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

Example: VLA and MFFE

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

Horn antennas

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

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

to receiver

1..16on/off delaystep

on/off delaystep

BDT Radio – 1b – CMV 2009/09/04

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