Signal Processing Instrumentation
(RF / Analog)
Ganesan Rajagopalan
Electronics Department
Arecibo Observatory
REU-Summer Student Seminars
14-June-2011
Talk outline
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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 Seminars
14-June-2011
Nature of Radio emission
• Radio Sources
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Thermal emission
Non-thermal synchrotron emission
Spectral line emission including Masers (partially polarized)
Pulsars
• Extremely weak, noise like signals
Power collected=S Ae B
S =Source flex density (watts/m^2/Hz)
Ae=Telescope effective area (m^2)
B=Bandwidth (Hz)
REU-Summer Student Seminars
14-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 Seminars
14-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:
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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 Seminars
14-June-2011
FRO N T
END
BAC K E
- ND
F
I /LO
D E TEC TO R
SG
I NAL
N
I
Dana
C OM PU TER
DG
I IT IZ ER
Phil / Luis
RADIO ASTRONOMY RECEIVER SYSTEM
REU-Summer Student Seminars
14-June-2011
Front-end of the Receiver
• Antenna
• Feed horn / dipole
• Polarizer &
• Low Noise Amplifier
REU-Summer Student Seminars
14-June-2011
X-POL.
ANTENNA
Y-POL.
ANTENNA
DUAL-POL.
(DOUBLE)
ANTENNA
SUMMER_06
DUAL-POLARIZATION ANTENNA
(EQUIVALENT TO TWO ANTENNAS)
REU-Summer Student Seminars
14-June-2011
FRONT
END
SIGNAL
IN
IF/LO
(T OT AL POWER) BACK-END
DETECTOR
DETECTOR
SIGNAL
IN
DIGITIZER
COMPUTER
DIGITIZER
rcvrsys.fc7
DUAL POLARIZATION RECEIVER
REU-Summer Student Seminars
14-June-2011
CO A X A
I L LN
I E :
O N LY O N E M O D E
PR N
I T ED C R
I C U IT
TR AC E :
O N LY O N E M O D E
CO A X A
I L LN
I E :
O N LY O N E M O D E
SQ U A R E W A V EG U D
I E :
TW O M O D E S
R E C T A NG U L A R
W A V EG U D
I E :
O N LY O N E M O D E
CR
I C U LA R W A V E G U D
I E :
TW O M O D E S
SINGLE AND DUAL-MODE TRANSMISSION LINES
REU-Summer Student Seminars
14-June-2011
CSIRO
POLARIZATION SEPARATORS
CSIRO
REU-Summer Student Seminars
14-June-2011
Low Noise Amplifier
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Equivalent Noise Temperature
Thermal and shot noise in transistors
Dependance on physical temperature
Cryogenic cooling improves sensitivity
REU-Summer Student Seminars
14-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 Seminars
14-June-2011
k=1.38 E-23 Watts/deg/Hz
POW ER = kT 2 B
POW ER = kT 1 B
F LI TER :
BAN DW DI TH = B
R@ T2
R@ T1
R E S SI TO R A T
290 D EG K.
(17 D EG C )
R EAD S -114dBm
ie. . 10 ^ ( -11 4. )mW .
F IL T E R :
1 M H Z B A N DW D
I TH
POW E R M E T E R
EXAM P LE
WHITE NOISE PRODUCED BY RESISTORS
REU-Summer Student Seminars
14-June-2011
S _ ni
S _ou t= G S _ ni + G kT am p
G am p
M
I AG NI AR Y R ES S
I TO R
AT TEM PER ATUR E
T am p
AM P L F
I E
I RW T
I H EQ U V
I A LEN T NO S
I E SO URC E
REU-Summer Student Seminars
14-June-2011
G 1G 2 T1
+ T2G 2
G 1 T1
G1
G2
T1
T1G 1G 2G 3
+ T2G 2G 3
+ T3G 3
T2
G3
T3
3 -AM P L F
I E
I R CASCADE
T 1G 1G 2G 3
+ T 2G 2G 3
+ T 3G 3
= (T 1 + T 2 G/ 1 +T 3 G/ 1G 2 ) G 1G 2G 3
G 1G 2G 3
EQU V
I A LEN T AM P L F
I ER
T 1+T 2 G/ 1+T 3 G/ 1G 2
NOISE ANALYSIS FOR CASCADED AMPLIFIERS
REU-Summer Student Seminars
14-June-2011
Berkshire Technologies, Inc.
