phased array systems - university of groningen

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Phased Array SystemsWim van Cappellen, October 10th, 2006

Phased Array SystemsPhased Array Systems

W. van CappellenASTRON


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OutlineOutline

• Introduction• Characteristics of phased arrays• Implementation• LOFAR• Phased Arrays in the SKA

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IntroductionIntroduction• A single antenna element has a relatively low

directivity• Directivity can be increased by increasing the

electrical size of the antenna1. Increase the size of the single element (horn, reflector)2. Combine multiple small elements into an array

• Phased array antennas consist of multiple stationary antenna elements, which are fed coherently and use variable phase or time-delay control at each element to scan a beam to given angles in space.


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IntroductionIntroduction

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THousandTHousand Element Array (THEA)Element Array (THEA)


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ArrayArray antennesantennes

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AdvantagesAdvantages

• Increased gain• Electronic beam steering (no moving parts)• Electronic beam shaping (nulls, sidelobe

levels)• Fast reaction time• Transient buffer• Simultaneous operation (multi beam)• Reliability, graceful degradation


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THEA meting van de THEA meting van de melkwegmelkweg

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GPS GPS satellietensatellieten


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DisadvantagesDisadvantages

• Complexity• Scan loss • Calibration is more complex• Distributed LNA’s, cooling difficult

θcos∝

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Radiation characteristicsRadiation characteristics

Total field is vector addition of single elementsPatterns depends on:

1. Geometrical configuration overall array2. Separation between elements3. Excitation amplitude of individual elements4. Excitation phase of individual elements5. Relative pattern of individual elements


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FarFar--field of a 2 element arrayfield of a 2 element array

⎭⎬⎫

+⎩⎨⎧

=+=−−−−

22

)]2/([

11

)]2/([0

21 coscos4

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θθπ

ηββ

θ re

relkIjaEEE

krjkrj

t)

λπ2

=k

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⎭⎬⎫

+⎩⎨⎧

=+=−−−−

22

)]2/([

11

)]2/([0

21 coscos4

21

θθπ

ηββ

θ re

relkIjaEEE

krjkrj

t)

rrr

drrr

≅≅

−≅≅

≅≅

21

21

21

cos2

θ

θθθ

(amplitude)

(phase)


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⎥⎦⎤

⎢⎣⎡ +⋅=

−

)cos(21cos2cos

40 βθθπ

ηθ kdr

lekIjaEjkr

t)

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⎥⎦⎤

⎢⎣⎡ +⋅=

−

)cos(21cos2cos

40 βθθπ

ηθ kdr

lekIjaEjkr

t)

Et = Esingle element X Array Factor

In the first order we can separate the element and array characteristics!

(mutual coupling is neglected)


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Array FactorArray Factor

• N-element linear array– complex weightings an

• In general

θψ

ψ

cos1

)1(

kd

eaAFN

n

njn

=

= ∑=

−

∑ ⋅= rrjkn

neaAFrr

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Properties of the array factorProperties of the array factor

• To scan to angle θ = θ0 :0

0

cos0cos

θββθ

kdkd

−==+

d

θ0d cosθ02d cosθ03d cosθ0


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Array factor of 4 elements arrayArray factor of 4 elements array

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Grating lobesGrating lobes

• When the spacing d > λ/2 multiple maxima can be formed

πθθλπ 2)cos(cos2

0 ⋅±=− md K,2,1,0=m

0coscos0cos

0 =−=+

θθβθkdkd

kd

dmλθθ += 0coscos


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wavelength

d

grating lobes

main lobe

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• Criterion for maximum element spacing(grating lobes at the horizon)

• Example: an array scanning up to θ = 30°requires d < 0.53 λ to avoid grating lobes

0cos11

θλ +≤

d


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Other beam characteristicsOther beam characteristics

3-dB Beamwidth

Directivity (Gain)

