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REU-Summer Student Seminars14-June-2011
Signal Processing Instrumentation(RF / Analog)
Ganesan Rajagopalan
Electronics Department
Arecibo Observatory
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
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)
REU-Summer Student Seminars14-June-2011
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.
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
RADIO ASTRONOMY RECEIVER SYSTEM
SIGNAL IN
FRONT END
IF/LO
DIGITIZERCOMPUTER
BACK-END
DETECTOR
Dana Phil / Luis
REU-Summer Student Seminars14-June-2011
Front-end of the Receiver
• Antenna
• Feed horn / dipole
• Polarizer &
• Low Noise Amplifier
REU-Summer Student Seminars14-June-2011
DUAL-POLARIZATION ANTENNA
(EQUIVALENT TO TWO ANTENNAS)
DUAL-POL. (DOUBLE)ANTENNA
Y-POL. ANTENNA
X-POL. ANTENNA
S U M M E R _06
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
CSIRO
CSIRO
POLARIZATION SEPARATORS
REU-Summer Student Seminars14-June-2011
Low Noise Amplifier
• Equivalent Noise Temperature
• Thermal and shot noise in transistors
• Dependance on physical temperature
• Cryogenic cooling improves sensitivity
REU-Summer Student Seminars14-June-2011
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.
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
Gamp
S_in S_out = G S_in + G kTamp
IMAGINARY RESISTORAT TEMPERATURETamp
AMPLIFIER WITH EQUIVALENT NOISE SOURCE
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
Berkshire Technologies, Inc.
REU-Summer Student Seminars14-June-2011
Sky Contribution(includes the cosmic microwave background)
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
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.
REU-Summer Student Seminars14-June-2011
Minimum detectable signal
BA
TsyskS
B
TsysT
e *)(
REU-Summer Student Seminars14-June-2011
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.
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
Aerial view of the telescope900 ton suspended platform held to within mm accuracy by laser
ranging
REU-Summer Student Seminars14-June-2011
Telescope Optics
REU-Summer Student Seminars14-June-2011
Shaped reflectors correct the spherical aberration & bring the focus to a point
inside the dome
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
Feed horns on a rotating turret on the focal plane
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
The front-end receivers and RF/IF signal processing
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
World record 2K Amplifiers uses Indium Phosphide transistors
Now, inside our 4-6 GHz Rx : ~ 7 K Rx temperature.
REU-Summer Student Seminars14-June-2011
Improved Tsys
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
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 %
REU-Summer Student Seminars14-June-2011
Arecibo L-band Feed Array
Inside view of ALFA
On it’s way up !
In place on the turret
REU-Summer Student Seminars14-June-2011
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.
REU-Summer Student Seminars14-June-2011
ALFA SYSTEM LEVEL DIAGRAM
REU-Summer Student Seminars14-June-2011
Wideband Single Pixel & Array Receivers
German Cortes in Ithaca is working on
• octave bandwidth single pixel receivers
• several possible array receiver configurations
REU-Summer Student Seminars14-June-2011
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
REU-Summer Student Seminars14-June-2011
The future is so exciting !
REU-Summer Student Seminars14-June-2011
Thanx
REU-Summer Student Seminars14-June-2011
Radio Receivers
Advancing Technology leads to miniaturization & adds to functionality
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