Front-end, Back-end, correlators in Radiastronomy
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Front-end, Back-end, correlators in Radiastronomy
First MCCT-SKADS Training School September, 23-29 2007, Medicina
Enzo Natale
IRA - INAF Firenze
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Topics
• Description of a cryo receiver
- Feed horn / coupling to the antenna
- Polarizer / OMT
- Low Noise Amplifier
- IF processor
• Receiver sensitivity
• How many receiver?
- Dense focal plane array
- Array of receivers
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Dewar
Layout of a cryogenic receiver
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Feed horn
• Mode launching section (return loss - crosspol)
• Flare section (taper - antenna illumination)
Performances
Return loss : > 30 dB
Insertion loss : <0.2 dB
Off axis crosspol : < -35 dB
Bandwidth : 30% or larger
Trasformation from free space to guided propagation
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Optical coupling to the antenna
The illumination efficiency (optical coupling) of the antenna is the ratio ofthe gain of the antenna to that of a uniformely illuminated aperture and isdetermined by the illumination function or “edge taper”, i.e. the level of
the illumination at the edge of the reflector compared to that of the center.
Edge taper Te = P(0) / P(re )
Te (dB) = -10 log10 (Te )
For gaussian illumination function
re / w = (Te (dB) ln 10 / 20)0.5
re : reflector radius
w: 1/e radius of the beam
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Normalized
Copolar and
Crosspolar
beam pattern at 22 GHz
Horn for 18-26 GHz Multibeam
Taper 9 dB at the edge of the subreflector (9.5°)
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Multibeam horn at the Gregorian focus of SRT
(simulation with GRASP by R. Nesti)
Maximum gain Gi = (4 g
g : geometrical area of the antenna
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G/T ratio
G/T = G/(TA + TR) ; TA : antenna temperature
TR : receiver temperature at window
Tatm = 265 K=f = 22 GHz
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To evaluate the performances achievable at the focus of a large antenna( D >> we report here some results based on the approximation of the electromagnetic field distribution in terms of Gaussian beam modes.
(Goldsmith: Quasioptical System, IEEE Press, 1998).
w0 waist (1/e)
wavelength
r radial distance
w(z) beam radius (1/e) at z
z distance from the waist
R curvature radius of the beam
Gaussian beams
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In this approximation, it can be shown that, for not too large flare angle afeed horn with aperture radius a and slant length R produces a gaussian beam whose waistradius is w = 0.644 a located inside the horn at a distance z approximatelye qual to 1/3 of the horn length. In these conditions about 98 % of the power radiated by the horn can be associated with the fundamental Gaussian beam mode.
Using the standard formulae for Gaussian beam mode propagation , it issimple to compute the antenna illumination (the edge taper) and consequentely thefull width to half maximum (FWHM) beam width in the sky of a in-focus system andunblocked aperture:
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Ortho Mode Transducer (OMT)
Differential Phase Shifter (DPS)
The feed horn is sensitive to both linear and circular polarizationsLinear polarizations are separated by the OMTCircular polarizations needs to be converted in linear (DPS)
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DPS
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Feed horn Coupler
DPSWaveguide to SMA
converter
Passive 18 - 26 GHz Front-end
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Turnstile junction(Navarrini, Plambeck IEEE MTT 45, Jan. 2006)
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Planar OMT(Engargiola,Navarrini, IEEE MTT 53, May 2005)
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Low Noise Amplifiers
Typical performances of a cryogenic LNA Gain : >= 30 dBGain flatness : ~ 1 - 2 dBInput return loss : < 15 dBBandwidth : 30% or largerPower Out @ 1dB Compression : +5dBmWorking temperature : ~ 20K
Noise temperature : ~ 18 - 30 K 18 - 26 GHz ~ 30 - 50 K 36 - 50 GHz
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Devices : GaAs, InP High Electron Mobility Transistors (HEMT) and Heterostructure FET (HFET)
: GaAs, InP Integrated circuits
1/f noise
(G / G)2 = N A f
N number of active devices
A constant (i.e. ~ 3.6 10-8 Hz-1 for InP HEMT
4 - 8 GHz2 stages GaAs HEMTNoise T : 5K(Alma Memo 421)
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LNA block diagram
(inclusion of coupler + calibration source at the input?)
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MMIC amplifier
chipmounted
Hybrid amplifier
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IF processor • accurate definition of the receiving band • conversion of the RF band to IF band for easy interfacing to the back-end.
