE Cast/Slice Convert to Power X 1 Y 1

Sascha Radio Frequency Interference

Sascha Sidelobe Mitigation Techniques

Sascha Noise Figure Measurements

Sascha D-PAD Analogue Components  LPDA Antenna (at boresight)

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

Sascha D-PAD Analogue Components  Gain Amplifier

Sascha D-PAD Analogue Components  Band-Pass Filter

Sascha D-PAD Analogue Components  Beam-Forming Combiner

Sascha D-PAD Analogue Components  Coaxial Cable A and C

Sascha D-PAD Analogue Components  Coaxial Cable B

Sascha D-PAD Analogue System  Total Gain

Sascha SKA Key Science Drivers

Sascha SKA Document Parameters

Sascha 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] Beam Power

Sascha Instantaneous Field of View  Fully Digital Dense A. Array  120deg  10,000deg 2  18deg  250deg 2  Hybrid Dense A. Array  120deg  10,000deg 2  28deg  625deg 2  18deg  250deg 2  60m Dish + Phased Array Feed  120deg+  10,000deg 2 +  18deg  250deg 2  Sparse High Gain A. Array  36deg  1,000deg 2  18deg  250deg 2

Sascha Effective Area  Effective Area: A eff (θ,φ) = G (θ,φ) λ c 2 /4π sparse limit Effective Area per Element dense up to f c = 700MHz dense up to f c = 1000MHz fcfc fcfc [SKA Memo 100] f c = 300MHz

Sascha Sky Brightness Temperature  T sky = 5e8 * f SKA Memo 95 by Germán Cortés Medellín.

Sascha Sensitivity  SKA Phase 1 Sensitivity

Sascha D-PAD Y-PolarisationX-Polarisation filter gain block 2-way 0° combiner antenna blade LNA 2-way 0° combiner amp2 coax antenna blade LNA 2-way 0° combiner amp2 coax gain block coax filter coax iADC ROACH F 10GbE GPU dispersion measure PC