REU-Summer Student Seminars
14-June-2011
Sky Contribution
(includes the cosmic microwave background)
REU-Summer Student Seminars
14-June-2011
Receiver Characterization
Receiver, 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 Seminars
14-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 * t )
independent samples in an integration time of “t” sec.
REU-Summer Student Seminars
14-June-2011
Minimum detectable signal
Tsys
T  
Bt
k Tsys
S  
( A )* Bt
e
REU-Summer Student Seminars
14-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 Seminars
14-June-2011
Typical Arecibo Receiver signal path
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Feed-horn at the focal plane
Polarizer (linear or circular pol splitters)
Front-end/
Noise injection Coupler
RF
Low Noise Amplifier
Filter (bandpass)
Post-Amplifier
Down-converter / Frequency Translator
IF/LO
Fiber-optic transmitter - receiver
More Down-converter / Frequency Translator
Digital
Sampler
Back-ends
Spectrometers / Total power recorders
REU-Summer Student Seminars
14-June-2011
Aerial view of the telescope
900 ton suspended platform held to within mm accuracy by laser
ranging
REU-Summer Student Seminars
14-June-2011
Telescope Optics
REU-Summer Student Seminars
14-June-2011
Shaped reflectors correct the spherical
aberration & bring the focus to a point
inside the dome
REU-Summer Student Seminars
14-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 Seminars
14-June-2011
Feed horns on a rotating turret on the
focal plane
REU-Summer Student Seminars
14-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 Seminars
14-June-2011
The front-end receivers and RF/IF signal
processing
REU-Summer Student Seminars
14-June-2011
DUAL BEAM DUAL POL. 6-8 GHz RECEIVER FRONT-END
Block Diagram of a single beam section
Band-defining
FILTER
AMPLIFIER
Dewar 15 K
30dB Coupler
LNA
FEEDHORN
Noise
Cal
inputs
OMT
Hi/lo Noise
Cal switching
circuitry
AMPLIFIER
30dB Coupler
LNA
Band-defining
FILTER
REU-Summer Student Seminars
14-June-2011
C-band high ( 6- 8 GHz) construction
• Caltech/JPL
InP LNA ~ 4 K
MMIC LNA
4-12 GHz
• Trx ~ 10 K
• Tsys ~ 25-30 K
• Polarizer, dewar
designed by Cornell
graduate student
J. Pandian
• Dual beam, in future
for continuum observations
REU-Summer Student Seminars
14-June-2011
World record 2K Amplifiers uses Indium
Phosphide transistors
Now, inside our 4-6 GHz Rx : ~ 7 K Rx temperature.
REU-Summer Student Seminars
14-June-2011
Improved Tsys
REU-Summer Student Seminars
14-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 Seminars
14-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 Seminars
14-June-2011
Arecibo L-band Feed Array
Inside view of ALFA
On it’s way up !
In place on the turret
REU-Summer Student Seminars
14-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 Seminars
14-June-2011
ALFA SYSTEM LEVEL DIAGRAM
REU-Summer Student Seminars
14-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 Seminars
14-June-2011
Arecibo’s FPA FOV Study
1. Caustics
2. Non Uniform
Plate Scale
-4.31
-3.17
-5.51
-3.92
-0.86
-0.86
-0.87
-1.16
-7.72
3. Non Uniform
Beam Power
levels
-2.93
-1.84
-1.56
-4.25
-3.35
-6.01
-6.96
-5.50
-10.04
~1700’’
4. How much
Incident Power
a PAF could
recover?
~1200’’
5. Optimum Ne?
REU-Summer Student Seminars
14-June-2011
The future is so exciting !
REU-Summer Student Seminars
14-June-2011
Thanx
REU-Summer Student Seminars
14-June-2011
Radio Receivers
Advancing Technology leads to
miniaturization & adds to functionality
REU-Summer Student Seminars
14-June-2011