Phased Array Systems - University of Groningen
Transcript of 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
21
θθπ
ηββ
θ re
relkIjaEEE
krjkrj
t)
λπ2
=k
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FarFar--field of a 2 element arrayfield of a 2 element array
⎭⎬⎫
+⎩⎨⎧
=+=−−−−
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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FarFar--field of a 2 element arrayfield of a 2 element array
⎥⎦⎤
⎢⎣⎡ +⋅=
−
)cos(21cos2cos
40 βθθπ
ηθ kdr
lekIjaEjkr
t)
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FarFar--field of a 2 element arrayfield of a 2 element array
⎥⎦⎤
⎢⎣⎡ +⋅=
−
)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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Grating lobesGrating lobes
wavelength
d
grating lobes
main lobe
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Grating lobesGrating lobes
• 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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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