Development of Focal-Plane Arrays and Beamforming Networks...
Transcript of Development of Focal-Plane Arrays and Beamforming Networks...
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Development of Focal-Plane Arrays andBeamforming Networks at DRAO
Bruce VeidtDominion Radio Astrophysical Observatory
Herzberg Institute of AstrophysicsNational Research Council of Canada
Penticton, BC
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Outline
� Key SKA and LAR specifications
� Vivaldi antenna work
� Beamforming network studies
� A possible development plan
� Concerns
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Key SKA and LAR Specifications
Frequency range 0.15 – 20 GHzPolarization both, 40 dB purityField of view 1 square-degree (@ 1.4 GHz)
scales with λ at higher freq’sNumber of beams > 100
Bandwidth (0.5+ν/5)GHz
Diameter 200 metresZenith angle 0 – 60◦
f/D 2.5Subtended angle 11 – 22◦
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LAR Geometry
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Why Focal-Plane Array?
� Make variable-ellipticity beam which is matched to foreshortenedreflector
� 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
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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
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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
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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 ∼200–800
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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
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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);
apill = twoD near field(fpat,hgrid, [0.23,0.55]);
phi steps = [0:10:90]; theta step= 1;
stack pat dB(polar far field(phi steps,theta step, apill));
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0 10 20 30 40 50 60 70 80 90
ampl
itude
, dB
i
theta, degrees
angle = 0.0angle = 10.0angle = 20.0angle = 30.0angle = 40.0angle = 50.0angle = 60.0angle = 70.0angle = 80.0angle = 90.0
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Focal Fields — Zenith (GRASP8)
� Frequency = 1.5 GHz
� Sampling interval = 0.16 m
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Focal Fields — 60 ◦
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Focal Fields — 60 ◦, Cross-pol
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Focal Fields — 60 ◦, Field-of-View Edge
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Test Array of Vivaldi Antennas
44 elements, dual polarized
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Electromagnetic Simulation of Small Vivaldi Array
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Mass-Reduction Method
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Calculated Pattern of Vivaldi Antenna
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Problem of Wideband Beamforming Networks
θ
x
x
Aperture Distribution Radiation Pattern
Frequency = 2 f
Frequency = f
θγ
γ/2
� Assume weights constant with frequency
� ⇒ Illumination is function of frequency
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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
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Coarse Channels
0.6 0.8 1. 1.2 1.4Wavelength, normalized
0.
5.
10.
15.
20.
Fig
ure
ofM
erit
Spectral Figures of Merit
� Figure of Merit: 290×ηillum×ηspill/(Tspill +Texcess)
� f/D = 0.4, 2.5
� Texcess= 9, 26K (upper, lower)
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One Possible BFN Architecture
LNA AGC
Sampler
FIRfilter
One beam in one sub-band
To other beams
Sub
-ban
ds
Oth
er e
lem
ents Σ
w
j-bits k-bits l-bits m-bits n-bits
{~3500 elements x 2 polarizations = 7000 ~1,000,000
Multipliers1800 Outputs
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Another BFN Architecture
(For acoustic beamforming, DeLap & Hero, 1993)
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Focal Fields
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-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Fie
ld M
agni
tude
Focal-Plane Displacement, m
Slice Through Focal-Plane Field Distribution
.75 GHz1 GHz
1.5 GHz
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FIR Filter Transfer Functions
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plitu
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egre
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Off-Axis Focal Fields
-4 -3 -2 -1 0 1 2 3 4-2
-1.5
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-0.5
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Off-Axis FIR Filter Transfer Functions
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plitu
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egre
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Frequency
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Status
� Directed by Bruce Veidt
� Vivaldi array design by Ed Reid (U of Alberta PhD student)
. objective is to have a 1×1 m2 array for experimentation
� Integrated low-noise amplifiers by Angel Garcia (U of Alberta MScstudent)
. 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
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A Possible Array Development Plan and Testbed
� Incremental approach
� Use DRAO 26-m dish astestbed?
. Test with astronomicalsources
. f/D = 0.31
. Feed angle = 160◦
. If fed at f/D = 0.36,◦ De f f = 22m
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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
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Antenna Element Design Plan
1. Understand Vivaldi-antenna design leading to a set of design rulesusable by designers
2. Understand inter-element coupling (impedance effects and noisecoupling 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
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Receiver Design Plan
1. Develop method to characterize the S- and noise-parameters ofHEMT devices
2. Understand feed-point impedance of Vivaldi
3. Adjust microstrip/stripline-to-slotline transition to optimally matchLNA to Vivaldi
4. Determine if LO injection is required (do we have to down-convert?)
5. Design LO/IF system if downconversion required
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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
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Digital Engineering Plan
1. Obtain and test commercial A/D’s (eg. Maxim MAX108 with 1.5Gs/s @ 8-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
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Algorithm Development Plan
1. Simple weighting scheme
2. “Optimized” weighting
3. Wide-band weights→ FIR filter weights
4. Interference mitigation
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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
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