Date post: | 19-Dec-2015 |
Category: |
Documents |
View: | 215 times |
Download: | 1 times |
BDT Radio – 1b – CMV 2009/09/04
Basic Detection Techniques
1b (2009/09/04): Single pixel feedsTheory:
Brightness function
Beam properties
Sensitivity, sky noise, system noise, Aeff/Tsys
Receiver systems, mixing, filtering
Case study: the LOFAR Low Band AntennaReference frequency
Bandwidth
Beampattern
BDT Radio – 1b – CMV 2009/09/04
EM waves
Directionality (RA, dec, spatial resolution)
Time (timing accuracy, time resolution)
Frequency (spectral resolution)
Flux (total intensity, polarization properties)
),,,,( mltf
V
U
Q
I
BDT Radio – 1b – CMV 2009/09/04
Coherent detectors
Responds to electric field ampl. of incident EM wavesActive dipole antenna
Dish + feed horn + LNA
Requires full receiver chain, up to A/D conversion
Phase is preserved
Separation of polarizations
Typically narrow bandBut tunable, and with high spectral resolution
For higher frequencies: needs frequency conversion schemes
BDT Radio – 1b – CMV 2009/09/04
Sensitivity
Key question:What’s the weakest source we can observe
Key issues:Define brightness of the source
Define measurement process
Define limiting factors in that process
BDT Radio – 1b – CMV 2009/09/04
Brightness function
Surface brightness:Power received /area /solid angle /bandwidth
Unit: W m-2 Hz-1 rad-2
Received power:
Power per unit bandwidth:
Power spectrum: w(v)
Total power: Integral over visible sky and band
Visible sky: limited by aperture
Band: limited by receiver
BDT Radio – 1b – CMV 2009/09/04
Point sources, extended sources
Point source: size < resolution of telescope
Extended source: size > resolution of telescope
Continuous emission: size > field of view
Flux density:
Unit: 1 Jansky (Jy) = 10-26 W m-2 Hz-1
BDT Radio – 1b – CMV 2009/09/04
Reception pattern of an antenna
Beam solid angle (A = A/A0)Measure of Field of View
Antenna theory: A0 Ωa = λ2
BDT Radio – 1b – CMV 2009/09/04
Black-body radiation
General: Planck’s radiation law
Radio frequencies (hf << kT):Rayleigh-Jeans law (or rather: approximation)
B = 2kT/λ2
BDT Radio – 1b – CMV 2009/09/04
Antenna temperature, system temperature
Express noise power received by antenna in terms of temperature of resistor needed to make it generate the same noise power.
Spectral power: w = kT/λ2 Aeff Ωa = kTObserved power: W = kT Δv
Observed flux density: S = 2kT / Aeff
Tsys = Tsky + TrecTsky and Tant: what’s in a name
After integration: B
TTT
recsky
BDT Radio – 1b – CMV 2009/09/04
Sensitivity
Source power from Ta:
Source power from flux:Antenna area A, efficiency a
Rx accepts 1/2 radiation from unpolarized source
Define scaling factor KK is antenna’s gain or “sensitivity”
unit: degree Jy1
aTkGP
SAGP a21
k
A
S
TK aa
2
BDT Radio – 1b – CMV 2009/09/04
System Equivalent Flux Density
Covers Tant, not Tsys
Define SEFD:
What’s in Tsys?3K background and Galactic radio emission Tbg
Atmospheric emission Tsky
Spill-over from the ground and other directions Tspill
Losses in feed and input waveguide Tloss
Receiver electronics Trx
At times: calibration source Tcal
][2
JyA
Tk
K
TSEFD
a
syssys
BDT Radio – 1b – CMV 2009/09/04
Receiver chain at WSRT
Recv.Recv.
Equal.Equal.
IVCIVC
ADCADC
Recv.Recv.
Equal.Equal.
IVCIVC
ADCADC
west east
TADU
TADU
LOLO LOLOVLBIVLBI PuMaPuMa
CorrelatorCorrelator
Ref. Ref.
BDT Radio – 1b – CMV 2009/09/04
Inside the MFFE
UHFlow UHFhigh 92 49 13 18/21 6 3.6
swsw
LO1 lowSynth.
LO1 lowSynth.
swsw
1200-2200 MHz
1 GHzIF1 IF1
LO1 highSynth.
LO1 highSynth.
swsw
2200-9600 MHz
1 GHz
1 GHz
LO2Synth.
LO2Synth.
900 MHz
2x IF out100 ± 80 MHz
Cryogenic
BDT Radio – 1b – CMV 2009/09/04
Wire antennas, vivaldi
BDT Radio – 1b – CMV 2009/09/04
RCU B oard
A /D
B ac k plane& RF S hie ld
D is tributedB eam form ing
GbE
Sta
tio
nG
bE
sw
itch
(2
4 p
ort
s)
Gb
E s
wit
ch f
ab
ric
(23
1 o
utp
uts
)
W A N fibrec onnec t ions
S y nc .
Delay
B uffering
Corre lator /B eam form ers
(B lue G ene /L)
Calibrat ionRF I M it igat ion
S torage
c alibrat ion
Im age c reat ion
Us er applic at ionsRSP board 24
Station 77
Centra l P roc es s ing F ac ilit ies
A rc hiveE x port and
G RID
RS P B oard
W A N
Ionos phere
Outputc ontro l
Filter
Filter
Filter
Filter
B eamformer
BDT Radio – 1b – CMV 2009/09/04
Sampling
I: 0 - 100 II: 100 - 200 III: 200- 300
200 M Hz clockNyquist Zones
0 100 200 300
160 M Hz clockNyquist Zones
frequency [M Hz]
o bse rv ationmo de I10 - 90
Filte rs30
10 90 110
optiona l
ob se rv ationmo de II
110 - 190
ob se rv ationmo de IV210 - 250
ob se rv ationmo de III170 - 230
I : 0-80 II: 80 - 160 III: 160 - 240