Front-end, Back-end, correlators in Radiastronomy

Front-end, Back-end, correlators in Radiastronomy

First MCCT-SKADS Training School September, 23-29 2007, Medicina

Enzo Natale

IRA - INAF Firenze

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

Dewar

Layout of a cryogenic receiver

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

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

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°)

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

G/T ratio

G/T = G/(TA + TR) ; TA : antenna temperature

TR : receiver temperature at window

Tatm = 265 K=f = 22 GHz

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

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:

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)

DPS

Feed horn Coupler

DPSWaveguide to SMA

converter

Passive 18 - 26 GHz Front-end

Turnstile junction(Navarrini, Plambeck IEEE MTT 45, Jan. 2006)

Planar OMT(Engargiola,Navarrini, IEEE MTT 53, May 2005)

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

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)

LNA block diagram

(inclusion of coupler + calibration source at the input?)

MMIC amplifier

chipmounted

Hybrid amplifier

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

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

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

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

In this case:

White noise (radiometric)1/f noise1/f2 noise

Allan plot

1/f2 noise

1/f noise

White noise 18 - 26 GHz receiver

B ~ 400 MHz

msec

Measuring system

Allan time

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

Dicke receiver

T / Tsys = ( G / G) (Ta - Tn )/ Tsys (Kraus, 1966)

For balanced systems Ta = Tn

T / Tsys = (2 / B

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

How many receivers?

Maximize observing efficiency of an antenna

Focal Plane Array

• Dense FPA (mainly for GHz)

• Array of single pixel receiver

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

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)

PHAROS antenna (3D model)

Beam former

Window problems :

• mechanical (16 mm plexiglas)

• thermal (radiation power due to the ambient ~ 45 Watt)

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

Sampling

Nyquist limit : 0.5 F (in the focal plane)

Actual sampling : 2 F

Undersampling 4

In practice

+ : Nyquist positionscircle : horn position

undersampling 5

18 - 26 GHz Multibeam

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

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)