Dana_Denis_talk_SDSS2013

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A Survey of Selected Radio Telescope Receiver Types
Dana Whitlow
Microwave Receiver Specialist, Arecibo Observatory
Denis Urbain
Microwave Receiver Specialist, Arecibo Observatory
In this talk we will consider several types of
receivers:
Single feed
Focal plane arrays
> Traditional (Arecibo ALFA, Parkes MB20)
> Phased array (AO-40; upcoming at Arecibo)
> Incoherent detector array (USRA SOFIA, GBT Mustang)
DEFINITION 1: COHERENT RECEIVER
Dual Polarization Feed, Typically a Circular
WG Feed with OrthoMode Transducer (OMT)
Polarization A
BPF
R
I
BPF
L
BPF
R
I
BPF
L
Low-Noise
Amplifiers
(cooled)
Polarization B
Local Oscillator
Power Detectors
(diodes or
ADCs + software)
COHERENT RECEIVER:
A receiver in which phase information is preserved through a gain process in the
signal chain, regardless of whether or not the phase information is ever actually
used. If your receiver has an amplifier prior to the detector, it is a coherent receiver.
Coherent receivers are subject to a fundamental quantum sensitivity limit:
Tnoise >= h /k
DEFINITION 2: INCOHERENT RECEIVER
Incoherent Detector
(normally a cryo-cooled
bolometer in radio or
IR astronomy)
Low-Noise
DC Amplifier
(cooled)
Data Acquisition System
INCOHERENT RECEIVER:
A receiver in which the first element in the signal chain is a direct (power) detector, in
which case phase information is destroyed. If the first active element in your receiver is
a heat detector, photocell (not used as a mixer), etc, you have an incoherent receiver.
The quantum sensitivity limit is said not to apply.
Single-beam versus Multi-beam
• Single beam (single pixel) operation
seems like a waste of a perfectly good
(well, almost) optical system. It’s
especially inefficient for survey work.
• Multiple beams permit considerably faster
survey work, but having them is definitely
an extra-cost (and extra-complication)
option.
G. Cortes-Medellin, K.F. Warnick, B. D. Jeffs, G. Rajagopalan,
P. Perillat, M. Elmer, D. Carter, V. Asthana, T. Webb, A. Vishwas.
“Field of View Characterization of Arecibo Radio Telescope with a
Phased Array Feed”.
IEEE Antennas and Prop Symposium,
Spokane, WA, Jul 2011
Optical Axis
"Ideal" reflector brings all rays
from a given direction to a focus
at a location which depends on
that direction.
Focal
"Plane"
Reflector
Because of diffraction, each
focus is a "blob", not a point.
The blob's size depends on
the wavelength and on the
angle of convergence, and
so does the depth of focus.
In a coherent receiver, the
feed- horn for the wavelength
of interest must be sized to fit
its blob.
BASIC TELESCOPE OPTICS
ALFA’s 7-ELEMENT CLOSE-PACKED FEED HORN ARRAY
Optical Axis
Unfortunately, the blobs arising
from off-axis beams are oversize
& distorted, placing them beyond
the realm of good capture by
practical feed horns.
Array of Independent
Conventional Receivers
and Back Ends
Array of Sized
Feedhorns in the
Focal "Plane"
Reflector
Also, the desire for close-packing
of the feedhorns' mouths constrains
against feed designs that produce
a very favorable footprint illumination
distribution on the main dish, leading
to inferior aperture efficiency.
The EM field distribution at the mouth
of such a horn is overly concentrated
so that its far field beam, which is
basically an image of the focal plane's
field distribution, is a fair bit narrower
than the beam-to-beam spacing
defined by the horns' non-overlapping
physical openings. Thus, there are
appreciable gaps between the beams.
"TRADITIONAL" FOCAL PLANE ARRAY
TRADITIONAL FOCAL PLANE ARRAY
• Receivers are independent, with no phase
connection.
• Therefore each feed must take individual
responsibility for matching its footprint to the
main reflector, setting a minimum size
requirement.
• Feeds of this size (always too large) cannot
adequately spatially sample the electromagnetic
field configuration at the focal plane to correct for
off-axis aberrations and permit creation of a
pattern of contiguous beams.
Example of Off-axis Aberration
(this is primarily “coma”)
FOCAL PLANE PHASED ARRAY
• Here the array comprises a grid of small antenna elements spaced
by slightly less than l/2, thereby meeting the Nyquist criterion for full
spatial sampling of the electric field configuration over the focal
plane. The elements are often implemented as shortened “halfwave” dipoles.
• The outputs of the elements are vector summed with complex
element- and beam-dependent weighting to produce the desired
beam(s) on the sky.
• Assuming that sufficient processing capability is available,
simultaneous production of many beams is possible.
• Beams can be well corrected for off-axis aberrations and (within
reason) focus errors.
• Within limits, pattern notches can be formed to mitigate RFI.
• But there’s a catch: electrical interactions and noise coupling
between the closely-packed elements seriously complicate the
design process and tend to degrade overall noise performance.
e
+
e
j
+
e
j
+
e
j
+
j
+
Array of Small
Antenna Elements
in the Focal "Plane"
Digital Beam Former
Not shown: a ton of pesky RF hardware:
amplifiers, filters. downconverters, etc, not
to mention a big pile of ADCs and other
digital hardware.
Beam 3 Out
Beam 4 Out
Beam 5 Out
+
+
Beam 0 Out
Beam 1 Out
Beam 2 Out
+
Each beam is formed
as a unique complexweighted sum of the
elements' individual
outputs.
Reflector
FOCAL PLANE PHASED ARRAY CONCEPT
BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY
BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY
INCOHERENT DETECTOR ARRAYS
• Incredibly, heat detectors (such as bolometers and arrays
thereof) can be made sensitive enough to be very useful
for astronomy.
• Greatest usefulness (for “radio” astronomy) is in the mmwave and sub-mm-wave regimes where fundamental
quantum behavior places severe limits on the noise
performance of coherent receivers.
• Incoherent detectors in general (including photon
detectors as well as bolometers) extend astronomy
upward in frequency all the way to the gamma ray regime.
• A variety of useful detection mechanisms are known and
used; all require cooling to sub-one-degree-Kelvin
temperatures to work. In fact, usually well below one
degree is required!
SOME ADVANTAGES OF INCOHERENT DETECTION
• Extends upper frequency limits of high-sensitivity radio
astronomy beyond current practical (and even
theoretical) limits of “conventional” (coherent) radio
telescope receivers.
• Uncouples the strict connection between beamwidth and
effective aperture area that is characteristic of coherent
receivers. This can sometimes be exploited to obtain a
sensitivity advantage if diffraction-limited angular
resolution is not required.
• Very wide pre-detection bandwidth (tens of GHz) is
available, which is really great for continuum work.
SOME ISSUES WITH INCOHERENT DETECTION
• No phase information is available from the detectors; thus neither
off-axis aberration correction nor participation in interferometry is
possible.
• Sensors are inherently insensitive to polarization.
• Spectroscopy is usually considered impractical since nothing can be
done post-detection, and versatile or tight pre-detection filtering is
extremely hard to implement. Some attempts have been made.
• Extraordinary care is required in the design and implementation of
the sensor (array) to keep out stray radiation everywhere in the
electromagnetic spectrum, since the inherent bandwidth of a thermal
sensor is essentially infinite. Accomplishing this adequately can be
much more challenging than it looks at first glance.
• Great attention is also required in the sensors’ output signal
handling circuitry to avoid microphonics, 1/f noise, etc.
• Cryogenic cooling is a challenge, especially in large arrays.
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