A Survey of Selected Radio Telescope Receiver Types

Dana Whitlow Microwave Receiver Specialist, Arecibo Observatory

Denis UrbainMicrowave Receiver Specialist, Arecibo Observatory

In this talk we will consider several types of

receivers:

Single feedFocal plane arrays > Traditional (Arecibo ALFA, Parkes MB20)

> Phased array (AO-40; upcoming at Arecibo)

> Incoherent detector array (USRA SOFIA, GBT Mustang)

R I

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B P F

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Dua l P o la riza tion F eed , T yp ica lly a C ircu la rW G F eed w ith O rthoM ode T ransduce r (O M T )

Loca l O sc il la to r

P o la riza tion A

P o la riza tion B

CO HERENT RECEIVER: A rece ive r in wh ich phase in fo rm ation is p reserved th rough a ga in p rocess in thesigna l cha in , regard less o f whe the r o r no t the phase in fo rm ation is eve r actua llyused . If you r rece ive r has an am p lifie r p rio r to the de tecto r, it is a coheren t rece ive r.Coheren t rece ive rs a re sub ject to a fundam enta l quan tum sensitivity lim it:Tno ise >= h /k

Low-No iseA m p lifie rs(coo led )

DEFIN IT IO N 1: CO HERENT RECEIVER

P ower De tecto rs(d iodes o r A DCs + so ftwa re )

Incohe ren t De tecto r(no rm a lly a c ryo -coo ledbo lom e te r in rad io o rIR astronom y)

INCO HERENT RECEIVER: A rece iver in wh ich the first e lem ent in the signa l cha in is a d irect (power) de tecto r, inwh ich case phase in fo rm ation is destroyed . If the first active e lem ent in your rece iver isa hea t de tecto r, pho toce ll (no t used as a m ixe r), e tc, you have an incoheren t rece iver.The quan tum sensitivity lim it is sa id no t to app ly.

Low-No iseDC A m p lifie r

(coo led )

D EFIN ITIO N 2 : IN C O H ER EN T R EC EIVER

Data Acquisition System

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 aPhased Array Feed”. IEEE Antennas and Prop Symposium,Spokane, WA, Jul 2011

"Ideal" re flector brings a ll raysfrom a g iven d irection to a focusat a location w hich depends onthat d irection.

Because of d iffraction, eachfocus is a "b lob", not a po int.The b lob 's size depends onthe w avelength and on theangle of convergence, andso does the depth of focus.

In a coherent receiver, thefeed- horn for the w avelengthof in terest m ust be sized to fitits b lob.

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R e fle c to r

Fo ca l"P la n e "

BASIC TELESCO PE O PTICS

ALFA’s 7-ELEMENT CLOSE-PACKED FEED HORN ARRAY

U nfortunately, the blobs arisingfrom off-axis beam s are oversize& distorted, placing them beyondthe realm of good capture bypractical feed horns.

Also, the desire for close-packingof the feedhorns' m ouths constrainsagainst feed designs that producea very favorable footprint illum inationdistribution on the m ain dish, leadingto inferior aperture efficiency.

The EM field distribution at the m outhof such a horn is overly concentratedso that its far field beam , w hich isbasically an im age of the focal plane'sfield distribution, is a fair bit narrow erthan the beam -to-beam spacingdefined by the horns' non-overlappingphysical openings. Thus, there areappreciable gaps betw een the beam s.

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R eflector

Array of SizedFeedhorns in theFocal "Plane"

Array of IndependentConventional Receiversand Back Ends

"TR A D IT IO N A L" FO C A L P LA N E A R R A Y

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 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 “half-wave” 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.

Reflecto r

Array of SmallAntenna Elements in the Focal "Plane"

Each beam is formedas a unique complex-weighted sum of theelements' individualoutputs.

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FOCAL PLANE PHASED ARRAY CONCEPT

D igita l Beam Form erNot shown: a ton of pesky RF hardware:am plifiers , filters . downconverters , etc , notto m ention a big pile of ADCs and otherdigital hardware.

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 mm-wave 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.