Development of Focal-Plane Arrays and Beamforming Networks at DRAO

Transcription

1 Development of Focal-Plane Arrays and Beamforming Networks at DRAO Bruce Veidt Dominion Radio Astrophysical Observatory Herzberg Institute of Astrophysics National Research Council of Canada Penticton, BC

2 Outline Key SKA and LAR specifications Vivaldi antenna work Beamforming network studies A possible development plan Concerns 1

3 Key SKA and LAR Specifications Frequency range Polarization Field of view GHz both, 40 db purity 1 square-degree (@ 1.4 GHz) scales with λ at higher freq s Number of beams > 100 Bandwidth (0.5 + ν/5)ghz Diameter Zenith angle 200 metres 0 60 f /D 2.5 Subtended angle

4 LAR Geometry 3

5 Why Focal-Plane Array? Make variable-ellipticity beam which is matched to foreshortened reflector Multiple overlapping beams this is how we get 1 square-degree with a large aperture Correct optically-induced polarization doubles size of beamformer Correct for astigmatism increases (slightly) size of array increases number of inputs to beamformer 4

6 Sampling Requirements Must avoid grating lobes might point at ground Electronically-steered phased array: spacing λ/2 Focal-plane arrays (not electronically steered): short focal-length systems: spacing λ/2 long focal-length systems: spacing λ LAR with Vivaldi antennas on square grid: spacing 0.8λ LAR focal-spot size (full-width, half-power) 2.5λ 5

7 Why Vivaldi s? Printed circuit fabrication Can be packed as close as λ/10 Bandwidth up to 5:1 Possible to integrate low-noise amplifier on board Get both polarizations 6

8 LAR Array Size 1 square-degree 1.13 diameter (at 1.4 GHz) 1.13 on sky 9.86 metre in focal plane (half-power level) Include skirts of beam + foreshortening effects 10.7m 12m Assume 0.8λ sampling 4000 elements/polarization Elements/focal spot

9 Simulation Tools Octave-based focal-plane array simulator (array theory) Focal-plane array analysis and design GRASP8W reflector antenna simulator (PO+PTD) Optical calculations Micro-Stripes general-purpose EM simulator (TLM) Vivaldi-element design 8

10 Focal-Plane Array Simulator za = 0; taper = 15; n tiers = 6; spacing =.9; hgrid = hex grid(n tiers, spacing); half ang = 11; flat rad =.5; fpat = flat top pat(half ang, flat rad, taper, za, hgrid); amplitude, dbi angle = 0.0 angle = 10.0 angle = 20.0 angle = 30.0 angle = 40.0 angle = 50.0 angle = 60.0 angle = 70.0 angle = 80.0 angle = 90.0 apill = twod near field(fpat, hgrid, [0.23,0.55]); phi steps = [0:10:90]; theta step = 1; theta, degrees stack pat db(polar far field(phi steps, theta step, apill)); 9

11 Focal Fields Zenith (GRASP8) Frequency = 1.5 GHz Sampling interval = 0.16 m 10

12 Focal Fields 60 11

13 Focal Fields 60, Cross-pol 12

14 Focal Fields 60, Field-of-View Edge 13

15 Test Array of Vivaldi Antennas 44 elements, dual polarized 14

16 Electromagnetic Simulation of Small Vivaldi Array 15

17 Mass-Reduction Method 16

18 Calculated Pattern of Vivaldi Antenna 17

19 Problem of Wideband Beamforming Networks Frequency = f x γ θ Frequency = 2 f x γ/2 θ Aperture Distribution Radiation Pattern Assume weights constant with frequency Illumination is function of frequency 18

20 Possible Solutions Break spectrum up into coarse channels (analog or digital) Design a digital filter with matched frequency response can work with wider bandwidth Combine two techniques channelize design channelizing filters to compensate Do Fourier Transform (fine channelization) before BFN 19

