reu-summer student seminars 14-june-2011 signal processing instrumentation (rf / analog) ganesan...

REU-Summer Student Seminars14-June-2011

Signal Processing Instrumentation(RF / Analog)

Ganesan Rajagopalan

Electronics Department

Arecibo Observatory


Talk outline

• Concept of Polarization & the need for dual polarization receivers

• Concept of Noise Figure / Noise temperature

• Basic Receiver architecture

• Dynamic range considerations

• Concept of System Temperature

• Super heterodyne down converter

• Techniques of receiver calibration (hot/cold loads, cal injection)

• Cryogenic Receiver Front-end design & construction

• Array receivers & telescopes in the near future

• FOV study using BYU Phased Array Feed


Nature of Radio emission

• Radio Sources– Thermal emission

– Non-thermal synchrotron emission

– Spectral line emission including Masers (partially polarized)

– Pulsars

• Extremely weak, noise like signals

Power collected=S Ae BS =Source flex density (watts/m^2/Hz)

Ae=Telescope effective area (m^2)

B=Bandwidth (Hz)


Arecibo’s Receivers & Transmitters enable really unique Science

• Aeronomy– Incoherent RADAR scatter studies of the ionosphere using 2 MW

Pulsed RADAR & receivers at 430 MHz & 48 MHz.

• Planetary Astronomy– Imaging of Planets, Moons, Asteroids, Comets etc. using 1 MW

CW Radar & receiver at 2380 MHz.

• Radio Astronomy– Galactic, Extra-galactic astronomy using ultra-sensitive receivers

from ~ 300 MHz - 10 GHz for & Surveys using the multi-beam ALFA receiver. Plans for a ~100 element Phased Array Feed.


Radio Astronomical requirements• High sensitivity, wide frequency coverage, data acquisition

with very high spectral, spatial and time resolution.

• Remember, our receivers have to detect signals that are several orders of magnitude weaker than typical signals from:– cell phone towers

– local FM station

– nearby TV station

– DirecTV geo-stationary satellite

– NASA Spacecraft in the solar system

• RFI –Radio Freq Interference from nearby radars, cell towers cause serious issues


RADIO ASTRONOMY RECEIVER SYSTEM

SIGNAL IN

FRONT END

IF/LO

DIGITIZERCOMPUTER

BACK-END

DETECTOR

Dana Phil / Luis


Front-end of the Receiver

• Antenna

• Feed horn / dipole

• Polarizer &

• Low Noise Amplifier


DUAL-POLARIZATION ANTENNA

(EQUIVALENT TO TWO ANTENNAS)

DUAL-POL. (DOUBLE)ANTENNA

Y-POL. ANTENNA

X-POL. ANTENNA

S U M M E R _06


DUAL POLARIZATION RECEIVER

SIGNAL IN

FRONT E ND

IF /L O

DETECTOR DIGITIZER

SIGNAL

IN

DIGITIZERDETECTOR

COMPUTER

(T O T A L P O W E R ) B A C K -E N D

r c v r s y s .fc 7


SINGLE AND DUAL-MODE TRANSMISSION LINES

SQUARE WAVEGUIDE:TWO MODES

CIRCULAR WAVEGUIDE:TWO MODES

RECTANGULAR WAVEGUIDE:ONLY ONE MODE

PRINTED CIRCUITTRACE:ONLY ONE MODE

COAXIAL LINE:ONLY ONE MODE

COAXIAL LINE:ONLY ONE MODE


CSIRO

CSIRO

POLARIZATION SEPARATORS


Low Noise Amplifier

• Equivalent Noise Temperature

• Thermal and shot noise in transistors

• Dependance on physical temperature

• Cryogenic cooling improves sensitivity


Receiver is only as good as the very first amplifier in the chain

• Treceiver is mostly determined by the noise added by the first amplifier in the chain

• Noise added consists of thermal noise (coupled from the resistances in the device) & shot noise (from the quantized and random nature of current flow) –additionally inter-valley scattering, 1/f noise

• Both thermal & shot-noise contributions go down with temperature. So, we cool the front-end of our receivers to ~ 15 K

• Cooling the front-end to ~15K is achieved by a form of adiabatic expansion using 99.999% pure Helium in a closed cycle compressor system.


WHITE NOISE PRODUCED BY RESISTORS

FILTER:1 MHZ BANDWIDTH

EXAMPLE

POWER METER

R @ T1 R @ T2

READS -114dBmi.e. 10̂ (-11.4) mW. RESISTOR AT

290 DEG.K (17 DEG C)

POWER = kT1 BPOWER = kT2 B

FILTER:BANDWIDTH = B

k=1.38 E-23 Watts/deg/Hz


Gamp

S_in S_out = G S_in + G kTamp

IMAGINARY RESISTORAT TEMPERATURETamp

AMPLIFIER WITH EQUIVALENT NOISE SOURCE


3-AMPLIFIER CASCADE

T1

G1

G1 T1G1 G2 T1+ T2 G2

T2

G2

T3

G3

= (T1 + T2/G1 +T3/G1G2 ) G1G2G3

T1+T2/G1+T3/G1G2

G1G2G3

EQUIVALENT AMPLIFER

T1 G1 G2 G3 + T2 G2 G3+ T3 G3

T1 G1 G2 G3 + T2 G2 G3+ T3 G3

NOISE ANALYSIS FOR CASCADED AMPLIFIERS


Berkshire Technologies, Inc.