Effective area

DBb

λθ 886.03 =

33

0cos400,32yx

Dθθθ

=

GAe πλ4

2

=

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Amplitude taperingAmplitude tapering

• Sidelobe reduction• Increases beamwidth

– Expressed in Beam Broadening Factor (Bb)• Lowers gain

– Expressed in taper efficiency ( ετ )– Uniform dense array:

∑∑= 2

2

n

n

aN

aτε


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Example: amplitude taperingExample: amplitude tapering

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Element configurationElement configuration

• Definition:– Dense array: element spacing < λ/2– Sparse array: element spacing > λ/2


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Element configurationElement configuration

Field of View ConstantSmaller

Many low onesHigher, smooth (angle, f)

Steep decrease with wavelengthsmooth (angle, f)

Depend on position

Few high onesHigher, not smooth (angle, f)

Steep decrease with wavelengthnot smooth (angle, f)

Constant for most elements

Grating lobesReceiver tempEffective area

Element patterns

Sparse

NoLower, smooth (angle, freq)

Constant over frequency, smooth over angleDepend on position

Large

Grating lobesReceiver tempEffective areaElement patternsField of View

Dense

Lowered by space taperLowered by gain taperSidelobes

IrregularRegular

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Station beam patternsStation beam patterns

• Regular array, d = λ Irregular array


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Effective areaEffective area

dense array sparse arrayλ/2

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Effective areaEffective area

Regular (λ/2 @ 50 MHz) Irregular


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Effects of element spacingEffects of element spacing

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NoiseNoise

• Array noise– Receiver temperature (LNA noise, line losses)– Noise coupling– External noise sources (ground, sky)


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ImplementationImplementation

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Generic Rx array systemGeneric Rx array system

T4T4T1T1 T2T2 T3T3

ΣΣ

LNA

Implementation can be analog, digital or hybrid


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Time delay Time delay vsvs phase shiftphase shift• Ideally, time delays are required• For a single frequency, a phase shift has the

same effect• Offset frequencies result in beam squint

(similar to scanning)• Fractional bandwidth depends on size of

array (L):


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Time delay Time delay vsvs phase shiftphase shift

Two solutions:

1. True Time Delay beam former2. Beam forming in narrow bands (LOFAR)

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True Time Delay True Time Delay beamformerbeamformer

• Optical switched delay implementation

Loss: 12 dBHxBxD: 5 x 35 x 45 cm

Fiber based beamformer


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Time delay Time delay vsvs phase shiftphase shiftMeasured beam pattern

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Analog Analog beamformersbeamformers


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Dynamic range / linearityDynamic range / linearity

• Spatial selectivity is obtained after beam forming

• All components in front of the beam former need to handle the full RFI spectrum

• Example: – WSRT 2 bit ADC (spatial filtering by telescope)– LOFAR 12 bit ADC (array)

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LOFAR LBA RFI spectrumLOFAR LBA RFI spectrum


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Clock & LO distributionClock & LO distribution

• All AD convertors need to sample at exactly the same time

• Clock & LO generation is relatively easy• Clock & LO distribution is difficult

• Inaccurate sample clock reduces the effective number of bits


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CalibrationCalibration

• Amplitude and phase of all signal chains must be calibrated to compensate for– Unequal line lengths– Component variations– Mutual coupling, edge effects

• Calibration on– External sources– Internal sources (e.g. pilot tone injection)

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Example calibration resultExample calibration result

−10

−9

−8

−7

−6

−5

−4

−3

−2

−1

0Measured relative ε amplitude [dB], f = 800 MHz.