21 Coarse Channels 20. Spectral Figures of Merit Figure of Merit Wavelength, normalized Figure of Merit: 290 η illum η spill /(T spill + T excess ) f /D = 0.4, 2.5 T excess = 9, 26K (upper, lower) 20

22 One Possible BFN Architecture LNA AGC { j-bits k-bits l-bits m-bits n-bits Sampler FIR filter Sub-bands w Other elements Σ To other beams One beam in one sub-band ~3500 elements x 2 polarizations = 7000 ~1,000,000 Multipliers 1800 Outputs 21

23 Another BFN Architecture (For acoustic beamforming, DeLap & Hero, 1993) 22

24 Focal Fields Slice Through Focal-Plane Field Distribution.75 GHz 1 GHz 1.5 GHz Field Magnitude Focal-Plane Displacement, m 23

25 FIR Filter Transfer Functions 0-5 Amplitude, db Frequency 150 Phase, degrees Frequency 24

26 Off-Axis Focal Fields

27 Off-Axis FIR Filter Transfer Functions 0 Amplitude, db Frequency 150 Phase, degrees Frequency 26

28 Status Directed by Bruce Veidt Vivaldi array design by Ed Reid (U of Alberta PhD student) objective is to have a 1 1 m 2 array for experimentation Integrated low-noise amplifiers by Angel Garcia (U of Alberta MSc student) develop design techniques and to design an integrated probe/lna for Synthesis Telescope Beamforming network research by Bruce Veidt investigate means to correct frequency aberrations of array examine possible architectures estimate processing power required 27

29 A Possible Array Development Plan and Testbed Incremental approach Use DRAO 26-m dish as testbed? Test with astronomical sources f /D = 0.31 Feed angle = 160 If fed at f /D = 0.36, D e f f = 22m 28

30 A Focal-Plane Array Development Plan 1. Array (elements + rx s) + analog (narrow-band) BFN (single beam) 2. Array + analog BFN + additional BFN s (multi-beam) 3. Array + analog fibre optic system to ground + analog BFN s 4. Array + FOTS + A/D + digital BFN (single beam) 5. Array + FOTS + A/D + DBFN (multi-beam) 6. Array + FOTS + A/D + programmable DBFN (single beam) 7. Array + FOTS + A/D + programmable DBFN (multi-beam) 8. Array + A/D + digital FOTS + PDBFN 29

31 Antenna Element Design Plan 1. Understand Vivaldi-antenna design leading to a set of design rules usable by designers 2. Understand inter-element coupling (impedance effects and noise coupling effects) 3. Understand microstrip/stripline-to-slotline transitions 4. Investigate methods to reduce weight of array (holes, length, alternative substrates) 5. Design modular element + support framework 30

32 Receiver Design Plan 1. Develop method to characterize the S- and noise-parameters of HEMT devices 2. Understand feed-point impedance of Vivaldi 3. Adjust microstrip/stripline-to-slotline transition to optimally match LNA to Vivaldi 4. Determine if LO injection is required (do we have to down-convert?) 5. Design LO/IF system if downconversion required 31

33 Fibre-Optic Transmission System Design Plan 1. Evaluate analog optical link 2. Evaluate digital optical link 3. Evaluate high data-rate FOTS 4. Investigate low-mass/low-power FOTS modulators 32

34 Digital Engineering Plan 1. Obtain and test commercial A/D s (eg. Maxim MAX108 with bits) 2. How to partition data amongst beams 3. A/D + digital BFN (simple: weights + summing) 4. Investigate how to make FIR filters with time-variable coefficients 5. A/D + FIR + DBFN 6. A/D + FIR + multi-beam DBFN 7. A/D + programmable DBFN 33

35 Algorithm Development Plan 1. Simple weighting scheme 2. Optimized weighting 3. Wide-band weights FIR filter weights 4. Interference mitigation 34

36 Concerns Noise coupling between elements Low-mass materials Modify conventional structures to reduce mass Something new, such as metal patterns on flexible insulating film Transition to slotline Conventional approach: integrated into antenna structure Modular approach Fabrication and assembly 35