Development of Focal-Plane Arrays andBeamforming Networks at DRAO

Bruce VeidtDominion Radio Astrophysical Observatory

Herzberg Institute of AstrophysicsNational Research Council of Canada

Penticton, BC

bruce.veidt@nrc.ca

Outline

� Key SKA and LAR specifications

� Vivaldi antenna work

� Beamforming network studies

� A possible development plan

� Concerns

1

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◦

2

LAR Geometry

3

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

4

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

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

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

7

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

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));

-20

-15

-10

-5

0

5

10

15

20

25

30

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

9

Focal Fields — Zenith (GRASP8)

� Frequency = 1.5 GHz

� Sampling interval = 0.16 m

10

Focal Fields — 60 ◦

11

Focal Fields — 60 ◦, Cross-pol

12

Focal Fields — 60 ◦, Field-of-View Edge

13

Test Array of Vivaldi Antennas

44 elements, dual polarized

14

Electromagnetic Simulation of Small Vivaldi Array

15

Mass-Reduction Method

16

Calculated Pattern of Vivaldi Antenna

17

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

18

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

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)

20

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

21

Another BFN Architecture

(For acoustic beamforming, DeLap & Hero, 1993)

22

Focal Fields

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

-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

23

FIR Filter Transfer Functions

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-5

0

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Am

plitu

de, d

B

Frequency

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-50

0

50

100

150

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Pha

se, d

egre

es

Frequency

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-5

0

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Am

plitu

de, d

B

Frequency

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-50

0

50

100

150

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Pha

se, d

egre

es

Frequency

-30

-25

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-5

0

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Am

plitu

de, d

B

Frequency

-150

-100

-50

0

50

100

150

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Pha

se, d

egre

es

Frequency

-30

-25

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-5

0

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Am

plitu

de, d

B

Frequency

-150

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-50

0

50

100

150

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Pha

se, d

egre

es

Frequency

-30

-25

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-5

0

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Am

plitu

de, d

B

Frequency

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0

50

100

150

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Pha

se, d

egre

es

Frequency

24

Off-Axis Focal Fields

-4 -3 -2 -1 0 1 2 3 4-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

25

Off-Axis FIR Filter Transfer Functions

-20

-15

-10

-5

0

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Am

plitu

de, d

B

Frequency

-150

-100

-50

0

50

100

150

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Pha

se, d

egre

es

Frequency

-20

-15

-10

-5

0

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Am

plitu

de, d

B

Frequency

-150

-100

-50

0

50

100

150

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Pha

se, d

egre

es

Frequency

26

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

27

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

28

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

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

30

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

31

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

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

33

Algorithm Development Plan

1. Simple weighting scheme

2. “Optimized” weighting

3. Wide-band weights→ FIR filter weights

4. Interference mitigation

34

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