−5.3 −3.9 −4.1 −5.1 −6.3 −5.2 −4.8 −5.0

−2.9 −2.1 −2.1 −3.6 −2.9 −3.6 −3.2 −1.8

−1.0 −0.4 −1.0 −1.1 −2.5 −1.1 −1.2 −1.9

−0.9 0.0 −0.6 −1.2 −1.2 −0.3 −0.0 −0.7

−1.7 −0.7 −1.2 −1.6 −2.5 −1.3 −0.9 −2.0

−1.9 −1.5 −1.3 −1.9 −2.1 −2.1 −1.7 −2.5

−4.5 −3.6 −4.0 −4.6 −5.0 −4.3 −3.3 −3.3

−5.3 −4.7 −3.5 −4.9 −5.2 −4.8 −3.7 −3.7

0 1 2 3 4 5 6 7

0

1

2

3

4

5

6

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Example of spatial filtering (LOFAR)Example of spatial filtering (LOFAR)

transmitter at horizon (26.75 MHz)

after filtering (26.75 MHz)no interference (26.89 MHz)

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47Low Frequency ArrayLow Frequency Array

From steel to software...From steel to software...


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Remote Station ArchitectureRemote Station Architecture

120-240 MHz

30-80 MHz

Optional10- … MHz

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Key NumbersKey Numbers

Description Unity

160 MHz 200 MHz

Subband width 156 195 kHz

Number of beamlets 206 165

Value for fs of

Description Value Unity

# subbands 512

Max. number of beams (B = 4 MHz) 8

Min. number of beams (B = 32 MHz) 1

A/D converter resolution 12 bit

Sample frequency 200 / 160 MHz

Number of polarizations 2

Output word width (complex) 16+16 bit

Aggregate output bandwidth 32 MHz

Output data rate 2048 Mbit/s

Transient buffer storage period 1 s

Input Input dataratedatarate::460 460 Gbit/sGbit/s

Output Output dataratedatarate::2 2 Gbit/sGbit/s


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High Band Antenna (120High Band Antenna (120--240 MHz)240 MHz)

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ReCeiverReCeiver UnitUnit


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Remote Station Processing Remote Station Processing BoardBoard

• 90 nm technology• 192, 18x18 bit multipliers

running @ 200 MHz• 1020 “balls” on chip

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BeamformingBeamforming ArchitectureArchitecture

X Y X Y X Y X Y

RX TX

FILTER

ADD

FILTER

ADD

FILTER

ADD

FILTER

ADD


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Initial Test StationInitial Test Station

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ProjectenProjecten sky seen by LOFARsky seen by LOFAR

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Cosmic ray detectionCosmic ray detectionTransient buffer to look back in time


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43.000.000.000.000 vermenigvuldigingen per seconde

250.000 CD roms per dag

6 CD roms per seconde

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Wide area networkWide area network• Data transport from stations and central core to central

processor facility• Dedicated fiber connection between core and central

processorCentral Processing

Facility

Central Core

up to 800 Gbps bandwidth10 GbE CWDM 8 channelslength ~70 km


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BG/LRack

BG/LRack

BG/LRack

BG/LRack

GbE

sw

itch

GbE

switc

hG

bEsw

itch

GbE

switc

hG

bEsw

itch

10G

bEsw

itch

10G

bEsw

itch

10G

bEsw

itch

10G

bEsw

itch

10G

bEsw

itch

Clu

ster

of s

erve

rs4B

G R

AM/n

ode

Infin

iban

d in

terc

onne

c t

Cluster of serversgeneral purpose nodesInfiniband interconnect

Clu

ste r

of s

erve

rs10

TB

RA I

D p

er n

ode

Infin

iban

d in

terc

onne

ct

Cluster of serversgeneral purpose nodesInfiniband interconnect

250 Tbyte/day

10 Tbyte/day

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Seismic array & LOFARSeismic array & LOFAR


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Phased Arrays in the SKAPhased Arrays in the SKA

• LOFAR – like array < 0.1 – 0.3 GHz

• Aperture array 0.3 – 1.0 GHz

• Small dishes 0.3 – 25 GHz~10 m diameter

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Phased Arrays in the SKAPhased Arrays in the SKA

phased array systems - university of groningen

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