Sky Contribution(includes the cosmic microwave background)


Receiver CharacterizationReceiver, Sky, Antenna & System temperatures

• Treceiver, mainly from the first stage amplifier– measured by hot / cold “Y” factor method– room temp /liquid nitrogen /sky as reference absorbers

• Tsky = Tatmosphere + Tbackground + ( Tsource )

• Tantenna = Tsky + Tspillover

• Tsystem = Tantenna + Treceiver


Minimum detectable signal

• From statistics, we know the error on a measurement goes down as the square root of the number of independent samples.

• In a radio receiver with bandwidth “B” Hz, we get (B * ) independent samples in an integration time of “” sec.


Minimum detectable signal

BA

TsyskS

B

TsysT

e *)(


A simple receiver diagram

• Power received at the antenna P =k Ta B

K=Boltzman constant (joules/Hz/kelvin)

Ta =Antenna Temp. (kelvin)

B=Bandwidth (Hz)

• Dual Polarization Rx. S Ae = k T

Increase in T due to the source is usually a fraction of the total system noise, for most sources. So, several integrations over time is needed.


Typical Arecibo Receiver signal path

• Feed-horn at the focal plane • Polarizer (linear or circular pol splitters)• Noise injection Coupler• Low Noise Amplifier • Filter (bandpass)• Post-Amplifier• Down-converter / Frequency Translator• Fiber-optic transmitter - receiver• More Down-converter / Frequency Translator• Sampler• Spectrometers / Total power recorders

Front-end/ RF

IF/LO

Digital Back-ends


Aerial view of the telescope900 ton suspended platform held to within mm accuracy by laser

ranging


Telescope Optics


Shaped reflectors correct the spherical aberration & bring the focus to a point

inside the dome


Receiver Front-end Design, Construction, Characterization & Operation

• Feed horn designed to illuminate the tertiary optimally, without picking up a lot of spill-over radiation

• Polarizer designed to isolate the two linear or left/right circular polarizations

• Low Noise Amplifier (LNA) designed to add the least possible amount of additional noise

• Dewar designed to cool the Polarizer, noise injection coupler & amplifiers

• Cryo compressors & Cryo pumps use 99.999% He in closed cycle refrigeration system


Feed horns on a rotating turret on the focal plane


Spill-over contribution adds to Tsys• Feed horn design is usually optimized to get the best

possible (G / T)

• Depending on Feed horn design, the added noise can be as high as 12 K for the dome receivers

• Treceiver contribution is < 10K for most of the cooled receivers

• ~ 8K from Sky+Atm, <12 K from spillover, < 10K from receiver adds up to system temp < 30 K


The front-end receivers and RF/IF signal processing


DUAL BEAM DUAL POL. 6-8 GHz RECEIVER FRONT-ENDBlock Diagram of a single beam section

O M T

F E E D HO R N

Ban d - d ef in in gF I L T E R

L N A

AM P L I F I E R

AM P L I F I E R

Hi/lo N o is eC al s w itc h in g

c ir c u itr y

D ew ar 15 K

N o is eC alinp uts

3 0 d B C o u p ler

3 0 d B C o u p ler

Ban d - d ef in in gF I L T E R

L N A


C-band high ( 6- 8 GHz) construction

• Caltech/JPLInP LNA ~ 4 K

• Trx ~ 10 K

• Tsys ~ 25-30 K

• Polarizer, dewardesigned by Cornellgraduate studentJ. Pandian

• Dual beam, in futurefor continuum observations

MMIC LNA

4-12 GHz


World record 2K Amplifiers uses Indium Phosphide transistors

Now, inside our 4-6 GHz Rx : ~ 7 K Rx temperature.


Improved Tsys


Strong RFI from military radars, communication services, satellites and local sources

• Linearity is the most important requirement

• RFI causes receiver saturation and recovery problems & intermods.

• Switch-in Filters to cut down, if possible

• RFI mitigation

• RADAR blanking

• Flag off bad data in S/W

• Ref antenna & cross-correlation techniques

RFI in 1.8 - 3.0 GHz band


Single Pixel vs. Array receivers

• Increase in mapping efficiency, ideal for large scale surveys.

• Gregorian optics limits the number of pixels

• Scanning losses increase as feed moves away from center.– ALFA outer beams’ gain is less by ~ 10 %


Arecibo L-band Feed Array

Inside view of ALFA

On it’s way up !

In place on the turret


The ALFA system

• The gain of the central beam is 11 K/Jy, the system temperature is about 25 K at 1400 MHz

• The gain of the outer beams is about 8 K/Jy

• The average beam size is 204 x 232 arc seconds. The six outer beams sit on an ellipse of 329 x 384 arc seconds.


ALFA SYSTEM LEVEL DIAGRAM


Wideband Single Pixel & Array Receivers

German Cortes in Ithaca is working on

• octave bandwidth single pixel receivers

• several possible array receiver configurations


Arecibo’s FPA FOV Study

-1.56

-1.84

-0.87

-0.86

-0.86

-1.16

-10.04

-5.50

-6.01

-3.35

-4.25

-2.93

-4.31

-3.17

-5.51

-3.92

-7.72

-6.96

2. Non Uniform Plate Scale

3. Non Uniform Beam Power

levels~1700’’

~1200’’

5. Optimum Ne?

4. How much Incident Power

a PAF could recover?

1. Caustics


The future is so exciting !


Thanx


Radio Receivers

Advancing Technology leads to miniaturization & adds to functionality

reu-summer student seminars 14-june-2011 signal processing instrumentation (rf / analog) ganesan...

Documents