IF Processor
Receiver type : superheterodyne
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The mixer
LO
RFRF = E sin (2st +LO = V sin (2LOt )
I = RF + LO)2 = • EV2
• sin[2 (2st +• sin [2 (2LOt )• E V sin[2s - LO)]• V sin[2s + LO)]
I
I = I0 [exp(q v /k T ) - 1]
For small v :
Iv)2
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Receiver Sensitivity
Tsys ON = Tbg + Tatm + Topt + Trec + Tsource
Tsys OFF = Tbg + Tatm + Topt + Trec
Tsys ON Tsys OFF s(t) = k B G Tsys
If the noises are white : rmsTsys (B radiometric noise) (Kraus)
Tbg = 2.7 K CMB*atm
Tatm = atmospheric emiss.
Topt = spillover
Trec = receiver
Tsource = Tsys ON - Tsys OFF = xs (ON source - x r (OFFsource = X
X(rmsdepends on the modulation type
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But in real detecting system (receiver + atmosphere + ..) the low frequency noise is not white:
• 1/f(electronics ( gain variation, ..)
• 1/f(1< drift, atmosphere
18 - 26 GHz receiver
B ~ 400 MHz
msec
Measuring system
Power spectral density
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In this case:
White noise (radiometric)1/f noise1/f2 noise
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Allan plot
1/f2 noise
1/f noise
White noise 18 - 26 GHz receiver
B ~ 400 MHz
msec
Measuring system
Allan time
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Mitigation of the 1/fnoise
• “high” (>> 1/Allan time) modulation frequency- ON Source / OFF Source- On The Fly- Two beams Dicke (equalized channels)
• gain stabilization (no effect on the atmospheric noise)
- Dicke receiver ( Modulation between sky end reference source)
- Correlation receiver- Noise injection receiver
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Dicke receiver
T / Tsys = ( G / G) (Ta - Tn )/ Tsys (Kraus, 1966)
For balanced systems Ta = Tn
T / Tsys = (2 / B
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Noise injection receiver
Tsys = Tn s1/(s2 - s1)
( Tsys)2 = (1/Bs22 + s12)/(s2 - s1 )2 )
s1 = kGBTsys during toff
s2 = kGB(Tsys + Tn) during ton Tn = xTsys
Tsys = (2 /
Bxx2 )0.5
W = ( Tsys ) / (2 / B
= xx2 )0.5
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How many receivers?
Maximize observing efficiency of an antenna
Focal Plane Array
• Dense FPA (mainly for GHz)
• Array of single pixel receiver
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Dense array
• Small array elements : about 0.5• Optimization of the beam properties -> high efficiecy low spillover• Multi beam capabilities -> increase FOV
survey speed• Electronic synthesis -> flexibility• Operating frequency -> up to ~ 8GHz
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PHAROS (PHased Arrays for Reflector Observing System)
Vivaldi array
13x13 elements
pitch 21 mm
Optimized for:
• prime focus 0.3-0.5 f/D
• 4 - 8 GHz
Antennas, LNA, beam former cryocooled
(PHAROS System Specification, Dec 2006)
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PHAROS antenna (3D model)
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Beam former
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Window problems :
• mechanical (16 mm plexiglas)
• thermal (radiation power due to the ambient ~ 45 Watt)
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Array of single pixel receivers
Current technology capabilities still prevent the use of dense arrays at frequencies higher than say 10 GHz. The only possibility is to build-up array by assembling together a certain number of single channel (dual polarization) receivers..For the sake of simplicity we briefly describe the structure of an hypothetical multibeamfor the 36 - 50 GHz band for Medicina antenna.
Antenna: D = 32 m feq = 97 m F = feq /D = 3.04
~ 80%Optical coupling efficiency : edge taper Te (dB)= 9 dB
FWHM = (1.02 + 0.0135 Te (dB) ) l /D = 45 arcsec @ = 7 mm
Beam at primary (wa ) : 0.5 D / [Te (dB) ln(10) /20]0.5 (from the definition of edge taper)
The illuminator (feed horn) must have
have w0 = feq / wa
located in the antenna focus
Horn radius R = w0 /0.644 = 21.4 mm
13.8 mm
~ F
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Sampling
Nyquist limit : 0.5 F (in the focal plane)
Actual sampling : 2 F
Undersampling 4
In practice
+ : Nyquist positionscircle : horn position
undersampling 5
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18 - 26 GHz Multibeam
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Correction of field curvature (Petzval surface)
Configuration Antennagain [dB]
Antennaefficiency[%]
#1 Feed in the Cassegrainfocus
82.09 78.53
#2 Feed shifted in thefocal plane
80.90 59.67
#3 Feed placed in thebest-focus point
81.54 69.07
(distance from the optical axis)
Best focus positionfrom the in axix focus
Medicina antenna = 7mm = 500 mm
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Conclusions
Multi beam to increase the observing efficiency - new solutions for “simpler” front-end- integration of cal. source in the LNA- IF integration
(Low cost ?) Integrated receiver (MMIC)